Replacing 2 wire mercury thermostat

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Furnace has G,C,W,Y,R wiring harness with R and W connected.

Thermostat has this (R,W) wiring:

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I had to say there was a C wire to proceed in app and then run a standard 5 wire thermostat cable. Through the wall (using the W, R and C)

The quick guide says a C wire is not required? The app says it is. Which is it?

I used the furnace manual as a guide to using C wire.

30 minute install not happening as I had to play hypotheticals with the app.

It worked during the test. Hopefully I won’t freeze over night.

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Furnace control board.

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Furnace manual.

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C is required to power the thermostat. the only time C is not required is if you are using Y, G, W, R and you use the C adapter. For heat only, the C wire is always required.

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Thanks. That’s what I understood. What I was getting at was that the Wyze product instructions should say “You only have a W and R wire. It looks like you have a two wire thermostat. You will need to run a C wire to your furnace in addition to the W and R” instead of “three or more wires are usually present” and not allowing you to proceed using the app.

The balanced setting dropped the temperature too low at night so I’ve adjusted it closer to comfort priority. I also went into the advanced settings and had to offset temperature and humidity.

I’m disappointed that their thermostat is not as straight forward as their cameras, but there is less cycling of the system so that’s a plus.

6

There are “smart” wifi thermostats that run on batteries and don’t require a C wire. Mine has run a couple of years on the same batteries.

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