As we look at modern systems and what specific problems they present, it’s important to understand the basic fundamentals associated with these systems.

Most of our modern heating equipment in some way or another involves electronics; along with the use of electronics is the use of flame rectification as a safety and flame-proving system. It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler; the same basic system is used to perform safe ignition, and then consistent operation, throughout the entire call for heat.

There are, however, different ways it is applied—from intermittent pilot application to direct spark ignition and including hot surface ignition. Each system has its own distinct advantages and problems. Next in this series of resolving burner issues related to these systems, we offer corrections and diagnostics. It can be easy to jump to conclusions and change parts until you hopefully solve the problem. That is, however, time-consuming and costly.

I invite you to visit our Facebook page, Timmie’s Tips On Gas, I look forward to seeing you there.

We are starting a new series titled Advanced Controls for Gas Heating. In future columns, we will break down and examine individual controls, leading up to universal replacement controls—one control that replaces many others, making servicing these systems much easier, as well as simplifying truck stock.

Electronic Furnace & Boiler Controls
I have observed the gas industry since I began working in the 1960s until the present day and we have come a long way. Gas company technicians back then were using test lights instead of meters and there were no electronics to speak of. My background was in electronics in the Navy, and I owned my own multimeter and was quite capable of using it correctly to diagnose and troubleshoot the rather simple gas system circuits.

The slow addition of electronics into gas systems has surely taken off today. There are currently numerous systems, using rather sophisticated electronic controls, to operate many of the required functions. Almost everything in homes today uses some kind of microprocessor control. The mandates for efficiency have driven the plumbing, heating and HVAC industry into the electronic age. While we could probably still do some of the things we do with electromechanical controls, it would be with great difficulty. These functions are much easier with microprocessor-type controls.

Let’s discuss electronic controls used on furnaces and boilers. On the furnace side, we have Electronic Fan Timers and Integrated Furnace Controls. On the boiler side, we have multi-zone panels and (to be covered in a future column) Integrated Boiler Controls.

The traditional fan-limit control for warm air furnace:

• Has both functions combined into one device (L4064)
• Is difficult to apply in horizontal and counterflow furnaces, and especially in multipoise furnaces.

Ø Location for the fan control won’t work for the limit and vice versa
Ø Two separate controls cost too much

• Even though it perhaps has made controlling duct temperatures easier, due to its ability to be adjusted for ON–OFF temperature, it was just not adaptable to modern furnaces.
• Time assist fan-limit control (L4064T, E)

Ø Fan turns on after 90 seconds whether the sensor feels heat or not
Ø Limit control works as usual

Time-only fan control:
• The solution is to abandon temperature altogether as the basis for switching the fan and go to time only
• Bimetal timers have been available for a long time to do this job.

Ø S876 is still in the Honeywell product line and is a good solution to fan switching problems
Ø There are still Carrier/Bryant/Day Night Payne furnaces that use a bimetal control

Electronic fan control:
• The ability to design and build reliable electronic fan controls made a substantial change in the appearance of warm air furnaces:

Ø Electronic, and later digital, controls resulted in very precise fan-on and fan-off timing. We could have exactly the right timing for each furnace design. On delays and off delays could be specified exactly.
Ø Application became much easier because the issue of locating the fan sensor was completely gone.
Ø By switching the fan at exactly the right time, there are some small, but measurable, improvements in efficiency.
Ø Because of the increased efficiencies, the protection of the heat exchanger due to thermal stress was also a factor in precise timing, in particular the fan-on times.

Electronic Fan Timers
Electronic fan timers have been around for a while. The ST9101 is used on some standing pilot furnaces and the ST9103 is used on oil furnaces. The ST9103 integrates control of burner and circulating fan operation in an oil furnace. The ST9101A; ST9120A-C, G; and ST9141A integrate control of the combustion blower and circulating fan operation in warm air gas furnaces.

The use of twinning on warm air systems today is also addressed by twinning features on these controls. It is important to realize that these controls are configured for application to specific appliance models. They are intended for direct replacement of OEM installed controls only; as noted in Figure 3 below (do not attempt to apply these controls except as direct replacement for specific Honeywell models noted).

Later in this series, we will address the use of universal replacement controls on both the ST9120U-1003 and the newest version, the ST9120U-1011. As the systems grow older, the development of universal replacement controls makes the service techs’ job a lot easier.

Figure 3 shows the controls and the particular original equipment manufacturer (OEM) that used them. As we address universal replacement controls in future columns, you will see that many of these are replaceable by a universal application. ICM

As we look at modern systems and what specific problems they present, it’s important to understand the basic fundamentals associated with these systems.

Most of our modern heating equipment in some way or another involves electronics; along with the use of electronics is the use of flame rectification as a safety and flame-proving system. It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler; the same basic system is used to perform safe ignition, and then consistent operation, throughout the entire call for heat.

There are, however, different ways it is applied—from intermittent pilot application to direct spark ignition and including hot surface ignition. Each system has its own distinct advantages and problems. Next in this series of resolving burner issues related to these systems, we offer corrections and diagnostics to help solve those problems.

We will start with the basics and go into operation, typical problems, diagnosis, troubleshooting procedures and a final solution to a particular problem. It can be easy to jump to conclusions and change parts until you hopefully solve the problem. That is, however, time-consuming and costly.

I invite you to visit our Facebook page, Timmie’s Tips On Gas, I look forward to seeing you there.

We are presently doing a series on Honeywell Smart Valve and will continue to cover Smart Valve Generation III, with step-by-step troubleshooting of these controls.

Gas Valve/Induced Draft Fan/Pressure Switches
The Gas Valve is connected to the combustion chamber through a series of tubing connections, which include the Induced Draft Fan, the Low Fire Pressure Switch and the High Fire Pressure Switch.

The gas valve has a knurled fitting at what would be the normal regulator vent to atmosphere. The knurled fitting is for the tubing to fit over; the valve is then connected to the combustion chamber so that the valve senses, not room pressure, but combustion chamber pressure. The two pressure switches are set up to be differential switches, which must sense a certain differential pressure in order for them to close. The differential pressure for the Low Fire Switch is .58″ W.C. and the High Fire Switch is 1.38″ W.C.

Looking at the diagram shown in Figure 1, when the initial W1 call for heat takes place, the SmartValve sends 120 VAC from P1-4 to C3-2 then from C3-1 on the Inducer In to Ind In on the EFT. Then 120 VAC is sent from Ind High on the EFT to the inducer, which comes up to high speed. This creates sufficient differential pressure to cause both Low Fire and High Fire pressure switches to close. The High Fire pressure switch will then pass DC voltage to the two-stage (black) solenoid, which will cause it to increase the gas pressure in order to reach High Fire setting. The complete tubing connection set-up is shown in Figure 1. The DC voltages expected are illustrated in Figure 3.

