Very worthwhile and detailed explanation of function and wiring. My Honeywell Vision Pro 8000 (for heat pump) goes into aux heat now (for no valid reason - outside temp no low enough) and never comes out of it. I pull the batteries and let it set for a minute and when I reinstall it, it goes back to the heat pump setting and functions as it is supposed to - until the next time - could be today, tomorrow or whenever. It's unreliable. I used to have the fan set on recirculate, which would leave the fan running at about 1/4 speed 24/7, but then the fan started running full speed and never shuts off, and that's also when the aux heat problem started. Those two things combined suggest to me that the thermostat is bad, so I'll get another one. Need the heat pump running, so I don't have down time to set up a test like you have done. I welcome your thoughts on this issue.
Could be a few things…but starting with the thermostat is fine. Could be a sticking relay/control for the fan and/or heat strips. Also, the defrost board could be “bad” and energizing the heat strips. Those would be a more complicated to diagnose issue.
Very worthwhile and detailed explanation of function and wiring. My Honeywell Vision Pro 8000 (for heat pump) goes into aux heat now (for no valid reason - outside temp no low enough) and never comes out of it. I pull the batteries and let it set for a minute and when I reinstall it, it goes back to the heat pump setting and functions as it is supposed to - until the next time - could be today, tomorrow or whenever. It's unreliable. I used to have the fan set on recirculate, which would leave the fan running at about 1/4 speed 24/7, but then the fan started running full speed and never shuts off, and that's also when the aux heat problem started. Those two things combined suggest to me that the thermostat is bad, so I'll get another one. Need the heat pump running, so I don't have down time to set up a test like you have done. I welcome your thoughts on this issue.
Could be a few things…but starting with the thermostat is fine. Could be a sticking relay/control for the fan and/or heat strips. Also, the defrost board could be “bad” and energizing the heat strips. Those would be a more complicated to diagnose issue.