Dual thermostats on one AC unit

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Well-Known Member
Jul 2, 2007
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Its been a while that I posted here. I am trying to chase down this issue at my GF house. It was 110+ degrees in Phoenix so I didn't farther purse it. Needed to run back to Calif.


The house has two thermostats with one AC unit (condenser). The first floor thermostat looks like its broken, no light lid up. First floor, AC is still blowing out cold air to a point you have to wear a jacket.
Second floor, the thermostat is in the hallway and its working, however the bedrooms are barely cool (not much cold air coming out of the vents in the bedrooms). Both thermostats are identical. Model is Honeywell TH8110U1003. When I swapped the thermostats the reverse effect would happen (cold in the second floor (bedrooms), and barely cold in the first floor. I notice when I pulled the bad thermostat out of the wall, the entire AC unit would not run (seems like they are dependent of each other despite one thermostat being bad).


Do I need to replace both new identical thermostats in this case? Any special situations I have to be aware? I have swapped out a single thermostat before. Batteries have been replaced. Attached is the wiring of the thermostat (both are identical).



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