Thermostats with radiant floor heating
I recently read an article comparing forced air and radiant heat energy use. The finding showed that the folk with radiant heat set their thermostats higher than for forced air. I hope that I am not being silly, but how can a thermostat on a wall respond to radiant heat? If one put a metal foil around the thermostat ,but let air flow occur, would the thermostat read a different number? I read in Mr Lyles masonry stove book that a cathedral that had radiant heating only had a 2 degree difference between the floor and ceiling (75 feet up). I appreciate that thermistors can be used but understand that this is not universal. The science confuses me. Thank you
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Wall thermostats read air temperature no matter the type of heating system.
Some electric resistance radiant floor heating products also include a second thermostat that either reads the temperature of the floor or of the electric resistance elements.
Typically it is believed that users of radiant heat can set there thermostat LOWER because of the contribution to thermal comfort of the radiant and conductive heat on the body (versus establishing thermal comfort only via air temperatures.)
Mike,
Although people who sell equipment for in-floor hydronic heat distribution systems (PEX tubing, staple-up plates, etc.) describe these systems as "radiant heat systems," the label is misleading.
All heating distribution systems distribute heat via all three heating mechanisms: convection, conduction, and radiation. So-called "radiant" heating systems do no depend on radiation alone. All of these systems are controlled by a wall thermostat which responds to changes in indoor air temperature.
Hello, I am an engineer with REHAU, a manufacturer of crosslinked polyethylene tubing and radiant heating systems since the early 1970’s. We are based in Leesburg, VA.
As Martin pointed out, radiant floor heating systems also make warm air, rising 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 80F 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 75F setting for comfort, while others required 66F. 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 setpoint. These thermostats do not feel the radiant energy but rather the air temperature in their surroundings. That's why we recommend setting them to 68F 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 setpoint with occasional flow of warm water through the floor, without overheating the space.
Lance, Thank you for your post. Very informative.
Thank you ALL! I know more but still remain a neophyte. Cheers Mike Legge
Furthermore, most modern hydronic heat sources, such as condensing boilers, feature outdoor reset as a standard feature responding to outdoor temperature swings by "re-setting" the supply water temperature inversely to the outdoor temperature virtually eliminating indoor ambient temperature swings in most homes.