Hi All My question is what type thermostat should I use for a radiant heat system? I currently have a Braeburn 3000. I believe that it is a generic thermostat for a forced air heat system. I have read where a radiant system should have a pulse width modulator thermostat. Am i correct in what i am reading Thanks Larry

What sort of radiant are you planning? A high mass system might benefit fom a PWM control as it helps the temperature creep up to the set point and helps avoid overshoot. Since a low mass system heats and cools quicker, it is not as prone to overshoot. At any rate, you want a remote temperature sensor to monitor floor temp directly. The placement of this sensor and how you apply it should be able to provide good control of your floor temp with a plain differential thermostat.

One thing I havn't seen discussed much, is what the long term effects the added on-off cycles of a PWM will have on the heating appliance...

Hi Ronmar

I have a single zone 3 loop system in my house, aprox 900 Sq feet. It seems that the temp will drop 2-3 degrees and overshoot around 1 degree depending on outside air temp. can you explain how the remote floor sensor will work? the best place on a floor to install it? my system is attached to my floor plywood between the floor joist with .058 aluminum plates, then 1/2 inch foil backed board 1 inch below the aluminum panels. the area that is heated is all tile.

Thanks
Larry

2-3 under and 1 degree over with a wall sensor only thermostat? That dosn't sound horrible. The trouble with a wall mount and radiant is that the floor surface has to get warm enough to warm the area and air around the thermostat to shut off the heat, and cool low enough at the thermostat that it turns the heat back on. The Hysteresis this distance from source to sensor causes allows the heat in the floor to fluxuate over a wider range than it needs to. Kind of like trying to adjust the water temperature while you are in the shower. With the flow delay between valve and your skin, overshoot is real easy It feels cold so you open the hot water tap a little, then a little more, then a little more as you don't feel any change, then all of a sudden you are boiling and closing the valve way too far/too fast and then you are freezing...

The floor sensor will cut out this heat transfer delay from floor to wall sensor, then you pick a floor temp to maintain that gats you to a desired comfort level. Ideally you would want the sensor in the stack(under a tile) but probably not directly over a tube. A spot on top of a tile might also work but you might have to experiment with location in relation to a tube to get the desired floor response. Then you could possibly hide the sensor under a baseboard along a wall that crosses perpendicular to the tubes to allow you to fine tune the sensor placement.

I am planning on switching to a Tekmar 519 with a floor sensor. My radiant floor is 3/8 porcelain tile heated by a geothermal system, max floor temp 81F. In my situation because my house is super insulated it takes approx 30 to 40 minutes between calls for heat. During that time my floor temp drops from 80 down to 65 or so. My idea for a floor sensor is to maintain a room set point of 72 and a minimum floor temp of at least 72. I am told that if the floor sensor is active it will take precedence over the air temp. To quote Tekmar the thermostat will try and maintain the desired room temperature while operating between the min and max settings of the floor sensor. I figure if I set my floor min at 72 and max at 85 with a room temp of 72 the system will call for heat when the floor gets down below 72 but will it shut down when the room temp gets to 72 or will it keep going trying to attain a floor temp of 85. My heating contractor tells me it will be the latter and the system will run continiously trying to attain a non-attainable temp and possibly overheating the room. Guess I'll have to wait till I'm finished to see. Just some thoughts for you.

Happy new year!

It is a slab temp limit controller. As such, it will only turn the heat on and off when the limits are exceeded. It will turn the heat on and off based on the indoor air temp when the slab temp is within limits. We prefer true PID slab temp control for our integrated passive solar and HR floor heating designs. In other words, tightly control the slab temp based on the outdoor temp, indoor temp and insulation factor of the building.

Bob: I'm curious - what is causing your floor temp to quickly drop to 65F when the room is ~72F? So much difference suggests minimal external side insulation (or more than I would expect R value between the floor sensor and the room).

The 552 is your answer.

PID is simply a mechanism that must be programmed to achieve the desired result, usually comfort or economy. For such a modest demand the 552 will give you options to experiment with. By simply setting a decent thermostat, say the Honeywell 1100 series to a temperature 2°F above the adjoining room you may have what you need. Programming to control idle and on times may also fit the bill.

Naturally the heat source has to be considered and may effect response time.

JonR, good question. I'm guessing but I think the heat the tile floor gives up slows down the overall heat loss in the room. Because of the super insulation in the room I assume that the heat loss from the tile to the room is far greater than the heat loss from the room to the outside. As I mentioned in my other thread, the space below that floor heat is insulated with R14 Roxul.

If the slab temp is becoming cooler than the indoor temp, it must be the result of heat loss to the outdoor temp. You are not as well insulated as you believe.

Bob: True, tile has minimal R value and therefor the back side of it should be close to the room air temp. What's the temperature on the other side of the Roxul? Does an IR gun reading of the tile show a substantial drop too?