Mixing Methods with Radiant Heating Systems

Written on: September 1, 2019 by George Carey

When designing a radiant heating system, it becomes obvious that this system has characteristics different from the conventional baseboard-type of heating system. One quick difference is the temperature of the water circulated through the tubing. Most radiant systems can be classified into two types.

The first is a “wet system” in which the tubing is installed into concrete. The second type is a “dry system” where the tubing is either stapled-up from underneath the floor or laid down on a sub-floor and the final flooring placed on top of it.

The average water temperatures are 110°–120°F for the concrete type and 130°–140°F for the staple-up application; of course, there are exceptions where hotter or cooler water may be required. Unfortunately, most oil-fired boilers can’t operate at such low temperatures without experiencing flue gas problems. The best way of overcoming this problem is by using some type of mixing device, which lowers the supply temperature to the radiant zone(s) while allowing the boiler loop to maintain a temperature high enough to satisfy its requirements. There are numerous mixing methods available.

Mixing concerns
Here are some common concerns regarding the subject
of mixing:


What is mixing?
Mixing is when you take some cooler return water and “mix” it with some hot boiler water to supply a temperature of water that is lower than the boiler temperature but warmer than the return water.

Are there different methods available for mixing?
You can use a two-way valve, a three-way valve, a four-way valve or a circulator. All four devices can be used to supply a mixed water temperature.

How does each of these methods work?
1. A two-way valve works on the injection principal. There is a boiler loop with a circulator and the radiant loop with its own circulator. These two circuits are inter-connected through a supply pipe and return pipe that are spaced close together. A two-way valve is located on the supply pipe and has a controller that measures the radiant loop’s supply water temperature. The controller will cycle the valve open and closed based on the water temperature in the radiant zone. When the valve is opened, it injects bursts of hot water into the radiant loop. There, it mixes with some cool return water from the radiant zone.

2. A three-way valve mixes cool return water with hot boiler water to supply a “mixed” temperature. It has three ports, one for the return water from the radiant zone, one for hot water from the boiler loop and a mixed port to supply the radiant zone. These valves can be manually set to maintain a fixed temperature or they can have an actuator that repositions the valve according to the load.

3. A four-way valve is very similar to a three-way valve except it has four ports instead of three. Two ports go to the boiler and two ports go to the radiant zone. This valve can be set manually or used with an actuator to modulate the water temperature based on the zone load.

4. The last method is with an injection pump. This method has been used since the early 1960s. Back then, a controller would cycle the pump on and off to inject bursts of hot water into the radiant zone. Today there are control companies that will control the speed of a water-lubricated, impedance-protected, wet rotor pump. Instead of turning the pump on and off, the control increases or decreases the speed of the pump.

How to choose
Here are some general concerns for mixing:

Is one mixing method preferred over the others?
Not really, all of these methods work, but each method does come with its own benefits as well as its own limitations.
1. For example, two-way valves should only be used for small loads where the amount of the injected flow rate is a small percentage of the radiant zone’s total flow rate, typically less than 25%.
2. Three-way, self-contained, thermostatic valves are relatively inexpensive but can only provide one fixed temperature. This causes the zone’s thermostat to cycle the zone pump on and off. This type of operation is fine for a small radiant zone but not recommended when the zones become larger.
3. Injection pumping with a variable speed controller has become popular over the past few years. This method of mixing, which uses common wet-rotor circulators, provides many benefits to a radiant system such as full temperature modulation and boiler return protection from cold water. It is limited only by the pumping capacity of these wet-rotor circulators, which are typically around 35–40 gpm. In a typical radiant system, that flow rate equates to approximately 1,000,000 BTU/H.
4. Three-way and four-way valves, when used with actuating motors, have been installed in many radiant systems very successfully for years. The actuator adjusts the valve’s position to supply the appropriate mixed water temperature based on the heating load of the zone. The only real limitation to this method—compared to the cost of a wet rotor circulator—is that the valve and actuator are more expensive than an injection pump.

What happens if I use only one pump with the mixing device?
There will only be one mixing point, which will control the supply water temperature to the radiant zone, but not the temperature of the water returning to the boiler. Besides, the flow rate through the boiler will vary, decreasing the efficiency of the boiler.

Why should I use two pumps?
With two pumps and a mixing device, you create two mixing points. This allows you to control the temperature of the water going back to the boiler as well as to the radiant zone. In addition, the second pump provides constant flow through the boiler, increasing the boiler’s efficiency.

Why should I be concerned with the temperature of the water going back to the boiler?
Most oil-fired boilers are of the non-condensing type. This means it is important that the flue gases, released from the combustion process, are vented out of the boiler. When the water in the boiler is at a temperature below the dewpoint of these flue gases, the gases will condense back into water inside the boiler. The results can be very damaging. In commercial applications, boiler thermal shock is another important reason to control the return water temperature.

Is there a preferred way to pipe the mixing devices and the two pumps?
The preferred method is to use primary-secondary pumping. This method, which has been around since the 1950s, prevents pumps from pumping in series with each other and prevents valves from having a hard time opening or closing against high head pumps. This piping technique also allows the valves and injection pumps to be properly sized to the loads they are intended to control.

What is primary-secondary pumping?
It is a pumping technique that is simple in both theory and application. It is based upon a simple rule that states: When two circuits are interconnected, flow in one will not cause flow in the other if the pressure drop in the piping common to both is eliminated.

How do you eliminate the pressure drop in the common pipe?
This is achieved by keeping the supply and return tees to the secondary circuit very close together! (Maximum four pipe diameters) This means you can have two circuits interconnected, (for example, a boiler loop and a radiant loop, each with its own pump) but the pumps from each circuit will not cause flow to occur in the other loop.

How do I properly size the mixing device?
The size of the pump or valve is based upon the required flow rate from the high temperature loop. This flow rate will then blend with a portion of the cooler return water to supply the desired “mix” water temperature. This is an example to calculate the necessary flow rate:

 1. Radiant zone load =100,000 BTU/H designed with a 20°F temperature drop.
2. Design radiant zone flow rate =10 gpm
3. Radiant design supply temperature = 120°F
4. Return temperature = 100°F.
5. Boiler loop supply temperature = 180°F
6. The temperature difference between the boiler loop supply and the radiant return loop is 80°F. To calculate the flow rate required; divide the BTU/H load of the radiant zone by the temperature difference (delta T) x 500. 100,000/ 80 x 500 = 2.5 gpm.
7. The required flow rate is only 2.5 gpm of 180°F boiler water. This water will mix with 7.5 gpm (10 gpm–2.5 gpm) of 100°F radiant return water to supply a design water temperature of 120°F of 10 gpm. Therefore, the control valve or injection pump should be sized for a flow rate of 2.5 gpm.

If you have any questions or comments, e-mail me at gcarey@fiainc.com or call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM