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Where heat loss surveys seem to go against practical reality

Air Source Heat Pumps (ASHPs)

Last Post by Batpred 12 minutes ago

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ian33a

(@ian33a)

Trusted Member Member

Joined: 4 years ago

Posts: 35

Topic starter 04/09/2026 5:58 pm [#3153]

We have a reasonably large, old house, improved significantly by the previous owner, with reasonable insulation (but some old stone and cob walls retained), with UFH and decent double glazing.

It's been a walk through treacle to get installers to come out and quote, but I finally have two to compare against. I also have a figure from a third (no report) and a stupidly high quote which we rejected. I also did my own heat loss survey using HeatPunk.

So, in order of size

Installer A : 14.7KW - Tape measure and HeatPunk - rejected on grounds of price

Installer B : 13.7KW - Lidar and Spruce

Myself : 12.3KW - Tape measure and HeatPunk

Installer C : 10.7KW - Lidar and Spruce

I've done a load of cross referencing (with much help from knowledgeable people on this forum, thank you once again) of my annual oil usage and wood stove : ~ 11.3 KW

Installer B is adamant that many of our rooms will be too cold unless we have a flow temperature of over 50 degrees and a 15KW Grant heat pump. He also says that our main bedroom is too cold and will need an additional radiator. That we currently run the UFH in that room, and most rooms, at about 45 and it is toasty in the winter makes no sense to us. He keeps saying that MCS requirements are very different to what we may find acceptable. This same installer is advocating ACH figures of between 0.7 and 1.7 (average 1.03), seems rather high? The house old, yes, but it not draughty. This is a third party doing the heat loss before passing it to a chosen installer to do the work - the installer will take overall responsibility from the point that the contract is signed, the heat loss surveyor walks away. Heat Pump offered is the 15KW Grant.

Installer C is in agreement with us that if the rooms are remaining warm enough in the winter with reasonable flow temperatures, then we are probably fine. His ACH figures range between 0.5 and 0.9, with an average of 0.55. This is an installer who will be doing the work through his own company using dedicated subcontractors. He was a heating engineer, is Heat Geek trained but doesn't operate through them anymore. Heat pump offered is the 12KW Vaillant mk 2. Flow temperature is designed for 45.

Fabric losses vary between B and C , 9,800 with B, 8,300 with C.

Ventilation losses : B is 3850 and C is 2370.

So the combination of fabric and ventilation losses combine to give the 27% difference between each proposal.

My gut feeling suggests to me that Installer C is the more practical (although I wonder if the figures are trimmed to meet the Vaillant spec) and that installer B is protecting himself and the installer that he passes the work on to.

I don't have the appetite to get another installer out, even if they agreed to come, it would be Christmas before we get to an installation point.

If you were betting on the right choice, assuming I have given enough information here, where would you put your money?

thanks

This topic was modified 1 hour ago by ian33a

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cathodeRay

(@cathoderay)

Famed Member Moderator

Joined: 5 years ago

Posts: 3007

04/09/2026 6:51 pm

@ian33a — a tricky one. Any or all of your hunches may be correct. Can you get the raw data Installers B and C used for their fabric heat losses, and look for discrepancies there? The air change differences are already explained, and I favour Installer C assessment.

I would also ask both installers B and C if they can explain the difference in results given they used the same method, and then scrutinise their explanations for anomalies.

How confident are you in your 11.3 kW loss based on your past oil boiler and wood burner use? Which way are any errors likely to have pushed the result?

Of course getting the heat loss estimate as accurate as possible is very important, but installer character and style also matters. You need someone you can work with. On the basis of the information given, on that basis perhaps Installer C is a better bet? But the final choice has to be yours, we can only suggest things to consider.

Midea 14kW (for now...) ASHP heating both building and DHW

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Batpred

(@batpred)

Noble Member Member

Joined: 2 years ago

Posts: 1129

04/09/2026 6:52 pm

You probably know this, but when going with a Heat Geek trained installer that is not operating through them, you could end up taking the risk of a not correctly sized pump.

With Heat Geek they give you a SCOP guarantee for heating and hot water (hot water being a very small share of energy use). So if your figure is below their guarantee (exact figure depends on what they calculate your typical flow temperature will be), they will swap the pump at their own cost.

If you have rads undersized in a room you care about, you will probably end up turning the temp up. Ultimately, the pump will end up operating at a higher temperature to keep it comfortable and so run with a lower efficiency..

We made sure we increased rad output on the rooms that were a bit low so that the whole thing will run more efficiently. So I would look at the specific advice they give in terms of upgrades (based on loss calculations, ideally very close to room level detail). They should calculate a flow design temperature at -2C external temp (for SE, less depending where you are in the country). This is what MCS requires them to do before they complete a proposal. I know many just get data from your epc (like spruce does) and give you an estimate. But after this, they ask for £250-500 for a proper heat loss report. For a detailed heat loss report, I found it money well spent.

This post was modified 9 minutes ago by Batpred

8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kW export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; Vaillant Arotherm Pro 7kW; 100Amp head, HA lab on mini PC

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