Figure 1 also shows the wiring diagram for this system. Prior to the main burner turning on, a three-second pre-purge will occur; then the igniter will light the pilot and the flame will be proved from the sensor. If the microamp signal is 1.5 microamps or greater, then the valve will be energized through the limit switch, rollout switch and low-pressure switch. The burner will turn on at High Fire setting and run until the delay is satisfied; the system will then shut down the DC signal to the solenoid and the system will go to Low Fire. The inducer will be running at low speed and the DATA signal from the SmartValve will be sent to the ST9162 EFT board and the fan [on delay] will start. In 30 seconds, the system fan will come on at low speed and the burner will be at Low Fire. If there is a subsequent “R” to “W2” High Fire call, then the DC signal will be sent to the two-stage solenoid due to the ST9162 sending a 120 VAC signal from the board to the inducer. Additionally, it will go up to high speed closing the high pressure switch. The system fan will then also go to high speed.

When the thermostat is satisfied, the power to the inducer will be shut down after a five-second post-purge. Timed from the gas valve de-energizing, the low heat fan speed de-energizes after the selected heat fan delay “off” time expires.

Figure 2 show various components and their location on the furnace. The condensing principle is accomplished by causing the water (vapor-laden) products of combustion to be drawn into the secondary heat exchanger by the inducer fan. This will, with the blower blowing across the heat exchanger, cause the vapor to condense; the latent heat will be drawn from it and then the air flow will continue to pass through the primary heat exchanger, picking up even more heat. The condensate is then dumped out of the condensate drain typically either into a neutralizer, a condensate pump or a combination of both. The electronically commutated motor (ECM) allows for variable speeds to control the cubic feet per minute (CFM) flow in order to cut down on the possibility of blowing cold air. The ECM motor is controlled by signals from the EFT board. Figure 2 further shows the location of the high and low-pressure switches, along with the two-speed inducer.

This system uses a DC blower called GE ECM motor. There will be a separate column on ECM motors, so we will be brief in our discussion here. This is not a regular 120 VAC motor that many may be familiar with, so special instructions are necessary in order to properly diagnose problems within this system.

The Modulating Furnace ECM Blower Motor
Motor Operation
The modulating furnace uses the GE ECM motor. The motor is a DC motor. Line voltage AC power is converted to DC by an inverter inside the motor electronics control. DC voltage powers the motor stator; therefore, it is a synchronous motor. Since it is a brushless motor, there are no brushes to wear out and it makes minimal noise.

The rotor uses a permanent magnet. This eliminates almost all rotor losses found on permanent split capacitor (PSC) induction motors. In fact, the magnetic pull can be felt when spinning the rotor by hand.

The motor has two inputs. One is 120-volt line power. This motor must be powered continuously with line voltage. It also has a 24-volt power input. Figure 4 shows how power flows to the ECM motor electronics, how that power flows to the blower motor and how the motor electronics send a signal back to the integrated furnace control board.

The airflow through the duct work is varied to meet the load demand. Airflow as low as 300 CFM can be achieved by the response of the brushless permanent magnet variable speed blower motor.

The ECM motor is wired from the 120 VAC source and the feed from the six-pin connector on the ST9162 EFT. Dual in-line package (DIP) switches must be set up at the time of installation. DIP switch settings will be covered in a future column.

One of the things to consider when diagnosing problems with the ECM motor is that, if the fan does not run on any thermostat call, then the fan is suspect. On the other hand, if the fan runs on all but one speed, then one would suspect the timer board. A quick test to see if the fan will run is to pull a wire off of one of the limits; this should cause the fan to turn “on” (see Figure 5).

The six-pin connector from the EFT board can be checked for proper voltage (Figure 6). The tests are done as AC tests, looking for around 14 VAC. The #3 pin is the Common Pin; Figure 7 shows the Common Pin relationships for various applications. ICM

Parts 1 & 2, from the Jul/Aug and Sep/Oct 2021 Indoor Comfort can be found here.

Part 3, from the Nov/Dec 2021 Indoor Comfort can be found here.

Part 4, from the Jan/Feb 2022 Indoor Comfort can be found here.

Part 5, from the Mar/Apr 2022 Indoor Comfort can be found here.

Part 6, from the May/Jun 2022 Indoor Comfort can be found here.

Part 7, from the Jul/Aug 2022 Indoor Comfort can be found here.

Part 8, from the Sep/Oct 2022 Indoor Comfort can be found here.

Part 9, from the Nov/Dec 2022 Indoor Comfort can be found here.

Part 10, from the Jan/Feb 2023 Indoor Comfort can be found here.

Part 11, from the Mar/Apr 2023 Indoor Comfort can be found here.

As we look at modern systems and what specific problems they present, it’s important to understand the basic fundamentals associated with these systems.

Most of our modern heating equipment in some way or another involves electronics; along with the use of electronics is the use of flame rectification as a safety and flame-proving system. It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler; the same basic system is used to perform safe ignition, and then consistent operation, throughout the entire call for heat.

There are, however, different ways it is applied, from intermittent pilot application to direct spark ignition and including hot surface ignition. Each system has its own distinct advantages and problems. Next in this series of resolving burner issues related to these systems, we offer corrections and diagnostics to help solve those problems.

We will start with the basics and go into operation, typical problems, diagnosis, troubleshooting procedures and a final solution to your particular problem. It can be easy to jump to conclusions and change parts until you hopefully solve the problem. That is, however, time-consuming and costly.

I invite you to visit our Facebook page, Timmie’s Tips On Gas, I look forward to seeing you there.

We are presently doing a series on Honeywell Smart Valve. Picking up where we left off in the last column, we will continue to cover Smart Valve Generation III, walking you through step-by-step troubleshooting with these controls. This article will need to be in perhaps two or three parts in order to illustrate the characteristics of this two-stage system and make them understood.

SV9540Q/ST9162 SmartValve Two Stage Heating
The system featured is the SmartValve™ using the SV9540Q two stage gas valve, used on Comfortmaker® furnace models NTVM/VNK. This series furnace is from International Comfort Products Corp. of Lewisburg, TN, and is now perhaps an older system but is still applicable for your education on these systems.

This instruction is not designed to be an installation or service manual for this furnace; rather, the purpose is to show the operation of a two-stage system. The troubleshooting of this system is also included and the features of this furnace will be presented to help provide an understanding of the two-stage system.

Unlike single-stage furnaces that deliver heat unevenly, this variable speed, two-stage gas furnace provides consistent heat and overall comfort. The difference is that the two-stage system produces heat for normal temperatures most of the time, automatically ramping up to higher heat production as needed. Unlike the single-stage, which heats fast and shuts down fast because it’s set for the coldest extremes, this system keeps the home warm without those annoying hot and cold spots.

