Heat Pump spikes to ~400% before shutting off. Is that normal?

Also, I should’ve said so earlier, but thank you to everyone who has offered help with a reply. I definitely appreciate all of you helping me zero in on the apparent source of this issue, and offering suggestions for replacement thermostats.

Not to go too promo on Ecobee, but I had originally had Honeywell thermostats in my house but moved to Nests fairly early on. But about 5 years ago, I moved to Ecobees for several reasons, none of them related to programming.

  • Remote sensors - motion / temp sensors in various rooms to help control heating cooling. Target different temperature in different rooms at different times of the day.
  • WiFi capabilities - access to local weather station data, etc., HomeKit. The Google acquisition harmed Nest’s interoperability for the mid-term.
  • Data access - REST API plus CSV data access.
  • Greater flexibility - easy to adjust various advanced heating/cooling parameters

Thanks for the testimonial/review. It definitely gives me some food for thought as I shop for a new thermostat.

How old is your HVAC system? Does it utilize R410A refrigerant? Throwing thermostats at it isn’t going to make it more efficient. You may be very close to system end of life. (10-15 years)

According to the American Council for an Energy- Efficient Economy, you should consider buying a heat pump that is at least 15 SEER and 8.5 HSPF. The most- efficient Energy Star- rated heat pumps are 18 to 27.5 SEER and 8.5 to 12.5 HSPF.Nov 21, 2019

@babblefish26 - I’m legitimately confused… Are you following the discussion at all? First you suggested that the heat pump was using 20kW to defrost on a 65° day, and now you’re suggesting that, when the problem is clearly that the system is being thrown into aux heat mode to achieve a temperature change, that the heat pump is simply too inefficient (which would cause it to run frequently and for long periods of time, which it’s not)… You’re providing useful information in the most general sense, but none of it seems relevant to the issue being discussed.

The heat pump recently turned 10 years old and has had regular maintenance (and uses R-410A refrigerant), but thanks to a blowout during a summer storm, the compressor is only about 5 years old (thankfully the previous owners paid for a 10-year warranty that covered ALL parts and labor), so it’s essentially a 5-year-old heat pump running on 10-year-old efficiency standards. There are certainly more efficient units available now than 10 years ago, but with any large appliance, replacing it too early is far less efficient than letting it serve its time.

The compressor unit can still ice up on a warm day. Heat pumps are a strange little beast.
Don’t forget a heat pump runs both summer and winter. AC is summer only; so twenty years is a nominal life. Heat pumps work twice that amount; so maybe 10-15 years is a good life expectancy.

I myself am sometimes puzzled with the logic the HVAC controller uses for defrost.
Another thought and this is something I recently observed. We lost power on a cold night last month; thermostat was set for 72. When power was restored, indoor temperature was down to 65. Auxiliary heat (propane) was utilized until temperature was returned to 72. Heat pump didn’t kick in until house was stable at 72.

Also my HVAC contractor suggested that I don’t make large changes in temperatures while in heat mode. (setback).

I am sorry that I have confused you. I do hope you get this resolved, maybe it is time to call in a pro.

I bought an Ecobee SmartThermostat Pro and just set it up a few hours ago. The heat has only run once, very briefly, since then, but the good news is that the heat cycle did NOT finish with a few minutes of aux heat! I can only imagine how much that errant behavior of the Honeywell thermostat has cost me over the last nine years! But at least from first appearance, I’ve solved that problem with the Ecobee.

It does seem to read a little colder than the old thermostat (It was 40° outside, and I had the HVAC turned off for about an hour while I switched out the thermostats, yet the temperature in the house supposedly went UP by two degrees in that time… suggesting the Ecobee is probably reading 2-3° cooler than the Honeywell), so I’ll probably have to play with the settings a little bit. But the immediate problem appears to be solved!

Thanks for your help, and thanks @kevin1 for the Ecobee recommendation. A couple hours of research convinced me that it’s a much better choice for me than the Nest, or any of the lesser-known smart thermostats.


Glad that the Ecobee seems to have solved your issue and that you are enjoying. Once you get it tuned to your liking, and there is a lot you can configure, you are only going to want to do more with it.

