Mother Nature has a collection of “rules” that govern how things work in this world—“High pressure goes to low pressure,” “Whatever goes up must come down,” “For every action there is an equal and opposite reaction” and probably her most famous if you are in the heating industry, “Heat goes to cold!” If you were to ask someone where does heat go? he/she would say heat rises. That’s not necessarily correct, since hot air rises, but heat goes to cold, always. Mother Nature hates an imbalance and, when it exists, she does everything in her power to equalize or balance it.
When your heating system delivers warmth to your house, it eventually leaves through the windows, roof and siding. Why does the heat leave? There is an imbalance between the temperature inside the house and the temperature outside. Heat goes to cold…always! The same thing happens in the Summertime. When it is very hot outside and your house is cooler inside, the heat outside wants to go to the cooler indoor temperature.
How do we typically make the indoor cooler? Most people would say the air conditioner, which is true, but how eludes most people. We use an air conditioning system that removes heat from the indoors and sends it outside. That’s because you can’t make cold. To create a cooler atmosphere, you have to remove the heat, which is the basis of refrigeration. Whenever you feel cold, it is caused by a lack of heat.
If we know that heat wants to go to cold, why do we call it a heat pump? Why do we have to pump the heat when heat normally goes to cold? The reason is a heat pump, rather than creating heat, simply moves it. For example, it can move thermal energy from the cooler outdoor air into the warmer inside room. It pushes heat in a direction counter to its normal flow (cold to hot rather than hot to cold), hence the word pump. A boiler or furnace burns fossil fuel to create heat. A heat pump simply uses an existing source of renewable energy, like the heat that exists in outdoor air. This can lead to a significantly reduced consumption of energy while providing comfort.
The definition of Refrigeration is the process in which work is done to move heat from one location to another. It may also be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere.
Refrigeration uses refrigerant to move heat as it changes state. Nowadays, the refrigerant of choice is R410A. Its properties allow the refrigerant to be a liquid well below freezing. It has a freezing point at -155°C which is equivalent to -247°F. It has a boiling point of -48.5°C which is equivalent to -55°F. As a liquid refrigerant, it absorbs heat when it evaporates into a vapor. When the refrigerant is in its vapor state, it contains all that energy; when it condenses back into a liquid, it rejects or expels the heat it originally absorbed.
You have to remember that phase change contains a significant amount of energy. For example, when you change the temperature of one pound of water from 211°F to 212°F, it requires one British Thermal Unit (BTU). When you change one pound of 212°F water to 212°F vapor (steam), it takes 970 BTUs that you “get back” when the vapor condenses back to its liquid state.
A heat pump incorporates the vapor-compression refrigeration cycle to move heat either away from an area where it’s not wanted (cooling) or moves heat into a space that needs it (heating). Because of the unique operating properties of R410A, an Air-to-Water or Air-to-Air heat pump has the ability to take heat (energy) out of the air that we would consider very cold but to the refrigerant considers it warm. This applies to the heating mode of the heat pump.
The cooling operation is identical to that of an air conditioner. Again, using refrigerant and the vapor-compression cycle, the cold liquid refrigerant flows through the air conditioning coil as room air blows across it. The heat from the air goes to the cold liquid refrigerant, thus leaving the air cooler than it was when it entered the coil. The absorbed heat “flashed” the cold liquid refrigerant into cool vapor, which will then flow outside to the compressor. There, the cool vapor will be compressed (by the compressor) into a high temperature vapor. The vapor, which is storing a lot of energy (the heat we wanted to remove from the home), is pumped through a condensing coil where a fan is blowing outside air across it. This outdoor air is hot, relative to our comfort, but much cooler than the temperature of the hot vapor refrigerant. The hot vapor transfers its energy/heat to the outside air, thus completing the process of removing heat from the house and condensing it back into a warm liquid.
Heat pumps have the unique ability to either heat or cool a home through a simple device called a reversing valve. There are four key components required:
• expansion valve
By adding the reversing valve, the heat pump can “reverse” the role of these key components and provide heating or cooling from the same compressor.
Another term unique to heat pumps is Coefficient of Performance (COP). This term expresses how efficient the heat pump is with regards to the amount of energy it uses relative to the amount of energy it delivers. The term was developed to compare heat pump systems according to their energy efficiency. A higher value implies a higher efficiency between the pump’s consumption of energy and its output. Design conditions will impact the heat pump’s COP performance factor. Air-to-Air and Air-to-Water heat pumps have in the past been negatively impacted in their performance by colder outdoor temperatures. However, with advances in compressor technology, specifically invertor-driven compressors, these Air-to-Air and Air-to-Water heat pumps are capable of extracting energy (heat) from very cold outdoor temperatures and transferring the energy to the heating medium (water or air).
