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heat pump min outdoor temp

digidiotdigidiot Raleigh, nc, usaMember


I have upstairs and downstairs heat pumps. The downstairs is trance so 16i and upstairs is trance xr13. After a service call to replace disable links is upstairs air handler, all was well for 2 days down to temp of I think 18f. Then it got down to 12f, and the downstairs couldn't keep up. Both heat pumps have ecobee3 thermostat.

I'm fairly certain what happened is the downstairs ecobee3 was set to 15f min outdoor temp on heat pump settings, and when it dropped below that, the compressor shut down and only heat strips were used and they couldn't keep up.

The only problem with the theory is the upstairs ecobee3 is set to 20f, and the upstairs stayed toasty while the temp started dropping.

Questions: can I drop or disable minimum outside temp so compressor will continue to help heat WITHOUT damaging trane heat pump? Also, why would an older less efficient unit heating more space upstairs work well with the setting at 20 when newer more efficient unit can't do it at 15 with less space to heat.

I mainly just don't want to damage heat pumps. I feel sure the setting change would fix heat problem in colder temps.



  • digidiotdigidiot Raleigh, nc, usaMember

    That is TRANE XL 16i. Not trance...

  • kevinmcckevinmcc Macomb, ILMember

    I have my minimum outside temp disabled. Your heat pump should have external thermostat to disable the heat pump when too cold. You can also drop the minimum outside temp to 0 F.

    If your return air hits the coil first and then hits you heat strips, then definitely turn on Aux Heat Simultaneous Operation. The heat pump will preheat the air before the strips finish heating the air. Should get your strips checked too, they either are not working correctly or are undersized.

  • digidiotdigidiot Raleigh, nc, usaMember

    Thanks for the reply!

    I've disabled that setting. I think with it off I probably will be fine.

    Thanks again.

  • earlgreyearlgrey PhilaMember

    On the Ecobee3, I have found that if "compressor min outdoor temperature" is disabled the thermostat will turn off the compressor any time it calls for aux heat (I have electric strip aux heat). In fact I just tested this today.

    I allowed the house to drop to 3 degrees below my set point and then pushed the temp back up to my normal set point. I have the compressor to aux delta set at 2 degrees and max aux heat temp set to 25. The outside temp was 20. So with a delta of 3 degrees from my set point and an outside temp of 20, I would expect the system to call for aux heat.

    With "compressor min outdoor temp" disabled, the compressor ran for 30 seconds at which point the compressor was turned off and the aux heat came on. I waited a few minutes and turned the system off and changed "compressor min outdoor temp" to zero degrees. When I turned the system back on, it ran both the compressor and the aux heat together, which is what it was supposed to do.

    Ecobee really needs allow a lower temp range for the compressor min temp. I have mentioned this to them in previous conversations, and I know others have as well, so maybe they will listen. Where I live we rarely go below zero. So for me it is not a huge problem.

    One issue I still have is that the aux heat runs too long. In today's testing, the thermostat ran the aux heat all the way to the set point. It should have cut the aux heat as soon as it past the aux heat delta at 2 degrees from set point. I have "aux min on time" set to 1 minute. Yet the system called for aux heat for 15 minutes. I have asked Ecobee about this before, but I never got a good answer.

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