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

While Sense so far has done a pretty poor job of identifying any power consuming devices correctly, I’m wondering if it may have helped me discover a problem with my heat pump. I’ve looked around on the web, and I can’t find anything indicating if this is expected behavior or not, but it certainly surprised me.

Our heat pump seems to use about 9-10kW when it’s running… but for 1-5 minutes before shutting off, the power consumption jumps WAY up to 19-20kW (these figures are absolute, so in reality, about 0.5-1kW less than that is really attributable to the heat pump).

These spikes are consistent, though their duration is not. The screen shot below shows a spike that’s less than 2 minutes long, but as I scroll back in my timeline, I see plenty of other times where it’s 2-5 minutes, and at least one recent instance where it was in the 6-7 minute range. It makes me suspect that, for some reason, the home’s heating system is switching into aux (resistive heating) mode. I live in Virginia, and temperatures have ranged from 35°F on cold nights to 75°F on the warmest recent days, so it’s never been so cold that the heat pump should ever need to fall back to aux/resistive mode in order to keep the house warm.

I’ve never monitored my power usage with this fine a temporal resolution before, so while I suspect this isn’t normal, I don’t know for sure. If this is not normal, does it indicate a problem with my thermostat (a Honeywell 7-day programmable unit), my furnace, or my heat pump?

Here’s a screen shot of the most recent instance:

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Does it spike like this every time it runs? It could be the defrost cycle running.

@ken2 - I think it happens every time. There are a handful of other 3-5kW plateaus that don’t include this spike, but I can’t say for sure what they are, and they’re a small minority compared to heat pump profiles that look like the screen shot above.

The screen shot above was from just before Noon today, when it was close to 60°F outside. I wouldn’t expect a defrost cycle to run when it’s that warm.

20kW is a big defroster!

You should be able to look at the specs on your heat pump and narrow things down.

Depends how big the aux heat strips are.

Do you have a breaker for the auxiliary heat strips? If so, flip the breaker off and watch it cycle a run. The only thing that would cause a spike that high would be the aux heat strips which are probably 20 kW in size.

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@ken2 - Our breaker box wasn’t very well-labeled when we moved in, and there are two unlabeled double-width (240V) breakers, one 30A, and one 60A. I’m guessing they both power the furnace’s resistive heating coils/strips. I’ve turn them off to see if the heat pump still operates, but without the spike at the end. Heat’s been off all afternoon (it’s 66°F and sunny, with the thermostat set to 67°F), but it should go on again this evening.

There are two other breakers, 40A & 20A, labeled “Heat Pump,” so hopefully those are responsible for the steady-ish 4-5kW consumption during most of the heating cycle (I suspect it uses double that in the winter), and the 60A & 30A circuits are responsible for the 15kW spike on top of that. That’s the only thing I can figure, since I’d need a 100A circuit to sustain 19-20kW for, in some cases, multiple minutes.

Assuming it is the Aux heat (I can’t imagine what else it would be), I don’t suppose you know if it’d be more likely a problem with the furnace itself or the programmable thermostat? I can easily replace the thermostat on my own, but I’d need to call someone in for the furnace (after society is no longer sheltering in place, since it’s not an emergency).

Did the thermostat go off when you tripped the breakers? If so the heat pump won’t come on. The bigger breaker is probably the aux heat strips. Just make sure the thermostat is on.

It could be the programmable thermostat, it’s be known to cause the issue. It could ether be bad or programmed wrong.

My brother owns a HVAC company so I know some stuff but not a repair tech.

I have a new heat pump system and will see the gas furnace kick in some time during the defrost cycle. It is sort of a comfort thingy; so your not blasting ice cold air into the house.

@ken2 - No, I wouldn’t expect the thermostat itself to be connected to any of the 240V breakers, and thankfully, it didn’t turn off when I tripped both of the unlabeled breakers. The heat has continued to work fine with those circuits disabled, and I haven’t seen any of the 20kW power spikes. Especially since we’re in spring, rapidly heading toward summer, I shouldn’t need the Aux heat at all for a long time, so I should be able to just leave those powered off until I have a chance to find a proper resolution (new thermostat or furnace service appointment).

I can program the thermostat to specific temperatures at specific times, and I can force it to only use Aux heat, but I don’t see anything that I could set that would cause this behavior, so I don’t think it’s programmed wrong. Either it has gone bad, or something is wrong with the furnace to cause this.

@babblefish26 - Since this spike is at the end of the heating cycle, not the beginning, I don’t understand why it would be used to prevent a blast of cold air from coming in. Also, I don’t think the 20kW usage penalty is worth it for not having 65°F outside air blowing in. That might be worthwhile in early February, but not in April. :slight_smile:

The thermostat gets its low voltage power from the air handler/furnace. So if it gets turned off the thermostat goes off.