Circulators have been around the hydronic heating industry since the 1930s. They were originally added to existing gravity hot water jobs to “boost” the heat around the system. These original circulators were often referred to as “three-piece” pumps because they had three distinct sections—the wet end or volute where the impeller is located, the motor end (which is the driving force) that would mount in a cradle and a bearing assembly that would connect the two ends together with a coupling assembly.
The motor assembly was completely separated from the wet end of the pump by a seal. The bearing assembly would be lubricated with oil to keep the bearings in good working condition. These pumps dominated the hydronics industry for decades and did a very good job.
Sometime in the 1970s, a new style of pump came onto the scene that changed forever the residential (and eventually light commercial) market. These “new” circulators used the system’s own water as its lubricant. There was no longer a need for a separate bearing assembly and seal. The physical size of these pumps was considerably smaller and they cost much less than the three-piece style.
At first there were many skeptics about the new pump’s ability to perform as well as the original, larger pumps. However, over time, the industry realized these new “water-lubricated” pumps worked quite well, lasted a long time, required virtually no maintenance and were less expensive.
Over the past 10 years, some pump manufacturers have been offering multi-speed pumps, which offer a different pump curve for each speed. The most common is a three-speed wet-rotor circulator, which offers three different performance curves. The benefit is that with one pump, three different curves are provided to meet various system conditions. From an inventory standpoint, you can stock—on the shelves or in the service truck—one circulator model that can meet many different system applications.
Imagine that instead of three speeds there are 10 speeds or even 50 speeds—for each speed change you could plot a new pump performance curve. The highest speed would represent the pump’s maximum performance and the lowest speed would represent the pump’s minimum performance. A variable speed pump can operate anywhere between these two points simply by varying the speed of the motor.
Any wet rotor pump with a permanent split capacitor motor can function over an extensive range of speeds with a variable speed controller. This device varies the frequency of the AC signal sent to the permanent split capacitor (PSC) motor. By varying the AC signal, the revolutions per minute (RPMs) of the motor (the speed) are changed, which directly changes the flow and head capacity of the pump. The changes, and therefore the pump curves, are unlimited between the fastest and slowest RPMs of the motor.
The new circulator: ECM
“Smart” pumps, the latest technology in pumping, have made their way into the North American hydronics market. They are called ECM pumps, which stands for “electronically commutated magnetic motor.” They are very different from the PSC motors we have been using in our wet rotor pumps. This new style motor is sometimes called a “brushless DC” motor. The rotor in this ECM motor has permanent magnets instead of wire windings that are separated from the system fluid. The magnets are located inside a stainless steel rotor can and react to the magnetic forces created by electromagnetic poles in the stator.
However, you may experience a problem with the new ECM circulators, as the onboard magnets can collect/attract iron oxide from the water in the system. Iron oxide is a chemical compound consisting of a mixture of oxygen and iron. Every hydronic system has some oxygen (from the system water H2O) and more often than not some type of iron (i.e. cast iron from circulator volutes, flow-control valves, cast iron boilers, cast iron radiators and black iron steel pipe).
Of course, every system can contain a different amount of iron oxide, which will influence whether the ECM circulator “attracts” enough of it to cause the pump to stop running. I have heard of several instances where the contractor isolated the pump, pulled the can from the wet end of the pump, cleaned off the buildup of iron oxide, put the pump back together and it ran fine afterwards. Other times they ended up having to replace the pump because too much damage had been done.
A simple solution
There is a simple solution to this and it is not to go back to the old-style wet rotor pump— that horse has already left the barn. The utilities have been playing a big part in creating incentive programs to promote the installation of “High Efficiency” pumps, just like how they played a big part in the industry’s adoption of modulating/condensing (Mod/Con) boilers.
Look to Europe, which has been installing high efficiency boilers for years and has outlawed PSC (permanent split capacitor wet rotor pumps) due to the fact that they consume too much electricity. Instead, they have been installing high performance dirt and magnetite separators for years.
It is just standard design practice over there—if you are putting in high efficiency boilers and ECM pumps (by law), then you want to ensure the quality of the water circulating through the system. As the industry continues to install these high, efficiency ECM pumps, it should become standard practice to also include a magnetic dirt and iron oxide collector to protect the new high efficiency equipment.
The efficiency of these “Greener” circulators was designed to meet the ever increasing efficiency standards that have made their way over to North America. Their “wire to water” efficiency is simply higher than the PSC wet rotor circulators. Their multiple application capabilities with the on-board microprocessors, and their reduction in wattage consumption, make them a very compelling alternative to what we have used in the past.
If you have any questions or comments, E-mail [email protected], call 800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM
As I am writing this column, the summer of 2021 is getting underway. It seems like a strange time to talk about steam systems and condensate handling equipment, but as you all know, the heating season will be here before we know it, and steam systems will be turning on as the cool nights eventually arrive. Condensate pumps, when used in steam systems, play an important part in the proper operation of that system.