The furnace is remarkably quiet, thanks to its variable speed circulation blower. It runs quietly in the slower speed, and gently—quietly—increases to the higher speed, as more heat is needed. You can even run the variable speed blower continuously at the lower speed to improve air quality around the clock. Pre-wiring also allows for addition of electronic air filters and humidification.

This 90+ system takes fuel efficiency to its highest level. The variable speed blower also adds an efficiency boost, cutting electrical costs up to 75% compared to standard blowers. This ECM DC voltage motor operates at 500 watts, at ½ speed 90 watts (economy).

Furnace Basics
The thermostat calls for heat and the gas valve is energized by the control system. The burners ignite and the induced draft fan draws the flame into and through the sealed primary heat exchanger. Then, the hot flue gases are pulled from the primary heat exchanger into the secondary heat exchanger, increasing efficiency. The variable speed blower moves another stream of air over the outside of both heat exchangers and brings the warm air into the home. The combustion products are safely vented outside.

This furnace features:
• Two-stage redundant gas valve
• Two-speed induced combustion fan
• Variable speed circulation blower motor

The Honeywell SV9541Q is a two-stage valve, which combines gas flow control and electronic intermittent pilot sequencing functions into a single unit. The Q3450 or 03480 Pilot hardware supplies the low voltage igniter, flame sensor and pilot burner. These ignition system controls provide all gas ignition safety functions by controlling gas flow, ignition source, and a 120 VAC or 240 VAC combustion air blower. The controls also monitor the appliance airflow proving switch and limit string to assure proper appliance operation, and provide pre-purge, post-purge and timed trial for pilot ignition with 100% shutoff and continuous retry. A diagnostic LED indicates system status.

Sequence of Operation
This system is unique as a two-stage system in that it starts on high fire and then, if there is no actual high fire call, it will go to low fire. This allows the development of some Delta “T” to develop in the plenum and duct system (as this is a condensing furnace, it also allows some development of flue gas vaporization). Figure 1 illustrates the sequence as outlined step-by-step below with a bar graph.
(A more specific application to the ComfortMaker Furnace will be illustrated in the next issue).

The sequence of operation is as follows:
• Thermostat call—W1
• Combustion air blower—high speed
• Air proving switches
• Pilot flame lit and proved
• Main burner—high fire
• After delay—low speed combustion blower and low fire
• Fan timer
• Circulating fan—low speed
• Thermostat call—W1/W2
• Combustion air blower—high speed
• Air proving switches
• Main burner—high fire
• Circulating fan—high speed

Controls & Accessories
Thermostat
The two-stage furnace control will operate with a two stage-heating thermostat and will provide two-stage heating operation. For two-stage thermostat installations, the R, W1 and W2 wires from the thermostat connect to the R, W1 and W2 connections on the furnace control as shown in Figure 2. During operation, the furnace will shift from low fire to high fire as requested by the thermostat. The thermostat heat anticipators should be adjusted to a .10 setting.

Line voltage connection is also shown in Figure 2; it is important to follow correct polarity, as electronic controls may not function correctly with reverse polarity.

Low voltage connections to furnace must be made on terminal board to fan control.

Optional Equipment
All wiring from furnace to optional equipment must conform to local codes or, in the absence of local codes, the applicable national codes. Install wiring in accordance with manufacturer’s instructions.

Humidifier/Electronic Air Cleaner
The furnace is wired for 120 VAC humidifier and/or electronic air cleaner connection.

The fan control is wired for 24 VAC normally open (N/O) dehumidistat connection. Connect dehumidistat to the Y terminal and the ¼” male quick connect Y2 terminal on the fan control (scan the QR code at left to see the complete Furnace Wiring Diagram). A 20% reduction of cooling airflow will occur when the Y2 dehumidistat terminal is energized during a call for cooling from the thermostat.

Fan Control
The fan control is preset at the factory with an adjustable blower On delay of 30 seconds in the heating mode. The blower Off timing is preset at 140 seconds. If desired, the fan On delay and Off delay can be reset (See Figure 3 and Figure 4 for location of dip switches) to obtain the longest delay times while still maintaining comfort levels (scan QR code at left for Furnace Wiring Diagram).

NOTE: To achieve maximum efficiency, it is recommended that the fan control be set to turn on at 30 seconds after the burners light.

The ST9162 can be used as a two-stage or single-stage. When the choice is made, the dip switches on the EFT must be set as shown in Figure 3.
The dip switch settings for Heat-On and Heat-Off delay are shown in Figure 5.

In the next issue, we will pick up with SmartValve Two Stage with Comfortmaker Sequence of Operation. ICM

Parts 1 & 2, from the Jul/Aug and Sep/Oct 2021 Indoor Comfort can be found here.

Part 3, from the Nov/Dec 2021 Indoor Comfort can be found here.

Part 4, from the Jan/Feb 2022 Indoor Comfort can be found here.

Part 5, from the Mar/Apr 2022 Indoor Comfort can be found here.

Part 6, from the May/Jun 2022 Indoor Comfort can be found here.

Part 7, from the Jul/Aug 2022 Indoor Comfort can be found here.

Part 8, from the Sep/Oct 2022 Indoor Comfort can be found here.

Part 9, from the Nov/Dec 2022 Indoor Comfort can be found here.

Part 10, from the Jan/Feb 2023 Indoor Comfort can be found here.

As we look at modern systems and what specific problems they present, it’s important to understand the basic fundamentals associated with these systems.

Most of our modern heating equipment in some way or another involves electronics; along with the use of electronics is the use of flame rectification as a safety and flame-proving system. It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler; the same basic system is used to perform safe ignition, and then consistent operation, throughout the entire call for heat.

There are, however, different ways it is applied, from intermittent pilot application to direct spark ignition and including hot surface ignition. Each system has its own distinct advantages and problems. Next in this series of resolving burner issues related to these systems, we will give you corrections and diagnostics to help solve those problems.

We will start with the basics and go into operation, typical problems, diagnosis, troubleshooting procedures and a final solution to your particular problem. It can be easy to jump to conclusions and change parts until you hopefully solve the problem with these systems. That is, however, time-consuming and costly.

I invite you to visit our new Facebook page, Timmie’s Tips On Gas. I look forward to seeing you there.

We are presently doing a series on Honeywell Smart Valve. Picking up where we left off in the last column, we will continue to cover Smart Valve Generation III, walking you through step-by-step troubleshooting with these controls.

