Vaillant Arotherm Owners Thread

Hi all,

Has anyone seen this power signature on their Arotherm (with assocaited instant COP change)? It has been happening for a little while now, a sawtooth-like pattern during periods of otherwise steady state running. The sawtooth nature suggests something intentional by the compressor rather than an adaptation to environmental factors. I’m running WC+active.


Looks very weird. I’ve never seen such fast power rises on my unit. Also there seems to be zero impact on heat output which makes me doubt that this is the compressor. Are you sure the power sensors are working ok? Weather compensation should only affect target flow temperature.

Is there anything else on the electrical circuit that is monitored? The step looks like maybe 100w, some other device, secondary circulation pump or something else inadvertently connected to the same supply? Given it is not impacting the heat generated, maybe it is not the heatpump consuming it? Does the time of activation give any clues? morning/evening - occupancy?

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I feel so stupid. I lashed a supplementary electric UFH mat to the feed for one of the manifolds and completely forgot I did it. Thanks both!


Maybe my ignorance, however I noticed few translation errors in the vaillant software: the cylinder offset charging temperature might be the hysterisis. In other words: if it is set at 10K, the boiler starts to be recharged when boiler temp is target temp - 10.

Yeah, I wasn’t entirely sure which was which but I definitely got it the right way around now. However, DHW charging happens at a fixed offset that is not impacted by the settings on the controller.

Just wondering what others out there who have a few seasons’ experience do about relatively heavy houses (mostly screed UFH, solid wall) and control strategy during warmer months. We seem to be approaching the time of year when the heating is backing off; I’m running WC in any case with a cut-off at OT over 17. However during summer proper the OT will regularly go below that at night, is it worth ‘trusting the WC’ and having the HP come on just for a few hours in the night to do not much to the slab (and burn electricity for no meaningful return), or better just to turn the whole heating off for the summer, on the understanding that the odd cold day in July will have to be taken on the chin. The house has modest solar gains but arguably not in the parts where it would most be appreciated.

My heating is typically off from May to September. The few times where temperature drops below the threshold don’t really hurt in my opinion. Even very modest solar gains are quite sufficient to overcome the night-time temperature drops for me. I probably would opt for a weather forecast based approach. If there’s a longer cold period upcoming just switch the heating back on via the app. Just a single cold night probably doesn’t hurt, especially in a quite solid house.

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Similar situation, I have just run ours for the odd day during April, manually switched off in the app. the rest of the time. I guess it depends on the thermal mass, losses & solar gain.

When I have run it, I have done so during the warmest part of the day (for efficiency), on the basis that it does not really make any appreciable immediate difference to the house temperature, the slab acts as a buffer. Building is fairly well sealed with MVHR, not sure how much impact this has.

Defrost behaviour… Is there any way to modify this, am I missing something in the setup?
Our unit just comes on and runs for the duration the system is below the trigger temperature. It does not seem to be doing anything clever, like just maintaining its own temperature.

The app. says it is not heating, but the system is pushing about 3kW an hour in to the building. Not really a problem, but not necessary either. The defrost strategy could just pump warm water from the building to maintain the pump temperature and then only fire up if the return falls below a predetermined temperature.

I have an issue in that the radiators get noisy when the hot water cycle ends. This sometimes wakes us up as we have the hot water cycle configured to run at night when we have cheap rate electricity.

The problem occurs when the hot water cycle ends and a bolus of hot water hits the radiators causing loud expansion noises. I thought I’d fixed this by extending the time between the ending of the hot water cycle and start of the heating cycle. Unfortunately, this appears not to be the case. While there is a 60 min delay between the HW cycle ending and the heating coming on (and the setback mode is set to ECO so the heating should be OFF), the 2-port valve switches flow and directs water to the rads as soon the hot water gets to the set temperature. This happens at varying times depending upon how long it takes for the water to get to temperature. It appears that when the HW cycle is satisfied within its allocated time period, it switches to heating regardless of the latter settings.

Anyone got any ideas how to prevent this? Thanks.

I dug into this on my system and think I can replicate what happens to you. Unfortunately it seems there is not much one can do.

Apparently the process for switching back from DHW mode to “off” is to first switch the three-way-valve from DHW to heating and only then turn off the circulation pump. Hence, there is a brief period where the hot water does get into the heating pipes even when there is no demand. For me, the length of pipes between heat pump and the radiators is long enough so this bolus does not reach the radiators unless the pump is being kept on due to heating demand.

Have a look at the end of this DHW cycle when there was no heating demand afterwards. The PT100 sensor is attached to the pipe going into the heating circuit and you can clearly see the temperature shoot up at the end of the cycle, indicating there was some leftover flow into the heating pipes. The object temperature sensor is an NCIR sensor at one of my radiators - you can see the bolus didn’t reach that far (TRV was 100% open so it could have reached there). For you, the pipes are probably short enough so the bolus reaches the radiators.

Unfortunately I don’t see how to alleviate this apart from TRVs on the affected radiators. It seems this is an inherent problem with the DHW switchover.

Hi Andre

Thanks for the reply, much appreciated. I thought that this might be the case so I’ll have to think of a plan B. I’ll contact the installler just in case they have some ideas.