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I have an oil furnace in my basement with only 3 wires for the thermostat only 2 being used. I just installed central air in my house but air handler is in the attic with 5 wires for the thermostat 4 being used. my question is I bought the Glass WiFi Thermostat can I hook both systems up to it or do I need to have 2 separate thermostats

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  • Read the manual! Most likely there are separate connections in your new thermostat for A/C cooling vs. the heating system. The usual headache is a lack of a "C" wire going back to your furnace and there are plenty of workarounds for that.
    – Carl Witthoft
    Oct 29, 2019 at 17:38
  • Can you post photos of the wiring at all places, as well as a link to the installation manual for your t-stat, and the wiring diagrams for your oil furnace and air handler? Also, is this "oil furnace" itself a forced-air unit, or does it heat water instead?
    – ThreePhaseEel
    Oct 29, 2019 at 23:34

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Looking at the installation instructions found here, I see two important details. First, the thermostat does have a connection for Rc and Rh. When you have two thermostats and two systems located in different areas, you normally have two low voltage transformers for the control systems. The red wire from the heat goes to Rh, and the red wire from cooling goes to Rc.

The other subtle but important bit is this note:

Note: If you have only one R wire, connect to RC.

This tells us that Rc is the "main" red wire connection, and the common wire (that is required) must also come from the cooling system.

So, yes, this thermostat should work, but when it comes to hooking up a C wire, that will need to come from the cooling system. If your heating and cooling system have separate fans (blowers), then you might need to wire the furnace so that the fan always comes on with the heat and you will not have separate heat fan control.

Check the manual for other details and even call their customer support because I don't have all the facts, but it at least seems it's worth a try or more research.

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Short answer is yes, but you'll need to think about it a bit -- you'll need to decide which system is going to supply the power to the system, and that system will need to be turned on all year.

Your furnace will connect to Rh and W Your air conditioner will connect to Y1 and Rc (most likely).

Your C wire will probably go from the AC unit.

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