Application
The SV9440, SV9540, and SV9640 SmartValve™
System Controls combine gas flow control and electronic intermittent pilot sequencing functions into a single unit. The Q3450 or Q3480 Pilot hardware supplies the low-voltage igniter, flame sensor and pilot burner much the same as Generation I and II. These ignition system controls provide all gas ignition safety functions by controlling gas flow, ignition source and 120 VAC or 240 VAC combustion air blowers. The controls also monitor the appliance airflow proving switch and limit string to assure proper appliance operation and provide pre-purge, post-purge and timed trial for pilot ignition with 100% shutoff and continuous retry. A diagnostic LED indicates system status.

These controls communicate directly with an electronic fan timer (ST9160 Electronic Fan Timer for single stage applications; ST9162 Electronic Fan Timer for two-stage applications) in typical forced warm air furnace applications. They also interface with the 208907 Terminal Board, providing compatibility with power-stealing thermostats or they directly interface with the appropriate power supplies and a system thermostat for additional appliance applications. When controlled directly by a thermostat, these controls do not provide a post-purge function because power to the control is removed when the thermostat call for heat ends.

The SV9440, SV9540 and SV9640 Systems are suitable for a wide range of fan-assisted combustion gas-fired appliances including furnaces, rooftop furnaces, boilers, unit heaters, infrared heaters, water heaters and commercial cooking appliances. The appliance manufacturer determines the specific application of the SmartValve System.

SmartValve System controls are available in a range of valve capacities. Scan this QR code to see:

Table 1: Valve capacity
Table 2: Gas capacity conversion factors
Table 3: Model number suffix designations (the suffix letter indicates temperature range and regulator type)

This is very similar in operation to Generation I and II using a pilot system. It has the 90-second trial for ignition with the five-minute shutdown before retry. Pre-purge time is 15 seconds; post-purge is 30 seconds (not available when SmartValve System Control is connected directly to the thermostat). Flame failure response time is 1.6 seconds at 2 μA.

Figure 1 illustrates a top view of the valves; the only difference with this valve is that the igniter output C2 is 24 VAC. There is also 24 VAC interrupter fed to the C1-8 pin that was not used on the SV9510/9520 series SmartValves.

Wiring Diagrams
As we look at Figure 2 with a forced warm air furnace, the wiring is very similar to wiring on the SV9510/9520 series SmartValve. The exceptions are the 24 VAC C2 igniter hook-up on the SmartValve and the use of C1-8 to P1-5 for 24 VAC interrupted 24 volts. The sequence of operation is the same as using the S9160 Electronic Fan Timer as shown in Figure 2.

Figure 3 shows SmartValve 9440/9540/9640 used with Honeywell R8285D Boiler Control Center. Once again, this is very similar in operation to the SV9510/9520 Series of SmartValve. The difference is 24 VAC being fed direct from the “R” terminal on the R8285D to C1-8 on the SmartValve. The R/DATA connection is the interrupted 24 volts as the 120 VAC is fed from the junction box direct to C3-2/C3-4. The rest of the sequence is similar to Figure 3.

Figure 4 shows a direct connection from the thermostat to SmartValve. This wiring setup could be used with a steam system by wiring the LWCO and Pressuretrol in place of the limit switch. The 120 VAC is fed direct to C3-2/C3-4 from the junction box. The 24 VAC is fed direct from the transformer to C1-8. The R/DATA/W is the interrupted circuit.

Figure 5 shows the SV9440/9540/9640 with the 208907 terminal board that allows compatibility with power stealing thermostats.

Figure 6 shows the various pin locations and the shape of the pins on both the EFT and the SmartValve.

Scan this QR code to see Figure 7, which is the sequence of operation for the ST9160 EFT or 208907 terminal boards. These boards are used with the SV9541 and SV9641 SmartValve™. When troubleshooting, make note of the igniter turning off after 30 seconds into the 90-second trail for ignition, and remaining off for approximately 25 seconds. It then should come back on for the final 30 seconds of the 90-second trail. The pilot valve is energized during the entire trail for ignition. If the system fails to light after the 90-second trial for ignition, it will go into a five-minute delay and then retry for another 90-second trial. This process will continue until the system finally lights and the main burner is on.

Scan this QR code to read more about troubleshooting with LED indicator assistance for ignition controls using ST9160 fan timer or 208907 terminal board. ICM

Parts 1 & 2, from the Jul/Aug and Sep/Oct 2021 Indoor Comfort can be found here.

Part 3, from the Nov/Dec 2021 Indoor Comfort can be found here.

Part 4, from the Jan/Feb 2022 Indoor Comfort can be found here.

Part 5, from the Mar/Apr 2022 Indoor Comfort can be found here.

Part 6, from the May/Jun 2022 Indoor Comfort can be found here.

Part 7, from the Jul/Aug 2022 Indoor Comfort can be found here.

Part 8, from the Sep/Oct 2022 Indoor Comfort can be found here.

Part 9, from the Nov/Dec 2022 Indoor Comfort can be found here.

As we look into some more modern systems and what specific problems they present, it is important to understand the basic fundamentals associated with these systems. Most of our modern heating equipment in some way or another involves electronics and the use of flame rectification as a safety and flame-proving system.

It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler—the same basic system is used to perform safe ignition followed by consistent operation throughout the entire call for heat.

There are, however, different ways the system is applied from intermittent pilot application to direct spark ignition and including hot surface ignition (HSI). Each has its own distinct advantages and problems. We’ll now attempt to resolve those burner problems related to these systems, as well as offer corrections and diagnostics.

We will start with the basics and then continue to operation, typical problems, diagnosis, troubleshooting procedures and a final solution to a particular problem. It’s easy to jump to conclusions with these systems and just change parts to hopefully solve a problem. That is, however, time-consuming and costly.

I invite you to visit our new Facebook page Timmie’s Tips on Gas. I look forward to seeing you there.

We are presently doing a series on Honeywell SmartValve; in this article we will cover SmartValve Third Generation and walk you through the step-by-step process of operation with these controls.

Something new has been added to be able to distinguish between controls and their functions. They are using different voltages and rectifications to carry DATA signals. This helps to define the difference electronically between limits and other controls in the system. This is illustrated in Figure 1: Simplified Wiring Diagram.

This is nothing to get concerned about, as a switch is either “open” or “closed” and the voltage readings are always this—across an open switch you will read “voltage” and across a closed switch you will read “zero.” To troubleshoot, as always, you can jump out switches.

Here are a few new things to keep in mind while troubleshooting:
• Air proving switch and limit string use 24 volt half wave rectified current
• Volt meter checks will give different readings

Figures 2 and 3 are from the November/December column. They are necessary in order to trace through the diagram.

With no call for heat in Figure 4, the voltages shown were recorded. To be able to see the effect of operation on the pressure switch as to voltage readings, I simulated blocked pressure switch tubing in Figure 5; all that changed was the readings—in both cases it was an open pressure switch.

