Radiant Heating System Explained: Injection Mixing

The ability to control temperature is a vital notion of any radiant heating system. A sensible control system ensures that the system functions efficiently and maximizes overall comfort of the radiant heated floors. One of the temperature control methods commonly used is injection mixing.

Injection mixing is an easy method to maintain the desired temperature and heat output by controlling the rate of hot water injected into the system. This method works best in low-temperature radiant floor heating systems.

As the water circulates in PEX pipe and outputs heat, it tends to get cooler, requiring the heat level to be raised in order to maintain a set temperature. The heat output is increased by injecting hot water into the system where it is mixed with the colder water at the "mixing point". Ideally, an exact amount of hot water will be supplied that when mixed will result in the desired temperature. An example of injection mixing would be injecting hot water at 190F into the loop with the current temperature of 100F, in order to increase it to the set level of 115F.

Hot water injection is not constant and the water enters the system only when additional heat input is needed. The flow of water injection is what controls the temperature of the whole circuit. Increase in injection flow rate will increase temperature and result in increased heat output of the radiant heating system. Naturally, the higher the temperature of the injected water, the lower flow is needed. As a result, the water is usually injected at a very low flow rate, since the temperature in the circuit needs to be raised only by a few degrees, while the temperature of injected water is much higher than that.

Since the loop is completely filled with water, injecting new water into it will result in the same amount of water that is already in the system to be pushed out through the return outlet at the same flow rate.

One of the most common ways of applying the injection mixing process is in systems with 2-way valves. Such systems consist of a distribution loop (regular PEX tubing circuits), as well as a boiler loop, which is run at high temperatures. The two loops are connected by supply and return injection risers, with a 2-way valve installed on a supply riser. The valve is connected to a sensor, which is attached to a PEX tube. As the temperature in the sensor starts to fall, it sends a signal to open the 2-way valve to allow inflow. The valve is shut off as soon as the sensor reads that the set temperature has been achieved. A flow restrictor is positioned between the two risers; its role is to create a pressure drop that would allow the hot water to enter the loop, once the valve is open.

The boiler loop is used to mix hot water with cold water from the return riser. The mixing increases the water temperature before it returns to the boiler and ensures that water with very low temperatures doesn’t enter the boiler.

Honeywell and Taco provide a wide variety of controls and valves that will suit most radiant heating systems.