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Wiring 1 thermostat to control 2 HVAC units (1 a heat pump!)

There are two HVAC units in question:
1. natural gas furnace / AC
2. heat pump

They're completely separate units with separate ductwork and separate compressors, wires, etc.

However, their ductwork all goes into the same 'great room' and their return/supply ducts all come from the same 'great room' in the house. Two AC units is merely a way to cool/heat more rapidly, since the house is in Las Vegas. They're not for separate zones.

Both units will always have the same temperature settings on their thermostats. Therefore, I'd like to only have one thermostat that controls them both.

The thermostat is an Ecobee3. I'm guessing that I can connect the heat pump to the thermostat (and tell the Ecobee that it's controlling a heat pump), then just run a jumper for the rest of the wires (Rc, G, Y1, W1, but not C) straight between the heat pump and furnace/AC unit, so they perform the same actions at the same time.

Any guesses if this would work, or am I risking shorting out the control boards in both units by trying to simplify this interface to only 1 thermostat? If both HVAC units were the same (both heat pumps or both non-heat pumps) it should work, but I'm not familiar with heat pumps and how their wiring might differ (other than an O wire to switch between heat/cool).



  • Jamie Hall
    Jamie Hall Member Posts: 23,278
    Got to admit that I'm kind of a belt and braces guy. If it were mine, I'd be very inclined to use a relays controlled by the thermostat to, in turn, control the two units, thus keeping their control circuits isolated from each other.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ratio
    ratio Member Posts: 3,625
    They can not share wiring like that - for electrical isolation there must be an interposing relay that the stat controls, which will then control the units with electrically isolated contacts.

    That said, I'm not sure about using one stat will work in that application. A heat pump is an air conditioner that works forwards and backwards, there is a valve inside that switches it from heat to cool. The upshot is that the compressor runs during heat as well as cool, and the reversing valve is energized for one mode and not the other (around here, we reverse for cool, but in Las Vegas it might reverse for heat), whereas the AC/furnace has a separate wire for each function. I'm sure it could be done with some relay logic, but that's getting complicated!

    I don't know the details of the equipment, but perhaps they could be wired up as separate stages, e.g. both fans come on together, then the HP heats or cools, and if it is unable to meet the load by itself the AC/furnace heats/cools as well.

  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    Lot of potential (sorry :)) for backfeed with this arrangement. I would look into a dual fuel control first, then find a stat that is compatible?
  • Spence
    Spence Member Posts: 316
    I would be happy to make a wiring diagram for you if you would like.
  • synosure
    synosure Member Posts: 6
    Really, Spence? That would be awesome if you can. I've wired a couple relays up there already, to turn zones on and off (on other unrelated HVAC units), so I don't mind adding a couple more.

    However, what Ratio said makes sense to me, and wasn't something I'd thought of, that normal HVAC doesn't turn the compressor on during heating, and so the 'turn compressor on' line would need a circuit made from a series of relays, only switching if both the 'turn the compressor on' and the 'run the heat pump in AC mode' lines were both active. Spence, if you think it's reasonably doable, I'd be happy to give it a try. The rest of the house already has 2 Ecobee3 thermostats, and I really don't want to spend $250 to add a third when I'm not even going to let it think for itself.
  • Spence
    Spence Member Posts: 316
    "Normal HVAC" does indeed turn on the compressor in heating if you are using the heat pump configuration in the thermostat. This is less complicated than you think.
  • Spence
    Spence Member Posts: 316
    Am I correct that the heat pump system has an air handler with no auxiliary heat?
  • synosure
    synosure Member Posts: 6
    Yes, the heat pump can do heat or ac via the compressor, but has no auxiliary heat installed in it.
  • Spence
    Spence Member Posts: 316
    Okey-dokey. Will get diagram out; hopefully by tomorrow afternoon if that works for you.
  • synosure
    synosure Member Posts: 6
    You rock, Spence, thanks muchly. Seriously, it's very appreciated.
  • Spence
    Spence Member Posts: 316
    In the spirit of the Wall, you are quite welcome!
  • Kgall21
    Kgall21 Member Posts: 1
    Hello. I'm looking for a similar application. Does anyone have the wiring diagram that Spence drew up? Thanks!
  • unclejohn
    unclejohn Member Posts: 1,833
    The easy diagram is two tstats. One heat pump and one Gas & A/C.
  • John Mills_5
    John Mills_5 Member Posts: 951
    I've done it using simple relays