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Help with heat pump sizing

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 4 weeks ago
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 amin
(@amin)
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Joined: 4 weeks ago
Posts: 2
Topic starter 14/12/2025 11:33 am  

I’m looking to install a heat pump but confused on how to size it and what units work best.

My calculated heat loss is 14kw (8kw fabric and 6kw ventilation based on 0.7 air changes). It’s an Edwardian house that we are refurbishing and extending and will end up around 380sqm. Design temp is -3 based on being in London.

I’ve been looking at mixture of units but I am confused by the heating output figures. Do I look at A7W35? Surely that doesn’t work at design temps of -3 where defrost is likely to bite? Also it seems as though A7W35 doesn’t match the quoted kw rating of the unit. I assume I’m looking at the wrong thing?

The units I’m considering/recommended by installers:
Steibel Eltron WCL AC25 has A7W35 of~8kw
Vailant arotherm 12kw has A7W35 of~8kw
Samsung EHS 16kw has A7W35 16
Grant Arona 15.5kw A7W35 16.6kw

Would appreciate a bit of help - thanks!



   
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 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 3888
14/12/2025 1:45 pm  

Posted by: @amin
↑

I’m looking to install a heat pump but confused on how to size it and what units work best.

My calculated heat loss is 14kw (8kw fabric and 6kw ventilation based on 0.7 air changes). It’s an Edwardian house that we are refurbishing and extending and will end up around 380sqm. Design temp is -3 based on being in London.

I’ve been looking at mixture of units but I am confused by the heating output figures. Do I look at A7W35? Surely that doesn’t work at design temps of -3 where defrost is likely to bite? Also it seems as though A7W35 doesn’t match the quoted kw rating of the unit. I assume I’m looking at the wrong thing?

The units I’m considering/recommended by installers:
Steibel Eltron WCL AC25 has A7W35 of~8kw
Vailant arotherm 12kw has A7W35 of~8kw
Samsung EHS 16kw has A7W35 16
Grant Arona 15.5kw A7W35 16.6kw

Would appreciate a bit of help - thanks!

Unfortunately you will almost certainly need to go a layer deeper and look at the detailed specs (which are not always easy to find) to work out what the heat pump will do at your design flow temp and design air temp.  Some of your figures are odd though eg Vaillant Arotherm 12kw has A7W35 of~8kw is incorrect see attached (but beware that Vaillant are allegedly not good at accounting for defrost so you may have to derate a bit around OAT0, particularly in the parts of London where its damp.

Sorry but there is no short circuit to this other than relying on the installer.  Personally in your situation I would do the latter to give you cover, but check their workings

arotherm-plus-spec-sheet-1892564.pdf
kl-06-e2-verze-01-18012023-2564719.pdf

 


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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 amin
(@amin)
New Member Member
Joined: 4 weeks ago
Posts: 2
Topic starter 14/12/2025 4:08 pm  

For the Vailant I’ve been looking at the values from pg49 of the operation manual attached and the page here

This seems different to the table in the doc you shared so I assume each is measuring something different?

I’m okay to rely on an installer but I also want to be able to verify and making sure I get something up to the task. 

 

 

arotherm-plus-vwl-105-125-a-s2-installation-operation-manual-0020330792-03-2806940.pdf

 


This post was modified 4 weeks ago by amin
This post was modified 4 weeks ago by JamesPa

   
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 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 3888
14/12/2025 4:22 pm  

@amin 

 

I dont think materially relative to this (I assume 'nominal' means the output at which they measure all the other various parameters)

 

image

 

But I agree its different to the page here, although the latter does also say 12.2kW at A-7W35.  Not sure what the figures for A7/W35 or A2W35 are meant to signify, its the first time I had seen this table (Vaillant do keep changing exactly what they publish!).  I cant see how they could possibly be max outputs because thats not how the thermodynamics works.    Maybe they are trying to give representative figures for COP.  A 12kW heat pump sized for -2C wont be running at anything like 12kW when its 7C, roughly half is about right but it sure is confusing.  That however wouldnt explain the numbers at +2.  Perhaps they are coming clean about the effects of deftost.

 

If you find out perhaps you could post here the answer, Im tempted to ask Vaillant myself!

 

PS I was tempted and I have.  There is also a planning guide here 


This post was modified 4 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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