heat pump wattage curves

We have an older commercial heat pump package unit. Just started using the heat pump mode for the first time and noticed something quite odd on the sense charts. Tracking the wattage of consumption while running shows a slowly increasing wattage over several minutes, that does eventually seem to level off. The AC mode does not show anything like this. Wondering if anyone else sees this ?
The screen shot below shows two such cycles. No other devices came on during the heat pump run.

that almost looks like your emergency heat strips are being used…? That’s a lot of juice.

Here is my heat pump in cooling mode:

And in heating mode:

Thanks for the reply!

It is not the backup heat strips, I tested those and they click on sequentially with a full and instant 5KW each. This is a gradual increase in wattage.

I do notice that in your chart, the heat mode one also shows a gradually increasing wattage while the cooling mode stays flat. certainly not as pronounced as mine, but similiar pattern. There must be something different about running in heat mode over cooling mode.

Subsequent tracking shows that the norm is the second trace in the pic listed above. The first one, I can not explain the continued rise, but after that all the rest look more like the second one , and like yours, that climbs and flattens out after a while.

An interesting point is, we would never see this without this monitor!

Interesting datapoint! A post down below shows a similiar pattern, and the determination was made that the compressor motor was seizing up!

But if that is the case, I dont understand why it would affect heat mode but not cool mode. The compressor should not know the difference as per my understanding. Therefore, it almost seems more like a problem in the reversing valve perhaps not opening all the way and restricting flow?

In our case, the heat pump has not been activated for a couple years (due to a thermostat issue). So I suppose it could be “sticky”.

My guess would be… that your head pressure is slowly increasing causing the compressor to pull more and more amps until it either hits the high pressure switch or the compressor overheats and shuts off. It looks like it’s pulling more than your startup amps when it stops.

Generally if you have a blockage within the line/ dryer or faulty TXV/ piston/ reversing valve … it does that in like 15 seconds with r410 and maybe a min or 2 with r22.

If it cools ok but does takes 5-10 mins to do this in your heat cycle… I would start looking at restrictions in your air flow such as a dirty indoor coil. The air flow across the indoor unit is very important in heat mode, the indoor unit becomes your condenser and has to dissipate all the heat on such a small surface area compared to when it’s using the outdoor unit. When it can’t get rid of the heat… then as it loops thru your system, you get more heat and your vapor becomes saturated which = really high head pressures that started normal and slowly raised


I think @ccook has zeroed in on the most likely scenario. I say this after having a similar issue with my Carrier heat pump in just the last day or so… Cooling mode still works fine, but reverse cycle heat mode fails (the outside unit shuts down) after a few minutes.

I had a very experienced HVAC tech come out this morning, and he diagnosed a failed TXV (expansion) valve here, which is mounted at the inside coils within my air handler unit. Witnessed normal pressures and current draw in cool mode, but the high side pressure in heating mode got up to ~500 psi before he shut it off! Obviously, that high of a head pressure will pull a lot of current, explaining the thermal or overcurrent shutdowns I reported while in heat mode. At least he said it proves that my compressor is still running strong after 11 years! He also observed that the coils still looked great, so he’s recommending we just replace the expansion valve.

1 Like

There’s some great feedback in here @dsalch.

We’ve actually had a couple reported instances of this same issue occurring in homes of Sense employees. Data Science mentioned that a tell tale sign of this (as @ccook did a great job of explaining) is the triangular rise in the wattage consumed by the AC, aligning with what @ccook noted as the compressor working harder and harder before triggering an overheat or high-pressure switch.


Yes, this is a great discussion!

So what has been said makes complete sense. Interestingly, this wattage draw thing is a valuable diagnosis tool!

In my case, the cool works as expected, the heat only sometimes keeps rising. Most runs it rises a bit and then flattens out. This seems to mean that there is something causing a blockage only sometimes, which is why I suspect the valve itself.

For the last few years, this heat pump has run in backup heat only, becuaee someone installed a nonheatpump thermostat. So this is the first time in unknown years that the reversing valve has been worked.

Another tidbit, this is a package unit, meaning both sides are in the same case (as in commercial rooftop style).

I’ll keep an eye as winter approaches. But it would be AWESOME if sense could alert me when this bad pattern happens!

