Just bought a new (<6 years old) house and it's two stories, but only has a single furnace in the attic and a single condenser outside. However, there are two thermostats that work (semi) independently of one another.

The master on the second floor seems to have more options than the first floor thermostat. They are Honeywell models and I know there is a damper control in the attic vents to turn off the air flow on one side or the other if it's thermostat is set to off.

I'm sure I cannot have heat upstairs and cool downstairs since it's still just one system. But I can control the temperature per floor with each thermostat.

So my question is can I replace this master/slave combo with a Sensi programmable on each? I had them at my previous home and love the ability to easily program and control with my phone.

Will they automatically talk to one another with regard to "heat on" for example? Is there something else in the attic/furnace area I should be looking at as well?

Any tips or advice would be helpful as well. :)

  • 1
    Can you post photos of the wiring at both thermostats and at the zone controller? Jan 17 '20 at 5:19
  • @ThreePhaseEel - Unfortunately not. Before I start messing with the system, want to make sure it's something that will work. Figured it was a standard modern setup (my newest home before this was built in 1998 and it seems a lot has changed since then)
    – klkitchens
    Jan 17 '20 at 5:21
  • You're creating a catch-22 for yourself, as we'll need to know what make/model the zone board is and how it's hooked up to the thermostats to figure out if the Sensi will work with your system or not Jan 17 '20 at 5:28
  • @ThreePhaseEel - You just introduced a new element into the equation though. The "zone board". Is that on the furnace?
    – klkitchens
    Jan 17 '20 at 5:44
  • It can be anywhere in the house, but it's likely to be near the furnace. You'll have to trace the wiring from the furnace to find it Jan 17 '20 at 12:40

The most common way that two thermostats work with one HVAC system is through the use of a zone controller like one of these. They can be installed near the air handler, but it could also be in a nearby closet so it can be monitored or adjusted without going in the attic.

The zone controller makes all the decisions about controlling dampers and calling for heat or cool, so the thermostats don't have to know they are not controlling their own independent system. The thermostats are "standard" and don't need any extra features or wire connections, so you should be able to easily replace them with a smart thermostat and follow the normal installation instructions.

You can verify this by finding the zone controller, or just look at the wiring on your current thermostats. If they have the typical R, G, Y, W, C wires and nothing extra then its a safe bet they are standard thermostats. If they have some additional wires (and you don't have a heat pump or multistage heat/cool) then it might take some more research. Also look up the model numbers of the current thermostats.

  • The zone controller is a Honeywell HZ311 TrueZone.
    – klkitchens
    Jan 22 '20 at 14:58
  • 1
    @klkitchens, yes, those are very common. You should have no trouble using the smart thermostats. You may need to make sure you have a good C wire, but other than that it should be plug and play.
    – JPhi1618
    Jan 22 '20 at 14:59
  • Thanks... was reading docs/comments on Amazon that confirmed this as well. Another question... my Honeywell thermostat in the bedroom appears to be hardwired for power. The battery compartment is empty. Never had one like that before. Do you know if the Sensi can get power this way too?
    – klkitchens
    Jan 22 '20 at 15:12
  • 1
    Themostats have gone through phases over the years... In the beginning, they didn't need power at all (no C-wire) and had no batteries because they were "dumb". Then programmable thermostats appeared that needed batteries to back up the programming or power a display. Now the "smart" thermostats need so much power that batteries are not enough. Some might have them as a backup power source, but the wifi thermostats that have seen take no batteries and require a C-wire for power.
    – JPhi1618
    Jan 22 '20 at 15:16
  • Awesome... thanks again for all your help. Will move my thermostats this weekend... excellent!
    – klkitchens
    Jan 22 '20 at 15:21

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