I'm assuming after reading the owner's manual that the batteries are required. I purchased this 80 dollar thermostat at a garage sale for 5 dollars in an unopened box. So i thought it might be good futureproofing for my home. I had a very nice high efficiency Rheem furnace installed a few years ago. Just using the old round dial thermostat.

All I want out of the new high tech programmable Honeywell for the time being is ultra basic set-and-forget heat. I don't care at all about losing settings in a power outage. I hate the idea of complicating my life with having to worry about batteries going out and replacing them regularly. Why can't I just put a 120vac/3vdc transformer in the basement and run 3vcd through two of the unused wires in the 18/5 and connect the wires to the battery terminals inside the thermostat? Does connecting the 'C' wire power this thermostat without the need for the battery backup?

Or is there a way that honeywell has provided for me to do this that I don't know about?

  • Removing the batteries would defeat the purpose of the batteries, which is to preserve settings during a power failure. Commented Jun 6, 2020 at 14:58

1 Answer 1


The manual says "C does not power the thermostat display or operations; batteries are always required", so there's no reason to doubt that.

Putting 3V on a pair of wires and running them to the battery terminals would probably work fine, but obviously it isn't intended and I have no idea if it's to code.

Less complicated options would be to just put batteries in there (they should only need replacing every couple years) or buying another thermostat that is intended to be powered by 24VAC from the R/C wires.

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