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I would like to install electric radiant heating in my bedroom floors. But all thermostats with floor sensors utilize a powerful relay that makes a loud clicking noise every time it comes on and off. Are there any alternatives that would avoid this problem?

Things I have considered so far:

  • Install a thermostat in the utility room, which is conveniently located below the bedroom. This will mitigate the clicking noise, but obviously not desirable, as the thermostat is now nowhere near the floor being heated.
  • Place a thermostat in the hallway. Don't think this is going to work, as I will not be replacing hallway flooring, and won't be able to get all the wires there.
  • Hack a thermostat to remove the relay, and wire a remote module to turn the heat on and off. Least preferred way, as I'll probably brick at least 1 thermostat in the process :) Plus having the remote module is extra cost
  • Hack a thermostat to replace mechanical relay with solid state relay. Not gonna work, as just 5 amp load will generate 5-8W of heat, which can't be easily dissipated from a wall box.

Essentially what I need is a signaling thermostat with floor sensor, and a remote "power module" with GFCI that is controlled by the thermostat signal. Or maybe someone invented silent relay thermostat. Thoughts?

Edit: another thought... Convert 240 VAC into DC somewhere with ability to dissipate hear... Run to a thermostat (assuming it'll work on DC), and hack it to replace relay with power MOSFET. That should still allow GFCI function to work, and MOSFET wouldn't have much power dissipation requirement.

EDIT 2: I do not need to control air temperature with this thermostat. Floor heat is for convenience only, not to warm up the room, so it will be set to a minimal setting just to be nice to touch. I have a forced air system for the air (and I expect it’ll work less when floor heat is on).

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  • Nice quiet mercury relay? But evidently nobody likes mercury anymore...
    – Ecnerwal
    Sep 28, 2021 at 20:21
  • @Ecnerwal the nice quiet mercury switch is still going to trigger a giant clacky relay...
    – brhans
    Sep 28, 2021 at 20:44
  • @brhans You haven't met the big powerful mercury relay. Not the little bitty switch you are familiar with. Lovely things. Possibly still the best solution to some cases.
    – Ecnerwal
    Sep 28, 2021 at 20:52
  • @Ecnerwal - :O I didn't know such a thing existed!
    – brhans
    Sep 28, 2021 at 21:07

2 Answers 2

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You could use a smart thermostat or wireless thermostat.

This type of device has two parts: the usual wall mounted thermostat with temperature display and buttons, and that wirelessly controls another device which contains the power relay.

So you can put the latter (noisy) one outside of your bedroom while the control unit is in your bedroom.

I don't know if that will be able to use the floor sensor though.

If your underfloor electric heating needs a specific thermostat with floor sensor, you could hack it and replace the relay with a solid state relay as you suggest, and use that to drive a large noisy contactor located somewhere else, for example in your main electrical panel, in series with the circuit breaker for the heating.

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  • I wasn’t able to find one like that. Do you know of one’s existence? The rest was already considered and is my least preferred option.
    – Serge
    Oct 30, 2021 at 13:15
  • Googling "wireless thermostat" I get quite a few candidates
    – bobflux
    Oct 30, 2021 at 17:17
  • LOL is only that was as easy. If you click through any of those, you’ll see that “wireless” means it has WiFi connectivity, nothing more. So if you know of one that fits the job - let me know. I wasn’t able to find one.
    – Serge
    Oct 31, 2021 at 18:50
  • Damn, lol. I'm French, so I googled "thermostat sans fil" and here's one. Well if you're in the US, wrong voltage. Here we say "thermostat connecté" if it uses wifi. But in English-speaking countries I guess with "wireless" you'll get wifi.... maybe try "RF thermostat"?
    – bobflux
    Oct 31, 2021 at 19:19
  • OK, here are some models from the UK one two, try "remote thermostat" or "RF" that kind of stuff
    – bobflux
    Oct 31, 2021 at 19:20
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Well, you have a dilemma. The problem is is that your thermostat not only measures floor temperature, but also ambient air temperature. The floor sensor is really there to stop your floor from getting too hot and burning your feet. The ambient air temperature sensor is there to regulate how much you want to heat your room. If you move the thermostat out of the room, it will continue to heat the room, never having it's setpoint satisfied.

A solution would be to use a 24 VAC HVAC control transformer, a standard 24v thermostat, some thermostat wire, and a 24 VAC HVAC contactor at a remote location. This, however does not solve the floor temperature problem. For this, you could use a high limit snap disc switch inline with the control wire. They come in a variety of preset temperatures or with an adjustable range, and some are N.O. and some are N.C.

All of this, of course, you'll want to put on a GFCI breaker, as you'll no longer have the built in GFCI protection from the original thermostat.

And, this should go without saying, non of this will be UL listed!

EDIT: some floor heating thermostats allow you to turn off the ambient temperature control and just use the floor sensor. You would have no control of the room temperature though and may regret it later.

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    With the schluter thermostat there is an option to have the heating controlled by the floor sensor only, instead of the room/floor default, would that solve the problem of moving it into the hallway? Pg 52 sccpublic.s3-external-1.amazonaws.com/sys-master/images/hf5/h5e/…
    – redlude97
    Sep 28, 2021 at 21:56
  • Yes, most have that as an option. I will add that to my answer. Wouldn't be able to control room temperature though.
    – DrSparks
    Sep 28, 2021 at 23:24
  • If you use a UL508 listed industrial control or lighting contactor instead of the Definite Purpose (HVAC) contactor, and a UL listed fixed high limit thermostat (EasyHeat FC-130 or equivalent), you could make the contactor-based solution work within Code Sep 29, 2021 at 3:36
  • Added a clarification to my original question that I don’t need to control air temperature. Snap disk is not going to work as I can’t set a schedule for it, or turn it off when electricity cost goes above a certain threshold (I’m on hourly pricing)
    – Serge
    Sep 29, 2021 at 4:45
  • Also, hallway problem is lack of a good way to have a runway for wires, not the location itself. Also, I think some codes require thermostat to be within line of sight of the floor they control as they function as an off switch as well.
    – Serge
    Sep 29, 2021 at 4:48

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