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Where heat loss surveys seem to go against practical reality

ian33a · 2026-09-04T16:58:20Z

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

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Air Source Heat Pumps (ASHPs)

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cathodeRay

(@cathoderay)

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Joined: 5 years ago

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07/09/2026 5:47 pm

Posted by: @rob-of-york

↑

Can you add up the energy used for DHW each day? If so then at least for the daily averages, you could get the total heat pump output across all hours and then subtract the DHW contribution.

I could but I would like to try to find a more generic solution, working on it now!

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

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cathodeRay

(@cathoderay)

Famed Member Moderator

Joined: 5 years ago

Posts: 3030

07/09/2026 6:09 pm

OK, this one is fairly easy, because most of what is needed is in a 24 hour data file (logs past 24 hour values every hour on the hour):

The sql used to get the data, variable names are ugly but just about human readable, T00: in the like clause gets the midnight value:

"select datetime,amb_24h_mean,round(htg_24h_kWh_out/24,2) from midea_24h_data.csv where datetime like '2026-01-__T00:%' order by datetime"

The htg_24h_kWh_out variable contains all the space heating for the 24h period, including the pesky hours that also has some DHW heating.

The IAT-OAT version will take a bit longer as the IAT is not in the 24h data file.

Using like '2026-__-__T00:%' will get all 2026 data, I can then plot individual months, 1st Jan to end of heating season, whatever.

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

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