A service call might be in order.

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Check out this YOUTUBE video… It has a good description of the defrost cycle.
The newer HAVC Heat Pump systems utilize a communicating thermostat (controller); no longer simple on/off. The defrost cycle is actually air-conditioning (cooling) mode. Sometimes the systems will call for heat so you don’t feel a blast of cold air coming out of the ducts.

@ken2 - Hmm… I think I might’ve been wrong. A bit after writing that reply, I noticed that the house was 65°, and the thermostat was trying to use Aux heat to get back up to 69°. I think I might’ve disabled the furnace entirely, not just the heating coils. I think the house might’ve just held enough heat overnight that I didn’t notice it was slowly cooling down. I’ve tried turning the 30A circuit back on and leaving the 60A circuit off, but neither the furnace fan nor the heat pump turned back on (I waited 15 minutes or so, since I know sometimes HVAC systems won’t cycle immediately), even though the thermostat was commanding heat. I had to turn the 60A circuit back on as well, so I guess I’m stuck with the expensive 20kW spike until I replace the thermostat or call in a service tech (after non-essential services are back up and running in the US).

@babblefish26 - I don’t believe that can explain what I’m observing. That video specifically talks about the cycle being used “when it’s cold out” (he said that twice within the first minute). Unless there’s something very wrong with the heat pump’s own thermostat (or whatever sensor it uses to estimate frost buildup), it shouldn’t need to use 240V @ 80-83A for 5 minutes to defrost anything when it’s 60-65°F out and the heat only needs to run briefly every so often (and if the “heat pump” breakers are properly labeled, which they might not be, it’s impossible for the heat pump’s defroster cycle to pull that much power, because they only total 60A across both of them).

Yeah… I think it’s the thermostat. I was just looking at the Sense timeline and saw that the house is using 20kW again. I ran over to the thermostat, and even though the house is AT the set temperature, it’s displaying “Aux Heat.” I can force aux heat on, but I can’t force it off, so I can’t tell the thermostat to stop using aux heat. I’ll have to order a replacement and install it, to see if that resolves this.

Failing that… service call!

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Sounds like you might of figured it out. What kind of thermostat do you have?

Installer should have lock out aux heat a fix outdoor temp setpoint. Time to tell us your thermostat model.

As I said in the OP, I’ve got a Honeywell 7-day programmable thermostat. There’s no model name on the unit, but if I pull it off the wall, the sticker inside the unit has a box with “RTH75000D1031” in it. It doesn’t specify whether that’s a model number or serial number, and a web search for “Honeywell RTH75000D1031” doesn’t return a single hit.

Are you stuck with a Honeywell thermostat due to specific features of your heat pump ? Or could you replace with something like an Ecobee that can give you more historic data about operation of your system ?

ps: Sense also has an integration with Ecobee so they can read your historic data and improve HVAC models for the devices providing that data.

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Model is RTH7500. You could search « rth7500 installation manual filetype:pdf ».
This thermostat does not have configurable aux heat behavior. I always suggest to use a thermostat with an outdoor sensor ( or wifi compatible that can retrieve your location outdoor temperature ). That way you can lock out aux heat when your heatpump is able to heat your home without the help of aux heat. Try searching « heat pump balance point » for more information.


@kevin1 - No, I’m not stuck with a Honeywell thermostat. It’s just what Home Depot was selling when we moved into this house 9 years ago. The previous owners had a simple single-setting thermostat, and I wanted something more efficient. The heat pump itself is Amana, and the furnace has no visible branding, but there’s a small sticker that says “Goodman Company, Houston, Texas” which appears to be Amana’s parent company. I expect that any brand thermostat would work just as well (or better, given the problem Sense helped me find!).

@HVAC_Marc - I dug the operating manual (“RTH7400/RTH7500 Series”) out of my household manuals storage pile, but got sidetracked before coming back to reply again, and you got to it first. :slight_smile: Thanks for the tips on specific features to look for in a replacement thermostat. The thermostat is almost dead-center in the house, so wiring it to an outside thermal probe is probably more trouble than it’s worth given the availability of wifi-connected devices.

I had not seriously considered something like a Nest or Ecobee, since their primary selling point seemed to be that they’re programmable thermostats for people who are too lazy to program a thermostat. But if they will actually operate the heat pump in a more efficient manner than a manually-programmed thermostat, that’s definitely worth the extra cost and the power draw of having another network connected device.