Be careful to wait for the prepurge and igniter warm up time when diagnosing problems. Depending on those times, it can be up to a minute before you will have burner ignition. These systems also have a very short trial for ignition. Be aware of the sequence of operation and watch for any deviations from the normal operation.

For a trouble diagnosis, refer to the specific LED code as outlined in Table 1: Troubleshooting with LED Indicator Assistance. Newer versions have the expanded 6 code diagnostics with the 10 flash indicating reverse polarity.

In addition to the codes and other diagnostics available, checks can be done with a multimeter. Using the diagram in Figure 3, the following checks should be made for Direct Spark Ignition (DSI) igniter troubleshooting:
• Wait for pre-purge and igniter warm-up
• Short trial for ignition
• Watch for any deviation from sequence of operation

Here are more in-depth directions for troubleshooting:
1. L1 to Neutral should have 120 VAC; L1 to ground should read 120 VAC; if not then polarity may be reversed.
2. XFMR (transformer) to Neutral should have 120 VAC.
3. COM – 24 VAC should have 24 volts AC.
4. With a call for heat from the thermostat or by jumping “R” to “W,” you should have 120 VAC at C3-1/C3-2 and the Combustion Air Blower should be running.
5. Accounting for the time for prepurge, you should have 120 VAC at C2-1/C2-3 and the igniter should be glowing and the Air Proving Switch should be closed.
6. Accounting for igniter warm-up time, there should be 24VAC at C1-7/C1-6 and the Smart Valve should be energized. If not, check the limit switch, roll out switch or air proving switch string for an open switch.
7. When the burner is up and running, you should have a DATA signal coming out of C1-4 and going into P1-2 on the EFT. After the fan delay time is satisfied, the Circulating Fan should be on—if not, check for 120 VAC from the HEAT terminal on the EFT; it should be set to Neutral. If you have power to the fan and it is not running, replace the fan.

Also keep in mind:
• Use troubleshooting table in spec sheet and LED
• Using both should lead you to the trouble

These LED codes are for the older version Smart-Valve, the newer ones have the extended code as shown in Table 1. There is also a procedure for troubleshooting without the codes just in case they are not working or the power was shut off before the technician arrived to fix the problem. It is illustrated in Table 2. ICM

Part 9, from the Nov/Dec 2022 Indoor Comfort

Parts 1 & 2, from the Jul/Aug and Sep/Oct 2021 Indoor Comfort can be found here.

Part 3, from the Nov/Dec 2021 Indoor Comfort can be found here.

Part 4, from the Jan/Feb 2022 Indoor Comfort can be found here.

Part 5, from the Mar/Apr 2022 Indoor Comfort can be found here.

Part 6, from the May/Jun 2022 Indoor Comfort can be found here.

Part 7, from the Jul/Aug 2022 Indoor Comfort can be found here.

Part 8, from the Sep/Oct 2022 Indoor Comfort can be found here.

As we look into some more modern systems and what specific problems they present, it is important to understand the basic fundamentals associated with these systems. Most of our modern heating equipment in some way or another involves electronics and the use of flame rectification as a safety and flame-proving system.

It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler—the same basic system is used to perform safe ignition followed by consistent operation throughout the entire call for heat.

There are, however, different ways the system is applied from intermittent pilot application to direct spark ignition and including hot surface ignition (HSI). Each has its own distinct advantages and problems. We’ll now attempt to resolve those burner problems related to these systems, as well as offer corrections and diagnostics.

We will start with the basics and then continue to operation, typical problems, diagnosis, troubleshooting procedures and a final solution to a particular problem. It’s easy to jump to conclusions with these systems and just change parts to hopefully solve a problem. That is, however, time-consuming and costly.

I invite you to visit our new Facebook page. I look forward to seeing you there.

We are presently doing a series on Honeywell SmartValve; in this article, we will cover SmartValve Third Generation and walk you through the step-by-step process of operation with these controls.

SV9510/SV9520 Sequence of Operation
Figure 1 is the sequence of operation for SV9510 and SV9520 SmartValve. The various diagrams featured in this article will show you the circuits involved.

Referring to Figure 1 and Figure 2 for the Sequence of Operation and to Figure 3 for a diagram, let’s go through the Sequence of Operation.

When power to both 120 VAC and 24 VAC is applied to the system, the LED on the SmartValve will be pulsing “in the Heartbeat mode”—bright-dim-bright-dim, etc. L1 (Hot) is fed to L1 on the EFT. Then 120 VAC is fed from XFMR (transformer) to an external 40 VA or 50 VA transformers, depending on which valve. From the transformer, 24 VAC is fed to the 24VAC/Com terminals on the EFT.

The dotted lines in Figure 3 are my additions to illustrate the printed circuit board feed from L1 to P1-1 120 VAC, and from neutral to P1-4, as well as the 24 VAC (Hot) from terminal 24VAC to P1-6 and the common (COM) to P1-3. The ST9160 Electronic Fan Timer will be covered in a later section.

The setup for this wiring shown in Figure 3 has the 120 VAC to the SmartValve interrupted and 24 VAC is fed direct on the “R” wire to C1-7 and the “C” to C1-6 on the SmartValve. On a call for heat, a circuit is completed through R and W on the ST9160 the air-proving switch, which must then prove open in 30 seconds. If it does, then 120 VAC is fed from P1-1 to C3-2 and back on neutral from C3-4 to P1-4. Then 120 VAC is fed from C3-1 to the Combustion Air Blower and back to neutral on C3-3. Then the prepurge begins.

At the end of the prepurge, the HSI warm-up starts. The igniter warm-up time for the Norton 271 (used with SV9510) is 17 seconds on the first try and 27 seconds on subsequent tries. The Norton 601 (used with SV9520) warm-up is five seconds on the first try and then 10 seconds on subsequent tries.

The main valve opens and ignition takes place. The flame rod proves the burner flame and the DATA signal is fed from C1-4 to P1-2 to start the fan-on time sequence. The fan will usually come on in 30 to 60 seconds.

When the call for heat ends, and R and W breaks, power to the Combustion Air Blower ceases after post-purge. The air proving switch opens and the DATA signal ceases; the Electronic Fan Timer goes to the dip switch set fan-off run time. At the end of that time, everything is ready for the next call for heat.

Figure 4 shows how the 208907 terminal board allows compatibility with power-stealing thermostats. Many of the control boards cannot be used with power-stealing thermostats unless there is an isolating relay used or a 100 ohm 10 watt resistor placed across W and C.

Figure 5 shows a typical wiring diagram with a Forced Hot Water System using a R8285D Honeywell Control Center. With this setup, the 120 VAC is fed direct from the junction box to C3-2 (Hot)/C3-4 (Neutral); in this case, the 24 VAC is interrupted.

