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Current situation:

My home is currently heated with central air. Not my first choice, but that's what I have. I have brand new 98% efficient modulating furnace controlled by a Nest thermostat(no AC). I have cold spots all over my house. We did the best we could with balancing on the first floor but the original install is just bad. Some rooms don't even have vents. My primary concern is the first floor. The room on the right side (living room gets quite warm, but the kitchen (left) stays rather cold. There is a large amount of windows to be sure, but I'm sure the major problem is the addition behind the stairs with no heating whatsoever. It is very cold in that room in the winter. Not only does it seem to keep the thermostat on in the winter until the upstairs is significantly hotter then the downstairs, due to the lack of vents on the left side of the home I believe it creates a temperature gradient preventing the kitchen from warming up.

 |[]-----|-[==]-|
 |  V    |      |                []    Door   
 |----  -|    ss|R-------|       [==]  Sliding door
 |R      |     s|        |       s     stairs 
 |       T      V        |       V     vent
 |                       |       R     return
 |                       |       T     thermostat
 |       |V    R| R      |
 |-------|--[]--|--------|

The solution?

The flooring is unfinished in that room and sits about 3/4" below the rest of the floor. I am debating using an electric heated floor, embedded in mortar, with a tile finish. It's 134SF, I already did the math and I'm anticipating an extra $30 - $36/mo on my electric bill in the colder months. My question is about integration. I did this once before and was told I needed an extra "special" thermostat. As you can see from my amazing drawing, my Nest thermostat is right next to this and I am told it can support both a heat pump. Would I be able to attach it to a 240V, two wire, radient flooring system instead. I realize I would need a relay. Has anyone done anything like this?

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  • How do you want the Nest to prioritize using the two heat sources? – ThreePhaseEel Dec 4 '19 at 2:10
  • I wouldn't necessarily want it to prioritize at all. If ambient temperature is 40F and I want to heat my home to 68F, I would want both units attempting to regulate to 60. The problem is, it just occurred to me what be so "special" about those thermostats they always try to sell you. I'm thinking the temperature feedback might be resistance based as opposed to a sensor. If that is the case I may have just answered my own question. – mreff555 Dec 4 '19 at 13:57
  • Thermistors are variable resistors but these would need to be a sensor as the are very tiny and cannot directly control power. – Ed Beal Dec 4 '19 at 16:38
  • @mreff555 -- one issue I can see here is that if you don't have a priority order, the thermostat may consider the setpoint prematurely satisfied because the radiant is heating the room the thermostat is in, but not anywhere else. You might be able to overcome this with a remote sensor...but this seems tricky, in any case. – ThreePhaseEel Dec 5 '19 at 1:09
  • I had a similar situation and solved it here: diy.stackexchange.com/questions/107216/… – Sealander Feb 29 '20 at 20:21
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The Nest can be the controller but you will probably need a separate contactor /GFCI power source unless the flooring system has low voltage control.

Some heated floor systems have there own thermostat /GFCI protection, if you wanted to control the floor heat from the nest additional controls would be needed. One of the best home made systems I have seen was a 2 stage control , radiant heat takes time to heat up so an engineer I worked with asked me if I could build a control using a SCR for low power and a TRIAC for high power (SCR’s only turn on for 50% when commanded on and TRIAC’s are on for 100%) this worked quite well at maintaining some warmth on low power and true warmth at high power, he provided the signals to fire the 2 solid state relays , I put them in a metal j box with a GFCI And it passed inspection so I know it can be done but I think he was doing other things with the smart thermostat and using a micro controller for the control voltage.

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  • Yes, it sounds like he must have been maintaining the desired temperature using an external sensor and your two circuits. Unfortunately kind of logicprobably goes beyond the capabilities of a Nest thermostat. – mreff555 Dec 4 '19 at 18:17
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    I am not sure I thought there were multiple outputs available but I haven’t installed one in a while and it only needed RC / RH outputs – Ed Beal Dec 4 '19 at 19:39

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