  • Add additional room sensors for better control and comfort
  • Use Ecobee’s web viewing to get a better idea of your usage dynamics. Or if you want even more useful analytics you can log into https://app.beestat.io/
  • Pull the data for analysis and also “give” the historic data to Sense to improve their HVAC models for your type of system. I did my own analysis for my AC with Sense vs. Ecobee data at the end of the last cooling season back in Oct. 19.
    Sense vs. Ecobee: Round II

I guess I spoke too soon… It’s no longer using the aux heat at the end of every heating cycle, like the previous thermostat was, but it’s still doing it with almost every one, even though I set the Ecobee to never use the heat pump if the outside temperature is above 35°. I guess I’ll have to call a service technician in after all.

Also odd… After connecting Sense to Ecobee, it recognizes my heat pump as A/C. I expected it to separate out heat & A/C, even though it’s the same device, since it has Ecobee’s data to know which mode it’s operating in.

Good luck with the service call. A couple things I discovered after I discovered about my system after installing the Ecobee.

  1. I have forced-air gas heat with central AC compressors and Ecobee did not have ultimate control over the furnace blowers, though I could change some timings and make the blower come on for a minimum amount of time per hour. But ultimately, the controller in the furnace still managed heating and cooling related blower cycles.
  2. I also discovered the my downstairs Ecobee going offline regularly during the winter. It turns out, that this was an indicator of my downstairs furnace being under-aspirated. Not enough supply airflow through the return was triggering the overtemp sensor inside the furnace, and shutting down the furnace controller for a 5 minute cool down - that had been going on regularly for 20 years in my house, since it was built, because previous thermostats (Honeywell and Nest) had backup power sources and we’re hiding this “power down” short cycles. I had the returns enlarged and everything works much better in terms of function and efficiency.

The bottom line is that the controller on HVAC units, furnace and AC, have feedback systems that the thermostats don’t have any say over.

Cool to see the detection of heat pump. I should set your expectations - Sense isn’t reading your Ecobee real time with the integration, it’s only regularly looking at historic data, maybe on a daily basis. That means that when Sense does it’s detection, it is really only looking at the waveform on your both your mains, not at the current reading on your Ecobee, so unless there’s a big difference in what the waveforms look like for heat and cool, Sense will only see a compressor. Watching the the Ecobee mode information in real-time isn’t possible because the Ecobee API only allows data sampling every 5 minutes at most.

One other point - some devices have multiple components - one of my AC units shows up as two Sense components, the compressor and the furnace blower. I don’t merge those two, because in the winter the furnace blower has other components on the heating side (a second startup blower and igniter).


Remember your heat pump will use your aux heat during a defrost cycle. Your thermostat will not be aware of this. This is for comfort. The electronic board in the heat pump will output 24volts to W1Out. W1Out will then activate a set of electric strip.

The controller i have setup at my home desactivate W1Out during unoccupied period (at night or when we’re not home). During occupied period a pwm signal keep the supply temp to 100F during defrost. However you can’t do that with any thermostat on the market.

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I finally got the answer to this question! We had our A/C checkup visit today, and I mentioned this issue to the service tech. He insisted that it was the thermostat until he opened the unit up and said that it’s got a Heat Pump Maximizer installed on it (the furnace and heat pump have both been here since before we bought the house, and without any expertise in this stuff, I had no way of knowing). This device is marketed to both make the heat more comfortable (hotter heat, vs. the cool heat of a heat pump) and more efficient, by kicking in aux heat based on a temperature sensor that sits in the intake fan. I’m having a hard time buying the claim that it’s better at both (efficiency and comfort), and also having a hard time buying that it offers any real benefit on warm days (if it’s even operating as intended). It seems to me that it’s like saying “flooring your car’s accelerator will improve your fuel efficiency because you’ll spend more time driving at a consistent speed” (which is most definitely not true).

Regardless, I had the tech disconnect the Heat Pump Maximizer, and I’ll see in the autumn how it affects the runtime and overall efficiency.


Great find.

Sense should file this one under: Sneaky HVAC energy-hog things to watch and alert for. @JustinAtSense

@ken2 was pointing in that direction but it goes to show that it’s good to know what the full spec on your HVAC system is. They are often very complex setups with sneaky options like defrosters and aux heaters. Feels to me like an HVAC tech with some Sense-as-a-test-tool knowledge could solve these things by becoming more familiar with the signatures.

It speaks volumes to dedicating a Sense to your HVAC! Ground truth knowledge is significantly easier to decrypt without the noise of other devices clouding the picture.


Glad you found it. Yeah, I’d disconnect that in a heartbeat.