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Summer usage. Why is my Vaillant Aerotherm using so much energy on "heating"?

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 3 weeks ago
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 DavidB
(@davidb)
Trusted Member Member
Joined: 3 months ago
Posts: 48
Topic starter 25/04/2026 9:01 am   [#2971]

Hi everyone,  Having had our 3.5 Aerotherm installed Sept 25, only now am I continually operating with heating off.  The following show usage over the past few days.  Can anyone explain why I am using so much “heating” energy when the system is off?  The second screenshot is particularly surprising showing 1kWhr of energy goes into heating to generate 200W of heat - while heating is off?

IMG 2343
IMG 2342

I checked the DHW only information and it shows a COP of 3.27 so the system appears to be using 1 kWhr which is being wasted?  Over the next 6 months, that strikes me quite a lot.


This topic was modified 3 weeks ago by Mars

   
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 AndrewJ
(@andrewj)
Reputable Member Member
Joined: 1 year ago
Posts: 211
25/04/2026 9:52 am  

I don’t know how the Aerotherm works so can’t comment on that.  Looking at the graph, and as long as I haven’t misinterpreted it, it seems to be showing a total usage of 1kW over 6 days plus a few hours today, 7 I think.  That would be 1kW / ((24x6)+7 =0.00662 kWh which is around 7wH.  The second screenshot seems to be showing an efficiency of 2.76 for heating and hot water which, for hot water only, seems reasonable (on the basis that power for “heating” is minuscule.  I expect there is an element of tickover as the system probably has some frost protection built in.  I dare say someone will be along to explain the Aerotherm and why it might be maintaining some heating input.  



   
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 DavidB
(@davidb)
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Joined: 3 months ago
Posts: 48
Topic starter 25/04/2026 11:12 am  

@andrewj Thanks for pointing out my obvious error - as you say 1kW over 5 days so not much. That said, I still don’t understand why any energy is going into heating when it’s off?



   
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JamesPa
 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 4865
25/04/2026 11:14 am  

Posted by: @davidb
↑

 

IMG 2343

 

So the red bars are 'DHW' and the yellow bars 'heating', corresponding to the colours at the bottom.  This shows 1kWh for 'DHW' per day and perhaps 0.15kWh per day for 'heating'.

I dont know what is assigned by the system as 'heating' but it could be several things.  'Tails' on the DHW run, for example - I wouldnt be surprised if the water pump is kept running for a while after the DHW valve shuts to dissipate heat and/or the compressor to run down slowly or lots of other 'endy' effects that it may 'count' as 'heating'.  

Another possibility, depending on where you are, is frost protection.  Here in the south east of england the mornings have been cold enough that I would definitely expect the automated frost protection, which runs unless you disconnect power to the ODU, to kick in.  That would almost certainly be counted as 'heating'.

Finally the electronics will itself consume some energy.  0.2kWh/day is 8W.  That's a reasonable quiescent consumption for modern electronics.  Your house probably runs (much like mine) at 250W total when everything is 'switched off' due to the various electronics that is not completely powered down.  8W is negligible in comparison.  If this is the major concern about your heating system you are doing well!

I did look at my Vaillant for June last year (when frost protection would not have been active).  Like you, I see about 0.2kWh/day assigned to 'heating' and it looks like this is even on days when it does no DHW run.  That suggests rather strongly that its the electronics.

 

 


This post was modified 3 weeks ago 4 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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