Not sure why this happened to our home while we were away, but all of a sudden this showed up and lasted a long time. Right now, it's all good. What should I check to make sure it won't be an issue for the performance during colder nights?

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  • Do you have a furnace that runs on gas or oil (dual fuel), or is your aux heat a bunch of heatstrips? Commented Dec 12, 2023 at 5:25

2 Answers 2


Simply put, a heat pump moves heat from outside air (or better, from the ground, which maintains a warmer temperature through winter than does the air.) This is more efficient than resistive heating, which converts electricity directly to heat, as in a clothes iron.

When the outside temperature is very low, heat pumps cannot produce sufficient heat by moving it in from outdoors, so must turn on auxiliary heat (e.g., resistive heating or fossil fuel furnace), which lowers efficiency, producing less heat per watt-hour used, or uses up fuel (the Ecobee alert would then be useful to remind one to refill oil or propane tank).

If the weather has been unusually cold, ignore the warning; perhaps turn it off ("Edit alert preferences"). If not, then your house might need more insulation or repairs to the heat pump.

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    support.ecobee.com/s/articles/… You may have your staging temp set too low, your compressor min outdoor temp set too high, or your max AUX temp set too high.
    – Huesmann
    Commented Dec 12, 2023 at 13:37

I assume this is a default setting, to provide a warning message in the stated situation. Whether or not it means anything depends on a few things, including whether or not you've had the system long enough to know how it normally operates (I will assume you have). How much it matters to you may also depend on how much more expensive / inefficient your backup heat is compared to the main.

If the weather was colder than normal (and/or you set the heat higher than normal), then this is just your thermostat telling you that your primary system isn't keeping up. As long as the total system maintains indoor comfort without breaking your budget, that's probably fine. If not, your main system could be undersized or you could be loosing efficiency somewhere (e.g. lack of air sealing or insulation on ducts where there should be). Some of this could be handled by DIY, other parts only by an HVAC specialist - but I won't speculate without more info.

If it wasn't colder than normal, then your main system isn't keeping up like it used to and may need some kind of service (simple filter cleaning to something requiring a tech).

  • “this is just be telling you that your primary system isn't keeping up” Do any thermostats actually work by running the heat pump for a while, noticing it’s not making a difference, and then falling back to the aux heat? I’ve only seen ones that switch modes based on outdoor temperature (either sensed locally or pulled from the internet). The former seems like it would waste a lot of power, and lead to missed schedules.
    – nobody
    Commented Dec 12, 2023 at 14:39
  • @nobody I have not researched what kinds of logic are out there on the market, but I would assume outside temp is commonly what the switch is based on since it is simple. You could imagine a smarter algorithm based on an efficiency or cost crossing point which could operate in this manner, providing optimum performance on some metric (e.g. try to reach the setpoint first using the more efficient method before going to a less efficient backup).
    – blarg
    Commented Dec 13, 2023 at 3:04
  • According to support.ecobee.com/s/articles/… it seems to do just that -- it triggers when the inside temperature gets too far below the set point (and the outside temperature is below another threshold.) The threshold difference is around 2-3 F, depending on a comfort setting. Commented Dec 13, 2023 at 4:44

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