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Greetings,

I've got a Honeywell TH8320 t-stat set up for dual heat with my Trane XL14i HP and Trane CL90 propane furnace. It's been working fine for many years. The compressor lockout is set for 30F controlled by an external temp. sensor. Normally I only get fossil fuel coming online at temps 30 or below but now the propane is coming on above the lockout.

I checked all of the setup functions and confirmed they were unchanged and reset the furnace - same behavior. I factory reset the t-stat and re-entered the parameters - same behavior. I confirmed the accuracy of the outdoor temp sensor. I also checked the voltages across the t-stat and they all seem fine. The only change to the system was the replacement of the furnace blower motor due to high bearing noise; that was done in warmer months when heat was not needed. Since the t-stat is set for fossil fuel aux heat there is no "auxiliary lockout" function - propane should not be available above the compressor lockout setpoint.

Either the t-stat's lost its mind or I'm missing something fundamental. Any ideas?
 
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