Heat loss calculations and how they work in conjunction with heat pump sizing

Mars · 2024-09-25T15:47:11Z

Kicking off a New Thread: Heat Loss Calculations We’ve just released the latest episode of Homeowners’ Q&A, where we dive into heat loss calculations and their importance in sizing heat pumps correctly. Our panel – featuring Michael Crook from Heat Pump Installers UK, Graham Hendra, and @jamespa (from the Renewable Heating Hub community) – discusses why accurate heat loss calculations are critical to ensuring your heat pump is the right size for your property. We cover common issues that arise from inaccurate calculations and how homeowners can avoid costly mistakes. James shares his personal story of how incorrect heat loss surveys nearly led to an oversized and inefficient heat pump installation. Special thanks to One Heat Loss for sponsoring this episode. They specialise in providing independent heat loss calculations for both homeowners and installers. For more information or to take advantage of their special offer for Renewable Heating Hub members, email your plans to info@oneheatloss.co.uk, mention Renewable Heating Hub, and receive an automatic discount! Got questions about heat loss calculations? Drop them below.

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AgentGeorge

(@agentgeorge)

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Joined: 8 months ago

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12/10/2025 8:53 am

@grahamf

I had a similar journey, had 2 people do heat loss calculations. The first was very professional and looked at solar gain for each south facing room, as my house is EW facing.

The second survey was done after I’d had the extension built, usually they decline to do it at that stage as there is no insulation or ceilings in the extension so heat loss will be massive, fair enough, but the calculation came out 1/3 higher than the thorough one.

I didn't want the large 8kW HP as it would block the rear access to the garden. The next option from Octopus was 6kW Cosy6 which I wanted as it used R290 gas and is more efficient.

Unfortunately my Heat Loss calcs said this wasn't enough, and Octopus told me (they actually used very harsh words) that if I insisted on having the smaller HP, then I wouldn't get the £7500 grant.

That was a shock: I had 2 options, suck it up or negotiate.

I showed them the professional Heat Loss calcs I’d had done, and asked to see their calcs, which they were very cagey with. After half hour of poring over the data, we finally concluded their calculation of my Heat Loss was 300W higher than the Cosy6 output, apparently the Cosy6 is not 6kW, its 5.6kW.

Light Bulb Moment! Thats why my professional Heat Loss calcs of 5.9kW wont work with the Cosy6 aka Cosy5.6!

We tweaked a few assumptions and a note was added that the survey was only valid when the finished insulation is installed.

My Cosy5.6kW has worked well all year, it had a hard January as the insulation was not correct and was clearly unable to cope with the unfinished insulated rooms. I opted to insulate higher than required, and am insulating the last outside wall with insulated plasterboard

I had already opted to fit PV on each side of the roof, maxing out the PV to 5kWh, adding a 5kW battery which charges on cheap rate electricity, gave me an average 12.3p kWh this past year

The comment about only heating 4 out of 5 bedrooms Ive read about and would suggest is not good. I have my unused bedrooms turned down to 18C

When you do your Heat Calcs, their are 3 types of wall:

Inside, Party, Outside

an Inside wall has NO cavity insulation so is determined to be a high heat transfer wall. Having an unheated room next to a heated room sucks heat from the heated room. The unheated room also causes thermal draughts.

The last piece of my puzzle for controlling heat is to box in the stair well and fit a door. This will keep the heat downstairs from escaping to the upstairs landing. I’m sure you've all noticed it get hotter as you climb the stairs.

When I had Gas CH with radiators downstairs, the upstairs landing was a massive heat trap, and a cold draught was always coming back down. Since having UFH with the Heat Pump, the heat is introduced in the floor and the heat doesn't convect up the stairs like happened with radiators; fitted under windows and covered with curtains so the heat warms the windows first, worst design flaw of a heating system

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