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ComfortZone, HVAC Technician
Category: HVAC
Satisfied Customers: 516
Experience:  10 years in the HVAC industry.
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My Trane XE1000 heatpump turns on auxiliary heat when on Auto Cool.

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My Trane XE1000 heatpump turns on auxiliary heat while set to AUTO COOL. I turned the mode switch to off and the fan kept running for hours.
ComfortZone :

This would be indicative of a bad sequencer in the airhandler, a bad control board in the airhandler or a bad thermostat. You say the fan continues to run even if the unit is off and the fan is set to auto at the thermostat. Is that correct?

Customer: replied 5 years ago.

Yesterday, after I turned off the compressor (set the thermostat to "off"), I left the fan on "auto". It continued to run for another hour or more. Now, this morning, the air handler fan is off. By the way, this issue was discussed on hvac-talk and one of the respondents said it had to do with "one thing" within the heatpump that usually works when the system is in heat mode but that it was a "head scratcher" at first. Additionally, I've read that some systems use the auxiliary heat when in defrost mode, so I was assuming the "one thing" had to do with a defrost sensor or some aspect of switch into defrost mode. Might my freon be low, the system freezing up, and the heatpump defrost being triggered?

The defrost cycle is controlled by the outdoor unit and should never run in the AC mode. It is true that when the unit goes into defrost the heat package inside comes on and the outdoor unit runs in the AC mode, but if the unit is in AC mode at the thermostat the heat should never come on.

The heat sequencers inside the airhandler is what brings on the heat and they are activated by the control board.
Customer: replied 5 years ago.
Should I continue to leave the system off to prevent catastrophic damage or additional damage or if I turn it back on and it works properly, is there a risk of catastrophic damage to the system?
I would get an HVAC tech come and check it out. If the heat is running during the AC cycle, something is wrong with the system. If you turn it back on and it works properly the problem may still exist and could happen at any time so I would turn it off until I had it checked out.
Customer: replied 5 years ago.

After being off for 12 hours, I turned it back on and the aux heat came on and the outside unit was buzzing but the condenser fan did not come on. I turned it off again but the A/H fan kept blowing until I turned of the circuit breakers to both A/H and compressor. I cycled the circuit breakers to both the air handler and the compressor. I turned it back on in auto-cool and now it is working normally. What are ALL the possible causes to trip it into auxiliary heat mode? At this point it doesn't seem like there is permanent damage...

Was it in heat or AC mode at first?
Customer: replied 5 years ago.
I turned it back on to auto-cool. The blue auxiliary heat light immediately came on (as soon as I switched from "off" to "cool"). I checked the compressor and it was "buzzing" and the condenser fan was not on.
The buzzing may simply have been the reversing valve solenoid. If your aux heat light is coming on during cool mode, you may also have a bad thermostat.
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