Understanding the operation of these pumps is pretty straightforward. The condensate pump is made up of a pump and motor with an impeller on the end of it, and a receiver that the pump and motor are mounted on (usually cast iron, but steel can also be used). Cast iron receivers are more rugged and last longer in corrosive environments; condensate usually has a low pH (which makes it acidic). Inside the receiver is a float assembly (not unlike a ballcock found in a toilet tank) that is connected to a floatoperated electrical switch, which is attached to the receiver. The receiver has an inlet opening near the top side of the receiver; it also has an opening on the top where a vent pipe is connected.
The condensate return piping from the system is connected to the inlet connection of the receiver; the vent pipe is used to vent the air from the system. As condensate and air travel along the return piping, they enter into the receiver through the inlet connection. The condensate falls to the bottom of the receiver while the air enters into the receiver and exits out through the vent pipe located at the top of the receiver.
Therefore, in effect, the vent pipe of the condensate pump is the main air vent for the system. Make sure that there are no water pockets in the return piping because that could prevent air from getting out of the system. Also, never plug the vent line because condensate pumps are not rated for pressure. If they become “pressurized” because of a blocked vent line, they could explode!
As the condensate continues to gather in the receiver, the float part of the assembly starts to “float” up/rise up with the rising level of condensate. At some point, the float-operated electrical switch “makes”—closing a set of contacts that turns the pump on. The condensate pump discharges the condensate to either the boiler directly or possibly to a boiler feed tank in the boiler room. Naturally, as the pump discharges the condensate from the receiver, the water level, as well as the float in the receiver lowers to a point where the switch “breaks.” This opens the contacts that turn the condensate pump off. This is basically what a condensate pump does—as returning condensate enters the receiver, it raises the float, which eventually turns the pump on. As the water level drops in the receiver, so does the float—eventually turning the pump off.
There are a couple of details that you need to pay attention to when piping a condensate pump into the system. On the inlet side of the receiver, it is good piping practice to install some type of strainer (a Y-strainer or basket strainer are two popular choices) just before the condensate enters into the receiver. There is sediment in old steam systems that the condensate will pick up as it flows back to the receiver. If the sediment makes its way onto the face of the pump’s seal, it may groove lines into the carbon and ceramic seal, causing it to leak. If the leak isn’t discovered quickly, the condensate will work its way into the bearings of the motor, causing it to eventually fail.
On the discharge side of the pump, there should be a check valve, a balancing valve and a service valve. The purpose of the service valve is to isolate the pump from the system; the service valve can be closed if you need to work on the pump. The check valve is needed so that whenever the pump is off, the water that is in the piping on the discharge side of the pump doesn’t fall back into the receiver. If the check wasn’t there, or if sediment gets underneath the flapper of the check, the water would flow back into the receiver causing the float to rise and bring the pump on. The pump would unload the receiver and shut off, and this cycle would “seesaw” back and forth endlessly.
If you happen to walk into a boiler room in the summertime and the steam boiler is off because it is used for heating, yet the condensate pump turns on and then turns off…and then turns on, and then turns off again…most likely the check valve has been compromised. Probably dirt or sediment is keeping the flapper from seating properly on its seat.
The balancing valve is used to provide a certain amount of “back pressure” on the pump. Standard stocked condensate pumps are rated to pump the condensate at a discharge pressure of 20 pound-force per square inch gauge (psig). However, in most applications, the boilers are running at very low pressure, typically 2–5 psig. When the pump turns on and it doesn’t “see” 20psig of pressure, it will run way out on its curve, pumping too much condensate, too fast. It can cause the check valve to chatter, creating unnecessary noise. By closing the balancing valve, you are creating the additional pressure the pump was designed to work against, thus slowing the flow rate down to where it can operate properly and quietly.
I get calls every now and then complaining that the condensate pump is turning on, but the pump isn’t pumping the water out of the receiver. In some cases, the water starts pouring out of the vent pipe. Usually the problem is related to the temperature of the condensate. I am not talking about the temperature that exceeds the material of construction or the pump’s seal rating (most are rated at 250°F). What I am referring to is when steam is allowed to enter into the return lines (usually bad traps that have failed in the open position) and elevate the returning condensate’s temperature above 185–190°F.
When the condensate becomes too hot and gets close to its boiling point, there isn’t enough pressure on the water to remain a liquid. When the pump turns on and the water enters into the eye of the impeller, it experiences a drop in pressure. Because the water is so close to its vapor pressure (boiling point), it flashes into vapor/steam. Of course, the impeller isn’t capable of pumping steam, so the impeller is spinning at 3,400 rotations per minute (rpm), and no water is discharging out of the receiver. This isn’t because of a bad pump; the water is simply too hot. There isn’t enough pressure on the water to keep it in its liquid state.