The reversing valve is a mechanical valve that operates like a seloniod . The center of the valve has multiple holes, when it slides to the heat mode or even sometimes the valve actually on the heat mode with no voltage and when you turn the AC on, it moves over when voltage is applied. It is highly unlikely that it can get stuck in a position where some Freon is partially going thru. Once it started fluid… the pressure would help the valve line up. Sometimes it will get stuck in one position or the other. Smacking them with a wrench or hammer is a bad idea since the outside is copper and it can be dented easily causing it to actually get stuck in the middle None the less, when the valve moves it aligns several holes and it actually reverses the high side and suction side. TVX valves / capillary tubes and dryers are normally where a blockage is found… meaning cooling would also be effected.

The saturation of the freon from being overheated makes much more sense for a slowly rising amp draw than a blockage or a malfunctioning valve. Some HVAC techs would also miss diagnose this as a “locking up compressor.” The reason being is because the compressor which normally compresses a vapor…once the Freon can’t cool down/ change back into a vapor, is now trying to compress a liquid which makes the compressor appear to be locking up.

Here is what it takes for an r22 unit to have a malfunction from a blockage of the coil in cooling mode. This one the line sets were freezing over in about 20 mins. Keep in mind this unit was still working up to a week or 2 before I took this apart. It was actually still cooling for about 20 mins

Here is another r22 unit (heatpump) that within 5-6 mins of a heat cycle was tripping the thermal overload which is like a circuit breaker build into the compressor. Once it cools down it reconnects and the compressor was kicking back in. Cleaning this coil … 100% fixed this unit. This unit worked fine in cooling mode, and in heat mode the air from the vent would get hot / cold and the fan would be running inside but the unit outside was off sometimes.

1 Like

Wow. That is amazing to see such slight blockage cause major problems! Thanks for the pics, I never would have thought.

Last summer we did a thorough cleaning of this unit, and there was some blockage on the edges that was too hard to get to (again, package unit). So that’s a worry now.

But wouldn’t that provide a more consistent problem? In my case it will vary dramatically from one run to another just minutes later. As you can see in the trace above one run had steadily increasing wattage until the unit cut off, but only a few minutes later it came back on and ran with only a slight increase that leveled off. How could blockage of airflow be so inconsistent?

Fascinating discussion, thanks so much for joining in!

1 Like

There are a lot of variables such as humidity, outdoor temps, and your freon starting temp. Let say the temp of the freon is 130 deg after its first loop coming out of the compressor and when it returns to be “recompresses” its 120… When it its recompressed, 133, then 123 in, 135 out, 125in, 137 out. So if its cooler inside… it can actually get rid of the heat better because the air going in is cooler. If its warmer outside its absorbing more heat from outside as well (as it should). As the freon gets hotter and hotter it also becomes more saturated which means the freon is still part liquid as its recompressed… each cycle being more and more liquid. More liquid (saturation) = More load… R410 this is more temperamental as far as saturation but r22 still works the same way.

The reason that it not overheat the first 2 cycles is the sun isn’t hitting the outdoor unit or its dropped a degree, but even the “working” cycle that you posted its still raising.

Here is a screen shot of my heatpump running thru a cycle, …
5:43am schedule temp changes from 69deg (nighttime) to 73deg (daytime while home)
Compressor/ outdoor and indoor fan start ~3k watts and drop to running… you can see mine has a slight increase for the first minute as the entire system warms up. It then is a flat line until the thermostat has brought the indoor temp up to 73deg… My indoor fan actually runs for another min 430watts, but sense see it as “other.”

This shows up as my meter for “heatpump” … flat

Here is my total meter during it cycling off time

Also some people are buying these higher MERV rated filters… Some of these will do the exact some thing (restrict air)… Especially if you have a 1’ filter.

To actually clean your indoor coil, I would use completely clean out a shopvac, put the hose on the inside to coil, then use an air compressor to blow backward thru the coils. If you have a flashlight on the inside… you will see more light as you run the air nozzle over the fins. Then look inside your vac to see how much you got out.


Thanks everyone for the valuable input. Here’s an update:

Remember that this is a commercial package unit. 3.5 ton.

Using sense, I have discovered that the blower fan is pulling about half the wattage it should according to the paper inside the unit. The paper specifies a 1hp motors pulling 5 amps at 230 volts. Sense only shows 600 watts when running. The motor is a replacement, but mounted I can’t see the label.

The heat pump is working now, but pulls about 1000 watts more than the label says (according to sense) when the wattage curve flattens out.

So I find myself wondering if the motor was replaced with a smaller one, resulting in lower wattage and less airflow, which is causing the lack of flow over the coils and therefore higher pressure and amps at the compressor.


But lower wattage would mean lower rpm, right? I have no way to test rpm.

Interesting note… Sense has been invaluable in studying the system to see the electric usage over time.

1 Like