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Four Heat Loss Surveys, Four Different Answers. How Do I Choose the Right Heat Pump Size?

 
Air Source Heat Pumps (ASHPs)
Last Post by Batpred 37 minutes ago
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 AnnaB
(@annab)
New Member Member
Joined: 2 months ago
Posts: 1
Topic starter 17/06/2026 10:27 am   [#3066]

We are installing an ASHP at the same time as doing a house renovation/extension. We live in a 1950's detached 3 bed, but will be doing a 2 storey extension and it will become 4 bedroom. There are 5 of us living in the house. I have been researching this for months and feel I have approached 4 reputable companies for quotes. They have all been and measured (where they can, as extension is still only plans) and issued estimated heat loss reports, although one company has not issued the full report as they say we would have that on receipt of a deposit. 

My problem is that they are vastly different in terms of estimated heat loss, estimated energy requirements and then the recommended heat pump size. The lowest heat loss given is 5.67kW at -1.5degrees and they recommend a Valiant Plus AroTherm 7kw. Another company has also suggested the Arotherm, even though their heat loss estimate is 8.41(also at -1.5 degree).The highest is 9.98kW from a company suggesting we get a Mitsubishi 11kW, however they have calculated this at -3 degrees, which I feel is an over estimate. We have also had a mitsubishi Ecodan 8.5kW form a company who estimated the heat loss at 7.99kW at -2 degrees. I am very confused! Can anyone offer at suggestions on how to choose?

We have also had very different estimated heating requirements ranging from 12,344kwh to 18, 579kwh. I am presuming that this is partly due to the different estimated heat losses, but what other information should they be using to calculate this? They have all given the same estimates for hot water requirements, but even so, they seem to have all based this on 4 person occupancy, whereas we are a family of 5.

I'd be really grateful for any advice on how to proceed.  Many thanks, Anna



   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 5031
17/06/2026 3:01 pm  

In short they will be making different assumptions about the fabric leading to different loss estimates.  The only ways to reconcile are either to inspect their calculations, do your own of do a sense check vs current fuel consumption. You do not want seriously oversized so it's worth doing what you can.

In terms of the latter:

What's your current annual gas consumption?  Do you have half hourly readings, does your house stay reasonably warm all the time or does it chill a lot eg at night?

Can you tell us a bit more about the extension to help estimate whether it will reduce, increase or leave loss about the same.

Re DHW if you want a bigger cylinder they will almost certainly oblige, the problem tends to come if you want a smaller one.


This post was modified 2 hours ago 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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Batpred
 Batpred
(@batpred)
Noble Member Member
Joined: 1 year ago
Posts: 1028
17/06/2026 4:28 pm  

@annab 

Regarding the brands, the ones you mention are very common. Mitsubishi make R32 and also the newer R290 models, Vaillant Arotherm are all R290. I stated a preference for R290 as it is the one that eventually will become mandatory. 

Installers often works with various brands. I mostly looked at the controls when choosing... 

In terms of installers, you would want to make sure they are registered with MCS. The external temperature should be very similar for all the heatloss calculations, as they are expected to follow the same standard. It is quite common for installers to charge anything from 250 to 500 or more for a heat loss calculation, but once you have one, they can usually quote based on that (even if, if you decide to progress with them, they will do their own survey). 


8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kw export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; 100Amp head, HA lab on mini PC


   
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