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.
Here are some common concerns regarding the subject
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.