Remember—this is an open system and the only available pressure is the height of the condensate that is sitting in the receiver, which is less than a foot. In a closed hot water heating system, you have a fill valve and an expansion tank that provide a lot of pressure, so the pumps have no problem circulating the 200°F water.
The answer to this problem is to lower the temperature of the condensate, so fix the radiator traps that are leaking and make sure the pressuretrol isn’t set too high. There is a relationship between the pressure of the steam and its corresponding temperature, which holds true to the temperature of the condensate. This is another reason why there is no benefit to cranking up the pressuretrol setting in a heating application. If you have any questions or comments, e-mail me at [email protected], call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM
Over this past year, Zoom presentations have become a normal way of offering training classes while the country (and world) has grappled with the COVID-19 pandemic. One of our more popular classes is on the subject of hydronic system components.
Of all the components we use in a hydronic system, the most frequent questions I get during Zoom presentations are about diaphragm expansion tanks and pressure-reducing valves (PRVs) and how they interact with each other. Consistently, the contractors have been asking about what the proper settings are for each device and why.
Before we start applying answers to those questions, it is important to really understand what the functions of these components in a hydronic heating system are.
A pressure-reducing valve is pretty much like its name implies—it takes the incoming street pressure and reduces it down to what is needed inside a particular building where it has been installed. The hot water heating system functions when the boiler and all of the piping and radiation are filled with water that comes from the city water main located in the basement.
This question comes up often…how do we know how much water is needed to fill the system? By using the pressure gauge on the boiler, we can determine when the system is completely filled with water. Water has weight and as you stack more water on top of itself, it weighs more. By using the pressure gauge on the boiler, we can determine how high up into the system the water has gone. The pressure gauge reads in pounds per square inch or psi. We know a column of water that is 2.31′ (or 28″) tall weighs 1lb per square inch at the bottom of that column.
The key to using a pressure gauge in determining the height of a column of water is the expression pounds “per square inch.” Whether the piping system has ¾” copper pipes or 4″ steel pipes, the measurement on the pressure gauge is the same…per square inch. A square inch is a square inch, it doesn’t change; therefore, a column of water 2.31′ tall weighs one pound per square inch.
To properly fill a hydronic system, measure from the boiler pressure gauge/PRV location up to the highest piece of piping or radiation (whichever is highest) in the building. Then take that number and divide by 2.31′ to convert to pressure in pounds per square inch.
However, don’t stop there—the pressure reading would ensure that the water is all the way to the top of the system, but what would the pressure be in the system at the highest point? It would be zero pounds per square inch, and if you had any high vents located at the top, how effective would they be? There would be 0lbs of pressure inside the system and 0lbs of pressure (atmosphere) on the outside of the system.
There is no motive force for any air bubbles to vent out of the system. To ensure that high vents will be able to do their job, the industry has standardized on adding an additional 4psi to the number required to get water up to the highest point. To establish the proper PRV setting for each application, measure in feet the distance between the boiler pressure gauge/PRV location to the highest pipe/radiation in the building, divide by 2.31′ and to that number add 4psi. The result will be the proper cold water fill pressure for that system. The key is to fill the system when it’s cold so that you will have an accurate reading from the pressure gauge.
Expansion tanks play a very important role in the proper operation of a hot water heating system; that function is very different from the PRV’s job but for both to be effective, they work together. To appreciate this relationship, you want to have a good understanding of what the expansion tank’s role is and how it does what it does.
When a system is completely filled with water and then is heated to the operating control’s high limit, there is anywhere from 3.5%–5.0% more water in the system because when heated, it expands. Here’s the problem—water is not compressible and so when this increase occurs, if there is no place to put this extra water, the relief valve on the boiler will open up and dump system water onto the floor.
This is where those diaphragm tanks come into play; air is a gas which is compressible and so the expansion tank becomes the place where the expanded water goes while keeping the pressure in the system below the relief valve’s setting. The air in the diaphragm tank acts like a spring, allowing the system water to push against it as it is heated and expands. The air in the tank is separated from the system water by using a butyl rubber that is flexible.
These tanks are different from the older-style steel compression tanks—in those tanks, the system water and air cushion came into direct contact with each other, and because of that, the tanks were larger than the diaphragm tanks. With a diaphragm tank, the air side is fully expanded, pushing the rubber diaphragm all the way against the other side of the tank. When connected to the system, the air side pressure is now seeing the system’s fill pressure. Remember, when cold, there should be no system water in the tank; for that to occur, the diaphragm tank’s air charge pressure must match the system’s fill pressure.
Diaphragm tanks are sized to accept the volume of expanding water in the system while keeping the pressure in the system below the relief valve’s setting. Normally PRVs and diaphragm expansion tanks come pre-set at 12 psi since most of the applications are for two-story buildings. If you have a system in a building that requires a higher pressure setting, the expansion tank must be pre-charged to the higher fill pressure setting.
If you did not match the air charge to the fill pressure, once the tank was attached to the system, a certain amount of cold system water would enter the tank. Remember, there should be no water in the expansion tank when the system is cold. The net result is the expansion tank acts like it is too small, causing the relief valve to open, discharging the excess pressure.
The only proper way to check the tank’s pre-charge setting is while it is disconnected from the system. If you were to check the pressure while the tank is attached to the system, it would be a faulty reading because the water pressure from the system is “squeezing” against the diaphragm. The gauge would just be reading the system pressure.
As you can see, each of these components has its own “job to do,” but to do them properly, they have to work together.
If you have any questions or comments, e-mail me at [email protected], call me at FIA 1- 800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM
The landscape in the heating industry has drastically changed. Over the years, when you entered a typical boiler room in the Northeast, it was very common to find a sectional cast iron boiler(s) and usually some type of indirect heating appliance for domestic hot water. It could be a side-arm heat exchanger with a storage tank or a storage tank with the heating coil installed directly inside.
Nowadays, it has become quite common to find high efficiency modulating and condensing gas-fired boilers (either liquefied petroleum [LP] or natural). These boilers are not your average “run of the mill” atmospheric gas boilers, either, because their efficiencies range between 90%–96%. They use a Neg/Reg gas valve and fan assembly, which means the amount of gas that flows into the burner for combustion is regulated by the fan assembly’s blower speed. The blower speed is controlled by an on-board micro- processor that is performing several internal calculations to determine the appropriate amount of British thermal units (BTUs) needed to satisfy the call. Hence the modulating part—it only uses the amount of gas necessary to satisfy whatever load it is currently seeing.
Most of the residential models have a “turn down” ratio of 10:1, meaning they can fire down to 10% of their total capacity and, of course, all the way up to 100% of their capacity. It has become quite common in larger residences to install two or more small “mod/con” boilers, that, when combined, can handle the home’s total load. However, more importantly during the normal course of the heating season, when the home is operating at “part load,” the boiler plant consumes just the amount of energy needed to satisfy the current load that the house is seeing.
The same holds true for commercial applications such as apartment buildings, condominiums, churches and schools. The larger commercial “mod/con” boilers also offer turn down ratios 10:1. That means with a couple of commercial boilers, you can fire down to 5% of the total BTU capacity of the boiler plant. With this type of turn down, building owners are experiencing fuel savings in range of 35%–40% and higher!
Another unique feature of these boilers is the venting options. The blower motor is designed to not only bring combustion air into the burner assembly but also vent the residual products out of the building. Most of the “mod/con” boiler manufacturers have approved their boilers to use several different vent materials. They are approved to be vented with PVC, CPVC, polypropylene and stainless steel vent pipe.
Each manufacturer provides very detailed instructions on the dos and don’ts of how to properly vent their boilers. Following these instructions is critical to allow the boilers to operate efficiently. Of course, all of this piping needs to be sealed tight to meet the venting codes.
These boilers encourage the condensing of their flue products, which is the exact opposite of traditional boilers. Their heat exchangers are designed to withstand the corrosive nature of the condensate that forms when the combustion products are condensed. Of course, this condensing action is where the additional efficiency points are obtained.
Some of the by products of this condensing can gather in the boiler’s heat exchanger. If allowed to accumulate, they will negatively impact the boiler’s efficiency performance, which is why most manufacturers suggest an annual inspection and cleaning of the heat exchanger, if necessary. Also, the venting should be inspected to make sure nothing has changed that could negatively impact the operation of the boiler. This means every “mod/con” boiler needs to be inspected every year.
One of the oilheat industry’s shining stars has been its reputation for service and maintenance. The need for these high efficiency boilers to be maintained is a perfect opportunity for a company that has a service department to offer service contracts to homeowners, commercial property owners and management companies. Most of the boiler manufacturers or their local representatives offer classes on servicing these new “mod/con” boilers.
A new style of “smart” pump has made its way into the North American hydronics mechanical rooms. These new circulators are ECM pumps. ECM stands for electronically commutated motor and they are very different from the PSC (permanent split capacitor) motors we are accustomed to with wet rotor pumps. The rotor in this ECM motor has permanent magnets instead of wire windings that are separated from the system fluid. The magnets are located inside a stainless steel rotor can and react to the magnetic forces created by electromagnetic poles in the stator.
A microprocessor, which “sits on-board” the pump, reverses the polarity of the stator poles rapidly (within milliseconds), forcing the rotor to be rotated in the proper direction. The faster these poles reverse their polarity, the faster the rotor spins, meaning the faster the impeller spins.
ECM circulators can provide four times more starting torque compared to a PSC wet rotor pump and incorporate a microprocessor that has software on-board, allowing the pump to perform many functions.
