Which valve is used to prevent radiant floor overheating by mixing hot supply water with cooler return water?

• A. 3 or 4 way mixing valve
• B. Pressure relief valve
• C. Draft control
• D. Thermostatic radiator valve

Answer: (A)

Radiant floor heating needs to keep the water going to the floor at a safe, comfortable temperature. To do that, the system blends very hot boiler water with cooler return water before it reaches the floor loops. This tempering is accomplished with a mixing valve that can be three-way or four-way.

A three-way mixing valve has hot supply, cooler return, and a mixed output. It blends the two streams to produce water at the desired temperature for the floor. A four-way version adds an extra path, which can provide more precise control or allow for additional circuit configurations, but the essential idea is the same: mix hot and cool water to keep the floor from overheating.

This approach is necessary because radiant floors run best at relatively lower temperatures than the boiler water produced for other heating needs. By tempering the water, you prevent overheating of the floor, protect the system components, and maintain comfortable floor temperatures.

The other devices mentioned don’t serve this tempering purpose. A pressure relief valve is for safety against overpressure, not temperature control. A draft control manages combustion air flow. A thermostatic radiator valve regulates air temperature on radiators, not radiant floor circuits.