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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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If you connected the C wire, you may have damaged the thermostat by providing too much current. Honeywell has the documentation for the RTH6350 on the product page: http://yourhome.honeywell.com/home/Products/Thermostats/5-2-Day-Programmable/RTH6350D.htm . The installation guide is here: https://customer.honeywell.com/request.cfm?form=69-2416ES . See page 23 and subsequent pages for the wiring diagrams.

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First try replacing the batteries. I have a slightly diff honeywell model and it takes 2 "AAA" batteries. The battery holder comes out the side of the unit, just press where it says press and pull out.

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This model of Honeywell thermostat uses the 24v from the unit to power it. One of three things must have happened.

1. You have lost power to the furnace, therefore no power to thermostat.
2. You only have 4 wires hooked up on the thermostat and not 5. A lot of older homes had just 4 wire running to the thermostat, which will not work for this thermostat. You should have a wire connected to the (C) or (B) terminal on the thermostat, which ever one they have, I can't remember off hand. These are the Common terminals. They have to have the common from the furnace 24v power to operate.
3. Faulty thermostat, which will obviously have to be replaced.

Hope this helps and gets you to cooler days!

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You will need the installation book that came with the thermostat. There is a way to enter into the programming of the thermostat from the home screen. From there, you will need to find what code is the temperature settings in the manual, they are usually all 3 digit numbers. Once you find the code for the setting your looking for which is the lowest temp for the thermostat, it will have a 75 to the right of it which you can set a lower number. Sorry I do not have the manual right now to just tell you exactly how, but hope this still helps!

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Hi,

Here is a tip that tells you how to get a manual for most any Honeywell product online...

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