Heat Pump starting current

s_gatorator · 2025-01-16T21:27:52Z

There's been a bit of discussion e.g. here and here, and very recently here originally, I think, introduced by @transparent about expecting high starting currents on heat pumps as pressures build up to operating pressure. The interest on the forum in HP starting current particularly relates to correctly sizing an inverter if the HP is running off backup battery storage (though for a grid-tied inverter supplying the HP/house in parallel with grid power, any current beyond the capacity of an inverter is then supplied by the grid) Looking at the manual for our NIBE F2040 HP, it has a specified maximum operating current of 23A (5.5kW) and a starting current of 5A. So it looks like high starting currents may be characteristic of some HPs but not others. Out of interest, I took 1-second readings from a current clamp on our HP starting up today, and it does appear to be a well-controlled rise: this corresponds with this pressure and flow temperature rise: In our discussions over @ivanhoew 's system (Harnitek 12 kw ashp), graphs of operating current are at a useful sample rate (2 per minute) there does not seem to be any sign of turn-on surges, so I'm wondering if I'm either looking at the wrong thing, or there can be very different behaviour between heat pumps, which will need to be understood when considering maximum current handling, particularly for inverters. Interested to hear what others experience and understanding is ..

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Air Source Heat Pumps (ASHPs)

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JohnR

(@johnr)

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25/01/2025 9:07 pm

I caught my Arotherm+ heat pump at the end of a defrost cycle this morning when the OAT was about 0C. I then checked the data provided by the Vaillant control box to see that the compressor power was ramping up steadily by about 1% every few seconds, so no sudden jump to a big load.

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Old_Scientist

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04/02/2025 8:32 pm

Posted by: @s_gatorator

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Posted by: @cathoderay

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More seriously, -7 to +5 degrees is really quite a large range, there must be places in Europe where the OAT is within that range for some if not much of the time. I wonder what happens there?

That's a very good question. I wonder if anyone on the forum has an idea, an interested contact, or experience of living the ASHP in such temperatures

I have seen mentioned two things that may help alleviate the effects of defrost cycles.

The first is having sufficiently large thermal mass within the system such that the amount of energy used, or taken out of the system for the defrost doesn't have a significant impact and is easily recoverable. The larger the system volume, the less the flow temps will drop during a defrost cycle.

The second is the size of the heat pump and how hard it is having to work. An otherwise oversized unit that is not working as hard may frost up less frequently? Similarly, a large two fan unit with double the surface area would presumably take longer to frost up than a unit half it's size delivering the same amount of heat? Sounds reasonable although I have no evidence to substantiate. Having an oversized heat pump also means the system has capacity to recover quicker from the inevitable drop in flow temperature, and without the system working flat out.

This post was modified 10 months ago by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.

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MPHB

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05/02/2025 4:31 pm

Bit late to the party maybe, but my Midea clone heatpump slowly raps up its electricity use at the beginning of a cycle. I have a designated Eastron SDM230 smart meter that I tend to - not very smart - look at every now an then. The HP draws 17W when standby and when the compressor is switched on the power gradually (50W steps give or take) goes to the required power demand (up to 2300W for my 10kW HP). What I have not yet witnessed though, is how power ramps up when the internal backup heater is switched on and I suspect that to me just a 3000W jump.

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