For example, one application may call for a constant pressure differential where the building is zoned with zone valves. Normally as valves close, the pump would develop additional head pressure across the remaining open zones, causing an increase in flow rate through these zones. This wastes energy as well as creates potential noise problems due to increased velocity. With this constant differential in pressure capability, as valves close, the pump momentary senses an increase in differential pressure and quickly slows down the pump’s speed to eliminate the change in pressure. The result is no change in flow rate through the remaining open zones, no wasted energy and no velocity noise problems.
Another application that the microprocessor can control is called proportional differential pressure. The circulator control is set for a specific design head loss for a system. When the zone valve (or valves) then closes, once the pressure differential starts to climb, the circulator reduces its motor speed. The difference here is proportional control instead of maintaining a set differential. It will lower the speed and thus pressure differential proportionally to the reduction in flow rate. The result is an increased reduction in energy consumption.
The efficiency of these Greener circulators is designed to meet the ever-increasing efficiency standards that have made their way to North America. Their “wire to water” efficiency is higher than the current PSC wet rotor circulators. In addition, their multiple application capabilities with on-board microprocessors, and their reduction in wattage use, make them a compelling alternative to current offerings. Contact me at [email protected] or 1-800-423-7187. ICM
This happens every year—an older hot water boiler fails and needs to be replaced. Not always, but most times, the other components accompanying the boiler get replaced as well. The circulators get upgraded; the flow control valves and the air separator get replaced; and, of course, a new expansion tank replaces the old one.
In most of these systems, there is an existing (albeit older) diaphragm expansion tank. In the process of upgrading the tank, typically you just get the model number of the existing tank and replace it with the same, newer version.
What happens if that hot water system you are working on has one of those old steel compression tanks? You know the ones—installed up near the ceiling, usually suspended with strapping of some sort in between the floor joists? We call them “old-style” because they were invented before the concept of using a flexible butyl membrane was introduced.
These steel tanks had air and water “touching” each other in the tank. The job of the air volume was to act like a spring on the system to maintain adequate pressure throughout the closed system. It was important to always have a certain volume of air in the tank to allow for the expanding system water to “squeeze” against while keeping the pressure below the relief valve’s setting. We certainly don’t want water splashing onto the boiler room floor from the relief valve every time the boiler heats up the system on a call for heat.
When the time comes to replace this “old-style” tank, you have two options. However, before you replace it, you must confirm a few things: the original tank worked properly (and was therefore sized properly) and there will be no changes to the application. Once these are confirmed you can:
1) Replace the old tank with the exact same size and style tank; or
2) Replace it with the common “diaphragm style” expansion tank that the industry has been frequently using for the last 40+ years. This style tank design has separate compartments for the air and system water that are separated by the flexible butyl diaphragm.
When expansion tanks are sized properly, formulas are used to come up with the correctly-sized tank and sometimes they can be intimidating and hard to follow. When converting from the old-style tank to a modern diaphragm tank, a lot of the “heavy lifting” has already been done for us—that is, how the original tank was selected. We just have to apply some information that would be pertinent to our particular system to select the correct diaphragm tank.
Before we get there, let’s talk about some terminology that deals with expansion tanks. Here are two common terms:
1) Full Acceptance Tanks: the tank is big enough to accept all of the system’s expanded water volume while keeping the pressure range within working conditions (fill and relief valve pressure). This would include the old steel tanks as well as most commercial bladder-style tanks.
2) Partial Acceptance Tanks: the diaphragm style has a limited amount of expanding system water storage capability. This amount of water that can be stored is called the acceptance volume. This style tank is the most common type used in residential applications.
It quickly becomes apparent that when sizing a diaphragm tank, there are two criteria that need to be satisfied:
1) The total tank volume has to be large enough to keep the system pressure within operating range.
2) The acceptance volume has to be as large as the system’s expansion volume. The actual amount of expansion volume must be known. Fortunately, the original tank was sized with this piece of information.
A sizing example
Let’s walk through a sizing example to see how you can select a replacement diaphragm tank once you know the size of the original old-style steel tank. For this example, the system’s fill pressure will be 12 pounds per square inch gauge (psig) and the relief valve setting is 30psig.