L1 Hot is also fed to Pin 3 on the R8285; also, 24 VAC is fed to Pin 4. On a call for heat from the thermostat, 24 VAC is fed from “R” through the thermostat to “G,” then to the relay coil and back to “C.” This brings in the two sets of relay contacts—1–3 sends 120 VAC to the circulator; 4–6 takes 24 VAC from “R” to “Y” through the aquastat control (high limit) to the R and DATA wire C1-7 and C1-4 (DATA). The sequence after that is the same as the sequence for Figure 3.

Figure 6 is an example of wiring direct to the thermostat. This setup could also be used for a steam system by adding a Low Water Cut-Off and Pressuretrol in place of the limit switch.
The 24 VAC is again interrupted and the 120 VAC is fed direct to C3-2 and C3-4. In many instances, a jumper from C1-4 to C1-7 may accomplish the wiring from the DATA to “R” wire. Pay particular attention to the triangle notes 1–6 in Figure 6.

In the next article we will discuss troubleshooting the SmartValve Third Generation. ICM

Part 8, from the Sep/Oct 2022 Indoor Comfort

Parts 1 & 2, from the Jul/Aug and Sep/Oct 2021 Indoor Comfort can be found here.

Part 3, from the Nov/Dec 2021 Indoor Comfort can be found here.

Part 4, from the Jan/Feb 2022 Indoor Comfort can be found here.

Part 5, from the Mar/Apr 2022 Indoor Comfort can be found here.

Part 6, from the May/Jun 2022 Indoor Comfort can be found here.

Part 7, from the Jul/Aug 2022 Indoor Comfort can be found here.

As we look into some more modern systems and what specific problems they present, it is important to understand the basic fundamentals associated with these systems. Most of our modern heating equipment in some way or another involves electronics and the use of flame rectification as a safety and flame-proving system.

It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler—the same basic system is used to perform safe ignition followed by consistent operation throughout the entire call for heat.

There are, however, different ways the system is applied from intermittent pilot application to direct spark ignition and including hot surface ignition (HSI). Each has its own distinct advantages and problems. We’ll now attempt to resolve those burner problems related to these systems, as well as offer corrections and diagnostics.

We will start with the basics and then continue to operation, typical problems, diagnosis, troubleshooting procedures and a final solution to a particular problem. It’s easy to jump to conclusions with these systems and just change parts to hopefully solve a problem. That is, however, time-consuming and costly.

I invite you to visit our new Facebook page Timmie’s Tips on Gas. I look forward to seeing you there.

We are presently doing a series on Honeywell Smart Valve™. We will pick up where we left off in the last article and cover Smart Valve™ The Third Generation. We will walk you through the step-by-step process of operation with these controls.

SmartValve™ Third Generation
The third generation of SmartValve™ became available to the original equipment manufacturers (OEMs) in late 1998/early 1999. It includes the SV9510/SV9520 and SV9610/SV9620.

The SmartValve™ works in conjunction with the Honeywell Electronic Fan Timer ST9160. It uses direct burner ignition line voltage igniters, such as the Norton 271, which has been used on HSI for many years. The new igniter used is the Norton 601.

The SV9540/SV9640 is also used with the EFT ST9160 control. It uses the same 24-volt igniter system used on SmartValve™ Generation I and II.

There is also a SmartValve™ SV9540Q used with Electronic Fan Time ST9162. This is a two-stage system that will be covered in a later discussion.

SV9510/SV9520; SV9610/SV9620 SmartValve™ System Controls
Application
The SV9510/SV9520 and SV9610/SV9620 SmartValve™ System Controls combine gas flow control and electronic direct main burner ignition sequencing functions into a single unit. The ignition source is a 120V HSI lighting the main burner flame. Two types of 120V igniters may be used, as seen in Table 1.

The control provides all gas ignition safety functions by controlling gas flow, ignition source and a 120 VAC combustion air blower. The control also monitors the appliance airflow-proving switch and limit string to assure proper appliance operation.

The SmartValve™ System Controls provide pre-purge, post-purge and timed trial for ignition with multiple ignition trials and auto reset from lockout. Diagnostic LED indicates system status.

The control communicates directly with the ST9160 Electronic Fan Timer (EFT) in typical forced warm air furnace applications. It will also interface with the 208907 Terminal Board, providing compatibility with power-stealing thermostats. Alternately, it directly interfaces with the appropriate power supplies and a system thermostat for additional appliance applications. When controlled directly by a thermostat, the control does not provide a post-purge function, as power to the control is removed when the thermostat’s call for heat ends.

This system is suitable for a wide range of fan-assisted, combustion, gas-fired appliances including furnaces, rooftop furnaces, boilers, unit heaters, infrared heaters, water heaters and commercial cooking appliances.

The specific application of the SmartValve™ System is the responsibility of the appliance manufacturer. See Table 2 for temperature ranges and regulator types.

Line voltage polarity sensing models monitor the line voltage input connection to assure line voltage polarity is correct. If line voltage polarity is incorrect, the LED diagnosis code will flash and the control will not respond to the call for heat. These models also provide added LED diagnostic codes (“6+”) to indicate the reason the control has moved to the lockout state.

Depending on models, various timings are available, such as 15 or 30 seconds pre-purge; post-purge five seconds (typical depending on model; this is not available when the SmartValve™ is connected directly to the thermostat); five, seven or nine seconds trial for ignition; 60-minute soft lockout; three retries; four trials before lockout. The other unusual feature is that the draft inducer is now controlled from the SmartValve™; depending on the version, this is accomplished by either interrupting the 120 volts or 24-volt signal. This will be defined as each diagram is discussed.

The capacities on SV9510, 20 and 40½” x ½” 200,000 BTUs maximum and on the SV9610, 20, 40¾” x ¾” is 415,000 BTUs.

Figure 1 is an illustration of the top view of the valves. The C3 connector is for 120-volt power, and this is also the feed for the draft inducer. The C2 connector is for the 120-volt HSI, either the Norton 271 Silicon Carbide in the case of the SV9510, or the Norton 601 120 volt Silicon Nitride Igniter used only with the SV9520 model. The C1 connector is the low voltage 24-volt AC hookup.

The designation of pin numbers for the C1 and C3 connectors on the SmartValve™ and the 6-pin connector on the ST9160 Electronic Fan Timer are illustrated in Figure 2.

Converting Ignition System Control
From Natural Gas to LP Gas Application (or LP Gas to Natural Gas Application)
Always change the main burner orifices as provided by the manufacturer of the equipment being converted. Ignition system controls are factory-set for natural (and manufactured) or LP gas. Do not attempt to use an ignition system control set for natural (manufactured) gas on LP gas, or an ignition system control set for LP gas on natural (manufactured) gas.

