Nest e74 error when generac switches on

Automatic standby generator installation questions. Wiring, placement, code requirements, etc.
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Cb5252
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Joined: Thu Oct 17, 2019 3:41 pm

Thu Oct 17, 2019 3:45 pm

The nest thermostat works fine for heat and air conditioning. However, when there is a power failure and the generac generator switches on, the nest thermostat reports an e74 error and there is no heat.

My generac service guy hasn’t encountered this before.

Anyone know the solution?
Tx
Chris
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Thu Oct 17, 2019 4:35 pm

I'm not super familiar with the Nest products, but after looking it appears that the error is for not having current on the Rh wire which would be the power to the stat. The R, Rh and Rc are all about the same and are 24V power to the stat which is then sent back out to the various 'switch legs' to tell the furnace and AC to do certain things. For example, connecting R to Y would make the AC come on.

If the Nest is losing power from Rh, it would make me think that the furnace isn't outputting power anymore on the 24V system. I'd more lean towards an issue with the furnace not liking generator power, or somehow it's not resetting and coming back on when the generator comes back on. A good test would be to see if the furnace has any error codes flashing as well.
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