1) The first formula establishes the total tank volume:
a. The formula is Vt pressurized = Vt standard (Pa/Pfill)
b. Where Vt psi = Total tank volume of pressurized tank
c. Vt standard = Size of existing old-style steel tank in gallons
d. Pa = Atmospheric pressure
e. Pfill = Fill pressure in absolute pressure (gauge pressure + atmospheric pressure)
i. 60 gals (old-style tank volume) x 14.7/(14.7 + 12)
ii. 60 gals x 14.7/26.7 = 60 gals x .551 = 33 gallons
iii. Vt psi = 33 gallons
2) The second formula establishes the actual system expansion volume:
a. Ve = acceptance volume
b. Ve = Vt (Pa/Pfill) – (Pa/Pmaxop)
c. Vt = Size of existing old steel tank in gallons
d. Pa = Atmospheric pressure
e. Pfill = Fill pressure in absolute pressure (gauge pressure + atmospheric pressure)
f. Pmaxop = Maximum Operating Psi (Relief Valve Setting + atmospheric pressure)
i. 60 (14.7/26.7) – (14.7/44.7)
ii. 60 x (.551 – .329)
iii. 60 x .222 = 13.3 gallons
g. Ve = 13.3 gallons
In this example, if the existing old-style steel tank had a volume of 60 gallons, the fill pressure requirements were for 12psig (most two-story residential applications) and the boiler’s relief valve was set for 30psig, the replacement diaphragm tank specifications would require a tank with a:
• Total tank volume of 33 gallons
• Acceptance volume of 13.3 gallons
Not that this happens every day, but here at FIA, we do come across this question often enough. Also, remember to pre-charge the diaphragm tank to the system’s required fill pressure before it gets connected to the heating system.
See Chart 1 for details on the diaphragm tanks’ acceptance volume and total tank volume for the various sizes; see Chart 2 for details on the dimensions and gallons for the old steel compression tanks. If you have any questions or comments, e-mail me at [email protected], call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_GCarey. ICM
A contractor asked me to visit an apartment building that was giving the property management company a lot of headaches with nuisance service calls, so I met him on the job.
What we saw when we walked into the boiler room was quite remarkable. The first thing that got my attention was the six, old pressure-reducing valves (PRVs) sitting on top of the boiler. The next item of interest was the expansion tank or lack thereof…there was a 3/4″ copper line piped off the top of the boiler and it went straight up into the sheet-rocked ceiling. However, we couldn’t see any expansion tank, only a piece of pipe!
Steam systems with various problems have been coming at me fast and furious, even though it is still the summertime as I write this column. I have noticed though, with all of the jobs, there is one constant—the person wrestling with the problem really doesn’t understand the nuances that a steam system brings to the table. These include:
• A steam system is filled with air anytime the system is off. If you want heat, you have to get rid of all the air before the steam can get in and heat the radiation.
• A steam system operates nothing like a water system…the steam, when manufactured in the boiler, desperately wants to turn back into water and it will whenever it touches something cooler than itself. If you don’t make enough steam, it will never reach the furthest radiators.
In our world of providing comfort and energy efficiency to our customers, there are certain formulas that are used on a regular basis. The most important one, when talking about a hydronic heating system, is GPM (gallons per minute). Heat is distributed from the boiler room out to where the people are via water. How much water determines the flow rate; the term we use is called GPM. An accurate heat loss reading in a building is very important to establish the design load conditions. Once the load is established, we can calculate the necessary flow rate.
GPM = Heat Load/ 500 ^T
GPM is the flow rate in gallons of water per minute. The heat load is expressed as BTU/H (British Thermal Units per hour), which is the heat loss of the building at design conditions. ^T is the temperature difference that occurs from the supply to the return when the water is circulated through the radiation. Five hundred is the constant for standard water properties at 60°F and it comes from multiplying the weight of one gallon of water at 60°F, which is 8.33 pounds by 60 minutes (one hour).
The complete calculation is:
GPM = BTU/H
8.33lb./gal x 60 min x ^t°F
The formula indicates a water temperature of 60°F. However, since 60°F water is too cool for a hot water heating system, and too warm for a chilled water system, you would think that to calculate the correct flow rate, the formula should be based upon more appropriate water temperatures for each type of system—for instance, based on things such as the specific heat of the water, the density changes that occur as the water changes in temperature and the water volume changes when it gets hotter or as it cools down. As you can see from the following example, the differences are so minimal that the standard formula works fine for all of our heating and cooling applications.
8.04 x 60 x 1.003 x 20 = 9,677 BTU/H
The net effect is not significant, but there is another factor that needs to be considered for a complete evaluation. As water temperature rises, water becomes less viscous, and therefore the pressure drop is reduced. When water is circulated at 200°F, the corresponding pressure drop, or “head loss,” is about 80% of water at 60°F for a typical small hydronic system.
When calculated using a system curve, the flow increases by about 10.5%. Now you can multiply the new heat conveyance just calculated by the percentage of flow increase:
1.105 x 9677 = 10,693 BTU/H
As you can see with regard to heat conveyance, the simple “round number” approach will result in design flows very close to the “temperature-corrected” flows, providing that the results from the “round number” approach isn’t corrected from the original 60°F base for both the heat conveyance and piping pressure drop. The plus and minus factors very closely offset one another.
The right circulator
GPM plays a major role in ensuring that your heating system performs as expected. You need the right sized circulator to be able to move the heat from the boiler and deliver it out to where the people live. In selecting the proper circulator, not only do you need to know the correct GPM, you also need to know the required pressure drop to circulate the necessary GPM.
