HVAC and consolidating thermostats question

[Shocker 150]

I know there are some HVAC people on here, just had a question about consolidating thermostats

 

So PSE&G customers can buy smart thermostats ridiculously cheap right now (psegmarketplace.com) and my parents were thinking about getting one or two. But their house has separate thermostats for the A/C and their radiant heat, and two zones, for four total t'stats. That instant rebate is limited to two per person.

 

The heat side is really simple, just two wires - 24v and Heat call. The A/C has whatever A/C needs - cool call, 24v, Fan and Compressor relay, IIRC (maybe the R is jumped to Rc for some reason as well...not at their house as I type this...)

 

Is there any reason I can't simply connect the two heat leads to the corresponding terminals on the A/C t'stat and consolidate to one t'stat for each zone? I can't imagine a problem since the heater is basically on or off...but I'm not looking to cause a problem either.

 

TIA

[Handyman 5,682]

My parents had two thermostats back in the day. They were fairly talented at running the AC and heat at the same time. 

On the plus side, they could really dial in the temp they wanted.

[maintenanceguy 510]

Assuming that these are really two separate zones, it is easy to have one thermostat do both AC and heat.  By "really separate", I mean AC #1 cools the same rooms that heater #1 heats.  If there is some strange overlap between zones, you can still do it but strange things might happen in the overlapping areas. 

The heaters wires go the the heating terminals on the new thermostat and the AC's wires go to the AC terminals on the thermostat.  A heating/cooling thermostat is really just two thermostats in one box.

[Zeke 5,504]

If the thermostat ‘s are close together, ( i e you can get both wires) I see no issues.

[Shocker 150]

Thanks. 

 

AC and heat for zone 1 are a few inches apart, then AC and heat for zone 2 are at the other end of the house. So yes, they should only be sensing the areas they control  

 

I just couldnt figure out why when they put in the AC they didn’t just hook up the heat to the same t’stat, especially since the wires are close. I can only assume laziness. 

[njJoniGuy 2,131]

Keep in mind that the heat and cool are each powered by different power sources (ie. multiple 24vac transformers)

Strange things can happen when you tie multiple loads to multiple different sources.

[Sniper 6,372]

15 minutes ago, njJoniGuy said:

Keep in mind that the heat and cool are each powered by different power sources (ie. multiple 24vac transformers)

Strange things can happen when you tie multiple loads to multiple different sources.

That was the first thing I was wondered too. Each system has a separate transformer, relay and control system. Mixing the two could be an interesting experience.

 

[njJoniGuy 2,131]

That's why people have used relay-based control systems.

Loads and power sources can be isolated from each other

[capt14k 2,052]

Not too many thermostats that will control a radiant heat system and AC.

 

2 zones in a radiant heat system is not enough unless it is a really small house. Radiant heat is best if house has multiple zones all separately controlled. My father had 16 in house I lived in through high school; plus another radiant system for the driveway.

 

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[Shocker 150]

10 hours ago, njJoniGuy said:

Keep in mind that the heat and cool are each powered by different power sources (ie. multiple 24vac transformers)

Strange things can happen when you tie multiple loads to multiple different sources.

Hmm that's an interesting point. So what happens if they were combined and the 24v from the heater transformer were connected to the relays for the A/C or vice versa?

 

Can that be mitigated by using one of those transformer kits to create a common wire?

 

 

...also, 16 zones?! Their heater circulator has many branches that can be independently tuned for flow (do you count each of those as a zone?) but only two thermostats.

[capt14k 2,052]

Hmm that's an interesting point. So what happens if they were combined and the 24v from the heater transformer were connected to the relays for the A/C or vice versa?  

Can that be mitigated by using one of those transformer kits to create a common wire?

 

 

...also, 16 zones?! Their heater circulator has many branches that can be independently tuned for flow (do you count each of those as a zone?) but only two thermostats.

 

No, each zone has it's own circulators and thermostat. Current house isn't as large. That one is 8 zones.  

 

I am not as familiar with the AC side. However we always had separate thermostats for AC. House with 16 zones heat had 4 zones of AC. Currently he has 2 zones of AC, but because it was a complete remodel he used Furnaces for Air Handlers to have temp heat. So the 2 AC thermostats also control backup hot air heat.

 

I also should note that previous house was remodel as well so not all zones radiant in both house. Most of the first floor and bathrooms radiant. Second floor and part of first floor cast iron baseboard, 16 zone house had 3rd floor with 3 rooms and 3 zones that was also cast iron baseboard. Cheaped out on new house and got a lawn service who plows the driveway instead of using radiant to melt the snow.

 

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[ted527 21]

Any thermostat that has a jumper between R and RC can be used to control 2 separate systems. Remove the R-RC jumper, heat uses R and W. Air conditioner uses RC , G and Y.

Years ago you needed a special sub base to do this, most newer digital thermostats are set up for it.