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How do you wire the Honeywell RTH8500D thermostat so as to operate it battery-free from the main power, yet have battery back up? Info online and in manual does not appear to address this, yet on package it says battery-free operation is optional.

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You would need to have a the hot wire from the transformer which would be the red one and connected to R/Rh/Rc. You would also need a common wire from the transformer connected to the C terminal on the contactor. It is usually a blue or brown wire. You would need to look at the transformer in the airhandler to see what color the common is. The secondary wires on the transformer are usually yellow and blue so you will need to look where these connect to the thermostat wires to see which is the common. Also your existing thermostat wire may not have enough wires in it to do this. Let me know if you have any questions.

Posted on Jul 13, 2010

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Posted on Jan 02, 2017

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