Ignition system controls with standard or slow opening regulators (SV9510M, H/ SV9520M, H, K) can be converted from one gas to the other with a conversion kit (ordered separately). Order part No. 393691 to convert from natural (manufactured) to LP gas; order part No. 394588 to convert from LP to natural (manufactured) gas.

Two-stage ignition system controls (those with N and Q suffix letters) can be converted using a conversion kit (ordered separately). Order part No. 396021 to convert from natural (manufactured) to LP gas; order part No. 396025 to convert from LP to natural (manufactured) gas.

IMPORTANT: Ignition system controls with step-opening regulators (SV9510P/ SV9520P) cannot be field-converted to LP or natural gas. See Table 3 for pressure settings for valves.

SV9510/SV9520 Sequence of Operation
Figure 3 is the sequence of operation for the SV9510 and SV9520 SmartValve™.

Training
Finally, I love to teach and would love to have you come and join us for some training. We also conduct seminars on the following topics and many others:

Five-Day Course
• Fundamentals of Gas
• Circuitry & Troubleshooting
• Hydronic Controls
• Electric Ignition Systems
• Advanced Electric Ignition Systems

One-Day Course
• Powerpile Systems

Three-Day Course
• Combustion Testing Design Gas Equipment
• Conversion Burners

Four-Day Course Designed for Your Specific Need
• Modulating/Condensing Boilers

For more information, call 401-437-0557 or write to:
Gas Appliance Service Training & Consulting
42 Village Drive, Riverside, RI 02915.
E-mail: timmcelwain@gastcri.com. ICM

Part 7, from the Jul/Aug 2022 Indoor Comfort

Parts 1 & 2, from the Jul/Aug and Sep/Oct 2021 Indoor Comfort can be found here.

Part 3, from the Nov/Dec 2021 Indoor Comfort can be found here.

Part 4, from the Jan/Feb 2022 Indoor Comfort can be found here.

Part 5, from the Mar/Apr 2022 Indoor Comfort can be found here.

Part 6, from the May/Jun 2022 Indoor Comfort can be found here.

As we look into some more modern systems and what specific problems they present, it is important to understand the basic fundamentals associated with these systems. Most of our modern heating equipment in some way or another involves electronics and the use of flame rectification as a safety and flame-proving system.
It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler—the same basic system is used to perform safe ignition followed by consistent operation throughout the entire call for heat.

There are, however, different ways the system is applied from intermittent pilot application to direct spark ignition and including hot surface ignition. Each has its own distinct advantages and problems. We’ll now attempt to resolve those burner problems related to these systems, as well as offer corrections and diagnostics.

We will start with the basics and then continue to operation, typical problems, diagnosis, troubleshooting procedures and a final solution to a particular problem. It’s easy to jump to conclusions with these systems and just change parts to hopefully solve a problem. That is, however, time-consuming and costly.

I invite you to visit our new Facebook page Timmie’s Tips on Gas. I look forward to seeing you there.

Forced hot water boiler wiring diagram
I want to emphasize once again how important knowing the sequence of operation of equipment is to proper diagnosis of problems. We are now going to go over sequence of operation on a forced hot water system. In the last article we covered forced warm air.

The system shown in Figure 2 is using a control center (transformer and relay) shown in Figure 1. The same control, when used on a forced warm air system, is called a “fan control center.” It is important to note that the wiring is different for a boiler versus a furnace. The use of a control center on a warm air furnace or boiler is a convenient place to connect wires and also makes for easier troubleshooting.

The transformer has the wiring connections attached. The “R” and “C” terminals are the only actual voltage connections. The other terminals are for wiring connections. The relay part of the control center is field replaceable. The best thing is to carry a SPDT relay, which can be used to replace another SPDT or SPST relay without any wiring changes in the junction box. Being able to stay out of the junction box is very important as the possibilities of connections coming loose are increased when you have to get into the “J” Box.

In addition to the “control center,” we have a Generation I SmartValve™ and a Honeywell D-892 vent damper. In Figure 2, the vent damper is shown with only the external wiring connections. It is sometimes difficult to imagine what is going on inside a control if the internal connections and components are not shown. I find that showing the internal operation of controls makes following the electrical path easier.

In Figure 3, we show the internal connections of the D892 vent damper. The sequence of operation we are going to follow is best understood using Figure 3. The purpose of the vent damper is to increase the efficiency of the equipment a little more by shutting down the standby loss from the boiler during the time when the thermostat is not calling. Anytime a vent damper is used, it must have an interlock circuit to ensure that the unit does not fire with the damper closed. This particular damper from Honeywell is powered closed and must have a continuous uninterrupted 24 volts at all times. When a call for heat takes place, the motor in the damper is de-energized and it will spring open. This, in turn, will activate the two N.O. sides of the switches (end switches).

In addition to power being required to the vent damper on a continuous basis, the SmartValve™ must also have an uninterrupted 24 volts at all times. When the SmartValve™ is used on a boiler application, the Electronic Fan Timer (EFT) output from the SmartValve™ is not used. The EFT output is only needed with a furnace.

Sequence of operation
The sequence of operation with this example is as follows:
1. Uninterrupted 24 VAC from R/C on Control Center is fed to SmartValve™.
2. 24 VAC to terminal 1 on the vent damper through the orange wire, through the right-hand side of relay Kl (closed) the NC set of S3 contacts to T2 through the motor to T1 and returning back to terminal 4 on the yellow wire. This powers the motor and damper is closed.
3. On a call for heat, the coil in the control center is powered and two sets of contacts close.
a. The left-hand set takes 120 VAC from L1 to Circulator Pump and back to L2.
b. The other set takes 24 VAC from R through spill switch, rollout switch and aquastat to terminal 2 on the vent damper.
4. The 24 VAC on 2 (black wire) goes to energize relay K1 in vent damper actuator and back to 4 on the yellow wire, the right-hand K1 switch is opened breaking power to the damper motor K1 switch now makes the left-hand side. The motor is now de-energized and the spring now opens the vent damper (mechanical action); the two normally open (N.O.) contacts close S1 and S2.
5. The relay stays powered through terminal 2 (black wire) during the entire call for heat unless one of the safeties opens.
6. The S1 and S2 (SPDT) set are in series and when the motor de-energizes and the spring opens the damper, they provide the path for 24 VAC through the left-hand side of K1 through S1 and S2 on the blue wire terminal to 3 on the damper powering the SmartValve™.
7. Now 24 VAC is fed out of terminal 3 on the vent damper to the interrupted leg of SmartValve™. The burner is now on.
8. When the call for heat ends, the control center relay coil is de-energized, as is also the K1 relay coil and the circulator shuts down. When K1 is de-energized, the K1 relay switches back to the right-hand side and the motor is powered closed. End switch contacts S1 and S2 are now tripped back to NC position.
9. Power is shut off to the interrupted leg of SmartValve™ and the burner shuts down.
It should be noted that switch S3 stays NC at all times unless the service switch is tripped; in that case it will de-energize the motor and the damper will now open manually.