As water flows through the pipes and radiation, it “rubs” against the pipe wall causing frictional resistance. This resistance can affect the performance of the heating system by reducing the desired flow rate from circulating, thus reducing the heating capacity of the system. By knowing what this resistance will be, you can select a circulator that can overcome the system’s pressure drop.
Typically, in today’s systems, we use “feet of head” to describe the amount of energy needed so that the required GPM is delivered out to the system. There are pipe sizing charts that have calculated the pressure drop in foot head of energy loss for any flow rate through any size pipe. There are standard piping practices in which the industry references that limit the amount of GPM for a given pipe size.
Tell me if you have ever heard someone say something like this: I think the room is under-heating because the water is moving too fast through the baseboard. If I use a smaller pump or at least throttle down on the pump’s flow rate, the baseboard will start heating better.
I have heard this statement numerous times over the years from technicians that were having problems with their customer’s heating system. I guess you could reason that with the water moving so fast, it doesn’t have enough time while inside the baseboard to “give up” its heat.
Instead of spending too much time with the math formulas, use your mind’s eye to visualize the following: as the stream of water is flowing through the baseboard, the outer edge of this stream is in direct contact with the tube’s inner wall. This “rubbing” against the wall creates drag, which means the water that is touching the inner wall of the tubing is moving slower than the “core” or inner stream. Because of this, the temperature of this outer layer of water becomes cooler than the inner stream. In fact, this drop in temperature impacts the rate of heat transfer—it slows it down.
Remember, one of the factors that affects convection is temperature difference. A good visual is to think of this outer layer as an insulator that impacts the rate of heat transfer from the hotter inner stream of water to the tubing’s inner wall. This is especially true when the speed of the water approaches laminar flow instead of turbulent flow. So, in effect, the faster the water flows through the tubing, the outer boundary layer or insulator becomes thinner, thus increasing the rate of heat transfer from the hotter inner “core” water to the tubing’s inner wall.
You can confirm this by taking a look at any baseboard manufacturer’s literature and checking out its capacity charts. They will typically publish their BTU output per linear foot based upon two flow rates: 1gpm (gallon per minute) and 4gpm. The BTU output is always higher in the 4gpm column.
Wind chill factor
Here is another way to consider this concept of more speed (faster flow rate) equaling more heat transfer (higher output). Consider the size of a hot water coil used in an air handler and the amount of BTUs it can provide. Now think about how much fin-tube baseboard you would need to install to provide the equivalent amount of BTUs.
The difference is that the speed at which the fan blows the air across the coil is much faster than the air that flows naturally across a baseboard. Everyone experiences this phenomenon each winter. The weatherman refers to it as the “wind chill” factor. He will tell you the actual temperature outside, but because of the wind chill, it will feel “X” degrees cooler. Why? The cold air is moving across our bodies much faster. This takes heat away from us much faster, so it feels colder than it really is.
A trick of the trade passed on to fellow technicians over the years is to turn up the Aquastat setting on the boiler to increase the water temperature. The reason technicians will do this is—if they have a room or a zone that isn’t quite heating up to the thermostat’s setting—by increasing the water temperature, they can increase the amount of BTUs per linear foot available from the baseboard (of course, this will only work if the boiler is big enough to offset the home’s actual heat loss).
Does faster water = more heat?
Knowing that a baseboard can provide more heat with hotter water, follow this next example to see why moving the water faster and not slowing it down will provide more heat.
Let’s use average design conditions when sizing for the amount of baseboard you would need to offset a room’s heat loss. Typically, we design a hot water heating system for a residential home at a 20°F temperature drop. That means the water would enter the radiation at 180°F and exit 20°F cooler at 160°F. The average water temperature in the radiation would then be considered 170°F. You would then check the BTU/h rating of the baseboard at this average water temperature and determine how many feet of baseboard the room needs to offset the heat loss.
What would happen if we increased the flow rate so that the water only took a 10°F temperature drop? If it entered the radiation at 180°F and came out at 170°F, then average water temperature would be 175°F. At this higher average water temperature, the baseboard would be capable of providing more BTU/h output. Taking it one step further, if the system only took a 5°F temperature drop, the average water temperature would be 177.5°F in the radiation, yielding even more BTU/h output.
You can now see that water cannot be moving too fast through your baseboard to prevent the BTUs from jumping off where needed. Of course, to achieve these tighter temperature drops while delivering the right amount of BTU/h, the flow rate has to increase accordingly.
Here comes the trade-off—with higher flow rates through a given pipe size, the pressure drop increases quite dramatically. The result can be a need for very large, high-headed circulators that cost more and use more electricity. Also, the higher flow rates increase the velocity of the water, and at some point, the velocity will create noise issues.
The point is not to design a system around these high flow rate/small temperature drops, but rather recognize that the next time someone tells you that they think the water is moving too fast and that is why the room is under-heating, you’ll know that is not the cause of the problem.
If you have any questions or comments, e-mail me at [email protected] or call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM