I’d love to know what is going on here. Noticed it a bit more recently, about 15 minutes before a hot water cycle, there’s a little spike in the circulation pump for 30 seconds… Only noticed it because the Mrs said the radiator in the bathroom (smallest in the house) was warm… I almost couldn’t believe that the pump running for barely a minute would do this.
the pump operates every 23-24 hours for 30 seconds to prevent seizing when not in operation for 23-24 hours
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Ah great to know… I’m guessing that’s a setting somewhere…
I’ve seen no setting for that
Same for my gaz boiler
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Thanks - now I’m looking back at recent days and weeks, on the days after a DHW cycle that only runs for 45minutes, a pump does the 24 hour anti-seize run. Whereas on the days when it goes for over an hour, the following day doesn’t have one.
German engeneering 
Very clever
I have let my heatpump decide for itself as to whether it needs to run or not over the summer. It has kicked in the odd day here and there, which is better than the years of the gas boiler and having the house feel slightly chilly the very odd day. It’s great now that it is all just automatic
Periodically but almost randomly my aroTHERM+ 5kW seems to cycle or circulate briefly (perhaps 20 mins) then stop. It’s been doing this for months or more. No errors being reported. I have HA logging data from ebusd which picks up heat pump power use up to 50W tapering to 25W then off during these periods of usually 1 cycle/day but three in last 24 hours. Vaillant flow/return temps climb from ~25º to ~50º swiftly and then taper off over hours. I have the heating/cooling and DHW all switched off and my DHW is coming via solar PV charged SunAmp UniQ HW 12+i- VT heat battery. I think the flow/return increasing is coming from a small amount of hot water being pushed out of the SunAmp when the cycling begins. I have various Shelly temp sensors on my SunAmp system which behave exactly as though the heat pump is running a little circulation cycle. I don’t have a DHW cylinder and don’t run legionnaire cycles. I’ve spoken to Vaillant renewables tech support who didn’t really understand my system or issue. Wondering if anyone else has seen this phenomenon or knows what might be happening? Have charts I can share if need be.
20 min would seem long, but for a few seconds each day the circulation pump runs to prevent it getting stuck. 50W sounds like active standby + pump, then 25W is approximately what the heat pump uses when it was recently active. After a few hours.mine drops to 10W in “passive” standby.
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Thx Andre 20 mins was a guess tbh and I just looked closely it’s more like 5-7 mins peaking briefly at 49kW and settling at around 11kW passive. That explains it perfectly and I suppose this becomes unnecessary in winter when it runs daily for longer periods.
It’s been close to 3 years now since i’ve owned my 5kW Arotherm Heat Pump and I’ve written quite a few informational and performance related articles over on my Energy Stats UK website.
Because the info is all in different articles it can be hard to find what you need.
So i’ve created an Arotherm landing page pointing you towards all the important articles in one place.
Hopefully this can help people quickly find what they are looking for.
Let me know if you think there is anything missing that you’d like to know more about.
Hopefully a good starting place article for homeowners and installers alike.
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Thanks Mick.
I’m planning on getting an Arotherm Plus 5kw unit. I was wondering if anyone has put the unit against the wall of a habitable room and what the noise levels are like – any low frequency vibrations for instance.
The terraced houses on our street are a somewhat unusual offset layout, so the best place for the unit is against the neighbouring property’s wall. It’s a 1960s uninsulated cavity wall; no windows in the wall but they do have an air brick at about 2.3m which ventilates directly into their downstairs room. Would they be disturbed by noise? Would they notice it at all?
I have seen a new 7kw in action and it seems very quiet even at full throttle doing the hot water. I did notice some low frequency noise as it was starting up, but it even that seemed hard to detect. But I guess there could be resonance in the wall/airbrick or ground vibration.
Grateful for any reports 
Hi Gary, just wondering if you found the hoses made a difference to noise transmission? I’m thinking of trying the same.
Hi everyone,
First time posting on here and just wanting to get some general thoughts on an issue I have, from people that will hopefully understand what is going on with my Vaillant system better than I do!
My setup is an Arotherm+ 5kW heat pump unit, running at 45 degree flow temp for -3 outside temp. Using a heat curve of 0.7 and Active room temperature modulation. System installed in June 2024, predicted overall minimum operating SCOP of 3.6.
150 litre Unistor unvented DHW tank being heated twice a day on Balanced mode.
No Open Energy Monitor system in place at this point.
Tracking the system using data from the Vaillant app, the system has supposedly achieved an overall SCOP of just 2.88 since install, with maximum daily heating COPs coming out at no more than 4 and DHW maxing out at around 3 during milder outdoor temperatures. Any cycling that happens on the unit appears normal, seemingly no functional issue in that regard.
This low SCOP has been in discussion with my installer and Vaillant for a few months now, such that my installer came and swapped the flow and return temperature sensors on the heat pump last week, as there was suspicion that one of them may have been faulty from install and causing the SCOP value to be underestimated. Almost instantly I have seen COP values in excess of 6 for heating and around 5 for DHW over the past few days.
I guess my question is, is it plausible that a potentially faulty sensor being switched now be over-reporting efficiency values and making the system appear more efficient than it actually is?
Any other thoughts, etc on this will be gratefully received!
Thanks, Pete
Most definitely. Those sensors are pretty shitty and can be off by a degree or more in either direction. You need to get a reading when your system is pumping water but the compressor bring off. Check my post history to see the journey I took to fix these sensors.
Thanks André, just been having a quick read through your experiences and I wish I’d found this forum before now!
So, if I am understanding correctly what I’ve been reading, when the compressor is off but the pump is running, I should ideally see identical flow and return temperatures? In that situation, if the flow temp remains higher than the return temp, then the COP will get reported as higher than actual, and if the return temp is higher than the flow temp, then the COP will be reported as lower than actual?
Have I understood that correctly? If so then I guess now is the ideal time of year to be looking at this, because obviously I’m getting long runs of no heating through the day at the moment. Should be easy to compare this week with last week, either side of the sensors being swapped around.
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Just as a follow up, my installer has looked at my graphs and confirmed that prior to the swap the return temp was 0.5 degrees higher than flow, and now it’s the flow that is 0.5 degrees higher than the return. And I’ve gone from having an underestimated COP to an overestimated COP, which I think conforms the problem of one of the sensors being faulty?
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You’re entirely correct in your assessment. I don’t think it qualifies as a “defect”, just poor tolerances. If you can get it fixed again that would still be great.
Hi. Unfortunately, my installer stopped replying after agreeing to take a look.
I’m going to pick it up again when the weather gets colder. I don’t really have an issue when the weather is warm as the compressor only ramps up to around 80 rps on the hot water run.