What is my heat pump doing, that uses so much electricity in such an erratic way?

Question

TL;DR: I'm currently in a fight with my heat pump, as its energy consumption seems way out of line. I want to know what it is doing, that is causing it to use so much electricity even when the outside unit isn't doing any heating at all.

System: Alpha Innotec LWD 50A/SX HTD-S. It's a combination of an outside unit that does the actual heating, and an inside unit, where the inside part also contains a large buffer vessel for hot water and a Luxtronik 2.0 control panel. It uses R290 as a coolant. I'm not exactly sure when it was installed (the apartment was newly built) but it's been in use since September 2021. The heating of the apartment is done with underfloor heating with hydronic elements.

It is combined with an Alpha Innotec RBE+ thermostat, which has a touch screen that is 'always on'.

The heat pump has several settings for heat/hot water, two of which are relevant: 'off', and 'automatic', the latter meaning you can set scheduled times, so e.g. it won't start heating up hot water in the middle of the night after an evening shower, or turn on the 'night' mode for the heating. Night mode for the heating means it will lower the "heating curve" by 3 degrees Celsius. Currently, I've scheduled day mode from 8 AM to 8 PM, and night mode from 8 PM to 8 AM the next day.

Monitoring: I'm currently monitoring my energy usage using a site/app called 'mijnenergiebundel' which came with the apartment. This software splits my energy usage into two categories: 'building related', which only reports any electricity usage when the outside unit of the heat pump is working, and 'domestic', which reports everything else, including the electricity usage of the inside unit, like the Luxtronik and RBE+.

I can also monitor when the outside unit of the heat pump turned off after being used, as the Luxtronik allows me to see the last five 'afschakelingen' (shut downs).

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Now, as long as I have the heating set to 'off', my energy usage looks something like this, a pretty constant usage of 0.05/0.06 kWh (the spike at 8 AM was me). Cutting all power to the inside unit of the heat pump makes this go down by 0.01/0.02 kWh, I'm guessing that's because the Luxtronik and RBE+ are 'always on' and cutting power means they're off too.

As soon as I put the heating to 'automatic' though, my energy usage looks much more erratic, like this - the highest peak there is 0.12 kWh, the two lowest at the end were immediately after the hot water was heated up, and are 'normal' at 0.05 kWh. This was an office day, so with the exception of 15-20 minutes of lamplight to get up in the morning, there was no other energy usage. I checked when I came home, and I did also not forget to turn off the Wi-Fi, leave a light on, or the fridge door open. The outside unit didn't turn on except for one time around 1 PM to heat up the hot water buffer.

So, on average, with the heating turned to 'automatic', the inside unit of my heat pump seems to be using 0.04 kWh extra each hour, so about 350 kWh a year. I'd really like to cut all that out, because with current energy prices that means about 200 euros I can save each year.

What could the inside unit of my heat pump have been doing that made it use so much electricity, so erratically? Is there anything I can do to make it behave, and go back to having just 0.05/0.06 kWh of standby consumption, while having the heating set to the 'automatic' schedule?

Answer 1

The heating of the apartment is done with underfloor heating with hydronic elements.

Then you have a circulation pump. That probably explains the "high" standby power use, as 50 W is otherwise excessive for what is a modern high efficiency unit.

I skimmed the manual for your unit and it says the pump must be controlled by the unit and never shut off externally. However, the manual suggests the duty cycle of the pump can be configured. The default is listed as 5 minutes on / 5 minutes off. From what I understand, you can go all the way to 1 minute on / 120 minutes off.

I haven't read through the whole thing and it's not clear, locally, if this setting applies only when heat is required or all the time. I suggest you give the manual a proper read first. If the setting is indeed all the time, you probably won't want to turn this too low or your heating will not work correctly. But you seem to have a good grasp on experimentation and taking measurements, so I think you can figure out what's optimal for your use.

Answer 2

As soon as I put the heating to 'automatic' though, my energy usage looks much more erratic, like this...

When the thermostat is set to 'automatic', it's pumping water through the floors as necessary to maintain temperature in the apartment. Why wouldn't you expect some sort of electricity usage for that? The "as necessary" portion is why the usage seems "erratic". This is the way all heating systems work.

If we were to presume that your lower graph were based only on heating use, then it still makes sense and doesn't seem all that erratic to me. It takes more to heat the apartment overnight because it's colder outside. Therefore (no matter how much insulation) there is a greater thermal difference between outside & inside and more heat is lost to the outside, requiring more heat to be produced inside.

During the day, usage may vary as the outside temperature varies, and even as cloud cover changes because it makes the air temp change.

If you really want to eliminate this usage, then you'll need to set the thermostat to 'off' and deal with the fact that your apartment will be cooler when you turn it back on than it was when you turned it 'off'.