Thermostat Settings and Radiant Floor Heating

Radiant Floor Heating

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At REHAU Academy, some of our most popular seminars address radiant floor heating (RFH) design and installation. Discussion in these seminars about thermostat settings is always interesting. Should a thermostat for a RFH system be set lower or higher than a thermostat for a forced air system? How does a thermostat on a wall respond to radiant heat? Consider the following:

 

 

RFH systems do make warm air, which rises gently off the floor thanks to natural convection. In most situations, a RFH system outputs approximately 60% of its heat via infrared radiation, and the remaining 40% via convection and conduction. Unlike standard hot-air convection heating where hot air rises rapidly to the ceiling, RFH keeps the warmest air in the lower 5-6 feet of a room, with the warmest air no warmer than the floor surface. Floor temperatures are less than 80°F in most residential RFH systems even on the coldest days, so the warmest air is cooler than that. Stratification is practically eliminated.

 

In years past, we tried radiant thermostats that did not sense air temp, but attempted to sense the radiant feel of the space. In real life, each room has its own “radiant image,” dependant primarily on whether the thermostat is facing a window or a wall. We found that some rooms required a 75°F setting for comfort, while others required 66°F. This caused confusion with some occupants who paid too much attention to the number on the dial, and not just on how they felt, so our current offering is more reliable in that sense.

 

Today, we have special thermostats which are calibrated for radiant heating, so that they are tuned to the slower response time of most radiant systems vs. air systems. This is done with anticipation setups in the programming, and helps to prevent overshooting the set point. These thermostats do not feel the radiant energy but rather the air temperature in their surroundings. That’s why we recommend setting them to 68°F for most rooms as a starting point for comfort. The more advanced thermostats also use PID (proportional, integral, derivative) logic to learn the response time of each zone.

 

Our thermostats can also use floor sensors, with settings for both minimum and maximum floor temperature. Many people like to use floor sensors to program a minimum floor temperature, such as in a tiled area where the occupant wants the tile to be slightly warm to the touch all winter long. In this case, the radiant system would maintain that floor set point with occasional flow of warm water through the floor, without overheating the space.

 

Read more about our radiant systems here.

 

By Lance MacNevin, manager, REHAU Academy