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Here in central Virginia, most houses use heat pumps with electric auxiliary heat. I have inquired with several different contractors about installing outdoor thermostats on my units to 1) disable the use of auxiliary heat above a set outdoor temperature and/or 2) stop the outdoor compressor from running below a set outdoor temperature. No one I talk to thinks it a good idea or they have not done it before.

Looking at the schematics and examining the current wiring it seems fairly simple and the parts are very cheap. It seems like running the compressor when it's too cold to do its job and running the expensive electric heat strips when the compressor is much more efficient and able to do the job just seem like a lot of wasted energy.

There are manufacturers that make outdoor thermostat controls that are designed to do exactly what I am looking for and can be purchased at my local HVAC supply. (one Example :TPI # LRD100A)

My question is why wouldn't you want to do this? Are there downsides that I'm not seeing? (besides possible personal comfort)

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  • Hello, and welcome to Home Improvement. My guess is that the system controllers aren't set up to do this, and there's no way to hack it in and expect consistent results. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Feb 19 at 20:01
  • I don’t think that we don’t want to but modifying the control electronics will void the warranty. I am an electrician with a degree in electronics plus a universal hvac license, making these changes would be easy but would void the warranty on the systems I have installed. – Ed Beal Feb 19 at 20:52
  • FYI, some smart thermostats do take the outdoor temperature into account. – UnhandledExcepSean Feb 19 at 21:33
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    Normally, heat source blending is managed by the thermostat, and there are quite a few thermostats that support outdoor sensors + compressor lockouts. Aux heat lockout is harder because it still needs to work when the compressor is out of service or defrosting... – ThreePhaseEel Feb 20 at 3:18
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When a heat pump is installed it should be set up to turn on the electric heat strips when it drops below the point where the heat pump is more efficient. This is calculated on the units performance which should be specified based on the predicted (Charted) lowest anticipated temperature for your area. If it is using the compressor it will defrost either according to a time schedule or, on more efficient models, when it senses the outdoor coil is too cold and needs to be defrosted. When it does this, it turns off the outdoor fan and reverses the flow of refrigerant into cooling mode. It activates the heat strips to keep the air inside running warm. When it reaches a set temperature or after a set period of time, it will change back into heating mode.
They showed us this in my Residential HVAC Technician Class last year. Should do everything you want it to. All in one universal replacement heat pump defrost controller with included sensors can be set up for demand or timed defrosts. https://climate.emerson.com/documents/47d01u-843-universal-heat-pump-defrost-control-specs-en-us-1569854.pdf About $81 on Amazon.

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The one place that I lived at where it had a heat pump did have a auto-stop on the compressor so that it would not run below freezing temps. If the temp dropped while the compressor was running, you would hear a hissing sound when it would shut itself off.

I grew up with gas heat so there was always hot air blowing regardless of the outdoor temp. I mentioned this to the HVAC guy and he wired the auxiliary heat to kick on every time the system kicked on. That way I didn't have to pay attention to the outdoor temperature and know when to flick the switch.

By doing it this way, I really didn't notice much change to the power bill as the system would run slightly shorter cycles.

Perhaps this could be done for yours.

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  • Your system wasn't running at a very good COP if you didn't notice your power bill changing... – ThreePhaseEel Mar 27 at 23:55
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I had a Nest thermostat at my last home and it does exactly what you're suggesting. Because it uses Wi-fi, there is no need for an outdoor sensor - it gets the temperature from the internet. The setpoints at which it locks out the aux heat (above a certain temp) and compressor (below a certain temp) are adjustable. So if you don't want that feature, just set them to extremes.

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