For sale: used Honeywell T4 Thermostat TH4210U2002

Am replacing my 2 month old T4's with T6's. I have two available. These are manually programmable, not smart capable.

Probably not appropriate to post then...I doubt anyone here is interested non controllable thermostats...

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That being said, I do recommend the installation of a backup mechanical (non-powered) thermostat at the air-handler/furnace. This provides a backup, relatively failsafe, mechanism to turn on heating (up north) or cooling (down south) if the thermostat fails, and no one is at home.

Up north, it can prevent water pipes from freezing/bursting, and down south, it can ensure that excessive heat/humidity don't build up creating an environment that fosters mold growth.

However, the T4 isn't a mechanical thermostat.

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Never really thought about that. What do you do? Just splice it in line or rig a relay of sorts in case of failure?

Wire it in parallel to the real thermostat. My mechanical thermostat is set to turn on cooling if the temperature exceeds ~80F.

Because I never let the house get above ~75-76F, so it has never come on.

Up north, I would set it to turn on heating at ~40-45F.