The resistor R1 is to prevent power stealing programmable thermostats from energizing the damper when there is no call for heat. This is one of the phenomena of some power-stealing-type thermostats. ICM

Part VI, from the May/Jun 2022 Indoor Comfort

Parts I & II, from the Jul/Aug and Sep/Oct 2021 Indoor Comfort can be found here.

Part III, from the Nov/Dec 2021 Indoor Comfort can be found here.

Part IV, from the Jan/Feb 2022 Indoor Comfort can be found here.

Part V, from the Mar/Apr 2022 Indoor Comfort can be found here.

As we look into some more modern systems and what specific problems they present, it is important to understand the basic fundamentals associated with these systems. Most of our modern heating equipment in some way or another involves electronics and the use of flame rectification as a safety and flame-proving system.

It doesn’t matter if it is a forced warm air furnace or a forced hot water boiler—the same basic system is used to perform safe ignition followed by consistent operation throughout the entire call for heat.

There are, however, different ways the system is applied from intermittent pilot application to direct spark ignition and including hot surface ignition. Each has its own distinct advantages and problems. We’ll now attempt to resolve those burner problems related to these systems, as well as offer corrections and diagnostics.

We will start with the basics and then continue to operation, typical problems, diagnosis, troubleshooting procedures and a final solution to a particular problem. It’s easy to jump to conclusions with these systems and just change parts to hopefully solve a problem. That is, however, time-consuming and costly.

I invite you to visit our new Facebook page Timmie’s Tips on Gas, I look forward to seeing you there

We are presently doing a series on Honeywell SmartValve™. In this article, we will cover the Smart-Valve™ being used in a Forced Warm Air System. The next article will cover SmartValve™ with a Forced Hot Water System.

Troubleshooting electronic system
There are many different systems in use today on both Forced Warm Air and Forced Hot Water Systems. Knowing how to diagnose and troubleshoot these systems is something everyone needs to be able to do. There are some basic steps to follow.

Figure 1 is a basic diagram of a SmartValve™ hookup; we will be going much deeper into the process of diagnoses than this diagram will allow.

One of the first things to know in order to properly troubleshoot a system is the sequence of operation. If you do not know how it is supposed to work, then how will you be able to fix it? In this segment we are going to look at a Forced Warm Air System.

This particular system is composed of a Honeywell SmartValve™ Generation I SV9500/9600 with a Honeywell Electronic Fan Timer ST9120A, B or C. Using the diagram shown in Figure 2 we will walk through a sequence of operation on this system.

It is interesting to note that modern systems all come with wiring diagrams, both connection diagrams and ladder diagrams, trouble trees, and in some cases we even have diagnostic LEDs to help us find the problems. These, along with a written-out sequence of operation, should make our jobs easier.

The diagram shown in Figure 2 is for a Forced Warm Air System; it shows some air conditioning components, which will not be covered in this instruction.

Sequence of operation
The sequence of operation is as follows:

1. There is 120 VAC fed to terminal S1 on EFT—Neutral wire to N1.

2. 120 VAC is fed from terminal S2 to the primary of a 24 VAC transformer—N4 is neutral. In order to be able to more easily follow this sequence, Figure 3 will illustrate pin numbers and shape of pins for the ST9120.

3. The secondary of transformer goes to X (hot) C (common) Note: on some EFTs, there is a feed from X–4 and C–2 on 6-pin connector.

4. There is an un-interrupted 24 VAC from X and C to SmartValve™ or from pin 4–24 V AC (Hot) to Pin 2–24VAC (common).

5. When thermostat calls for heat R to W
a. 120 VAC is fed out of DI (draft inducer) to combustion blower neutral to N3.
b. The combustion air blower will cause the pressure switch to make COM to NO.
c. 24 VAC out from X to Pin 1 on 6-pin connector.
d. Pin 1 through limit and roll out switch to pin 6 to R (thermostat) through printed circuit board.
e. R to thermostat back to W to pin 3 on 6-pin connector.
f. Pin 3 to pressure switch and auxiliary limit to thermostat terminal on SmartValve™.

6. SmartValve™ is powered 24 VAC to the igniter, igniter glows and lights pilot gas—pilot proves back on clear (or sometimes black) wire a .3 microamp signal or greater. The main valve opens and lights from pilot burner. At this time, a data signal is superimposed on a voltage signal and fed to the EFT pin 5 on 6-pin connector.

7. Different ST9120 EFT boards will have dip switches for fan delay on and fan delay off (three dip switches) or just fan off delay (two dip switches). The 2-dip switch model has a fixed fan on time of 30 seconds. The 3-dip switch model has fan on times of 30 or 60 seconds. The off time on both can be 60, 100, 140 or 180 seconds. The data signal back to the EFT will trigger the fan on timing.

8. The fan comes on and runs until the thermostat is satisfied. Depending on the connection at the motor the fan will run on low, medium low, medium high or high speed. For heating, the HEAT terminal on EFT should be connected to low on motor and back to N2 on EFT. (Note: M1 and M2 on EFT are for unused motor leads).

9. When the thermostat is satisfied, the circuit to EFT is broken, the SmartValve™ shuts down and the fan-off delay starts (based on dip switch settings).

10. Fan shuts off—waiting for next call for heat. This completes the sequence of operation. It is the first step in troubleshooting and must be understood in order to further grasp how the system operates.

Figure 4 shows how the sequence takes place coming off the ST9120 control. It should be noted that the Auxiliary Limit is often difficult to find on Forced Warm Air Systems. It is sometimes located on the outlet side of the unit. Its purpose is to protect the heat exchanger from becoming damaged by excessive temperature. The whole idea of higher efficiency systems is to hold as much heat in the unit versus losing heat up the vent. This is why the system fan has to come on so  soon.

Educational opportunities
Finally, I love to teach and would love to have you join us for some training. We conduct seminars on the following topics and many others:

One-Day Course:
• Powerpile Systems

Three-Day Course:
• Combustion Testing Design Gas Equipment
• Conversion Burners

Four-Day Course Designed for Your Specific Need:
• Modulating/Condensing Boilers

Five-Day Course:
• Fundamentals of Gas
• Circuitry & Troubleshooting
• Hydronic Controls
• Electric Ignition Systems
• Advanced Electric Ignition Systems

For more information, Tel: 401-437-0557; E-mail: timmcelwain@gastcri.com or write to: Gas Appliance Service Training & Consulting, 42 Village Dr., Riverside, RI 02915. ICM