Can I use a nest thermostat for two heating sources (Furnace and electric floor heating)

Question

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?

Answer 1

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.