[FrostFlame64]

So I'm a DIYer renovating my house with a tricky thermostat wiring issue that I KNOW should be theoretically possible, but I'm having difficulty figuring out the last steps.

For Context: My house is a single story manufactured home that has forced air heating and cooling. I replaced the thermostat with a Nest 3rd Gen Learning Thermostat, which works perfectly fine.

I then decided to replace the carpet in my living/dining room with hardwood floors (an area that covers ~70% of my house), and while doing so I installed radiant heating via electric panels. I used a RC840T switching relay so that my Nest thermostat could control the radiant heat. This also works perfectly fine.

So independently, both systems can be controlled by a Nest Thermostat. But my goal is to get them both together working under a SINGLE thermostat, treating the heating systems as a Two Stage (something known as Hybrid Radiant Heating). My problem is trying to figure out how to resolve wiring them together. This seems to be a situation that will require an Isolation Relay, but I'm still not sure how to resolve this.

Any help that could be provided would be appreciated. Worst case scenario would require me to have 2 separate thermostats, which is what I'm REALLY trying to avoid.

 

[FrostFlame64]

For reference, here's a diagram of my wires:
[Dropbox url]/s/7dfllt23pm0qzrt/Wiring%20Diagram.png?dl=0

I would post it as an image, but it won't let me ><.

 

[bfrabel]

So I'm a DIYer renovating my house with a tricky thermostat wiring issue that I KNOW should be theoretically possible, but I'm having difficulty figuring out the last steps.

For Context: My house is a single story manufactured home that has forced air heating and cooling. I replaced the thermostat with a Nest 3rd Gen Learning Thermostat, which works perfectly fine.

I then decided to replace the carpet in my living/dining room with hardwood floors (an area that covers ~70% of my house), and while doing so I installed radiant heating via electric panels. I used a RC840T switching relay so that my Nest thermostat could control the radiant heat. This also works perfectly fine.

So independently, both systems can be controlled by a Nest Thermostat. But my goal is to get them both together working under a SINGLE thermostat, treating the heating systems as a Two Stage (something known as Hybrid Radiant Heating). My problem is trying to figure out how to resolve wiring them together. This seems to be a situation that will require an Isolation Relay, but I'm still not sure how to resolve this.

Any help that could be provided would be appreciated. Worst case scenario would require me to have 2 separate thermostats, which is what I'm REALLY trying to avoid.

Can't guarantee that this will work with a nest, but I'll give a whack at explaining how I would make it work with a known-to-be less crappy t-stat.

I'm assuming you have 2 separate 24v power sources and you are wanting this to work as a true 2 stage setup, as in when you need heat, only one system will normally turn on, unless that system can't keep up, then eventually the other one will kick in also. If so, here's how I would do it...

Wire up primary system as normal, with wires from R, W1, and C hooked up (along with G and Y1 if you also need fan and cooling to work).

Bring wires W2 and C to the "coil" of your relay. If the switching side of the relay is already wired to bring on your radiant panels, then you're done.

This should work with most t-stats, but nests are known to be special bastards, so all bets might be off with one of those.

Good luck!

 

[user_12345a]

What kind of forced air system do you have?

If it's not straight electric resistance heat, using electric in floor as a first stage could dramatically increase the heating bills.

I wouldn't do it unless you have very cheap electricity.

Resistance heat is the devil.

 

[bfrabel]

If you're really just wanting both systems to turn on and off at the same time, follow the above instructions pretty much the same, but instead of wiring the 2nd unit to W2 and C, use W1 instead.

Either way I should mention that you'll be using the 24v power from the first system to power both systems (or at least the first system and the relay for the 2nd system).

 

[Geochurchi]

Hi, the scheme seems to work, but a set of N/O contacts on the relay should be connected to the R / W terminals on the RC840T relay in place of a Tstat.
Geo

 

[FrostFlame64]

What kind of forced air system do you have?

It uses Natural Gas for the Forced Air heating. I know that the electric radiant heating will naturally increase my electric bill, but I'm hoping that making it hybrid with the natural gas will lower my heating bill overall. I'm fairly certain it will just from the fact that I like having cooler temps anyways, so with the radiant heat keeping most of the warmth down on my level I can lower my heat and still be comfortable (as opposed to my forced air heating, where the "temp" I set is only at head level, thus being cooler near the floor).

 

[user_12345a]

If you set this up so the gas heat is the first stage, the radiant will not come on to maintain - gas heat probably has more than enough capacity.

If you set this up so that the electric heat is the first stage, the electric bills will go up dramatically.

2-stage thermostats don't bring on second stage until first can not keep up when trying to maintain unless you find one with an upstage timer.

I would keep the systems separate and use the electric only on special occasions. Even if you can lower the temp setting a bit with the electric radiant, it will still cost a heck of a lot more than gas forced air unless you have very cheap electricity.

 

[beenthere]

If you were using a good Honeywell thermostat with CPH settings. You could set up the first stage radiant at 6 CPH, and the gas furnace CPH at 3. This would shorten the radiant on intervals and bring on the gas furnace quicker.

 

[FrostFlame64]

Thanks for the advice everyone! This thread has been very helpful.