The results of a HTC test will not get weather compensation totally corect unless the relatively heatlose of each room matches the relative sizing of all radators, but it would be a good starting point.
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Imagine you have oversized a gas boiler and its minimum modulation is above the heat load of the house, that kind of short cycling would make any stated efficiency figure completely wrong.
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I was in exactly this situation: 24 kW boiler, 30 years old, without modulation capabilities, non-condensing. Based on gas usage I estimated a worst-case 5500 W heat loss at design temperature for 100% assumed efficiency. With my heat pump Iâm now sitting at 4000-4500 W measured via heat metering, so around 25% higher efficiency as the boiler, which fits expectations.
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We have the issue that efficiency of gas boilers often increase when it is colder (due to reduced cycling) but maximum output of heatpumps reduce when colder.
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I wish companies / gov bodies were better at gathering feedback from existing installs and applying that to refining their prediction models.
Itâs the same in many industries though.
Theyâd find that we are happily using 33-30C LWT for outdoor temps of 0 to +5.
Indoor temps at a nice 21 - 21.5C downstairs.
They predicted Design Flow Temperature
50C at -1.5.
I have a feeling we might struggle to get LWTs low enough when its 10-15C outside!
Weâll see.
Just be Madoka turning off and on old school other wise.
Certainly wonât be cold anyway!
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Hi all.
Just a quick update.
After 3 months of waiting for a design heat geek decided that my heat loss doesnât âconform to their business modelâ
They did say via email that my local heat geek would design a system, but after waiting for an answer it was a no from him.
I got the impression that my local heat geek only want to fit vaillant ASHP only.
My time wasnât totally wasted.
Heat geek told me he used 1.8 ACH for my house and 2 ACH for my bathrooms. This have me a heat loss of 30kw.
so a heat loss of 20-24kw was achievable via heat geeks software once he reduced the ACH. Funny that.
Regarding heat geek using a HTC test to monitor my heat loss it was a no.
I think my worst case heat loss is what TrystanLea suggested.
Going off my gas and wood burning usage this year, my heat loss is around 15kw.
At the start of the ASHP journey I contacted a company in November 2024 who had designed a 22kw mitsubishi Cascade system.
Iâm waiting for them to carry out their own heat loss survey. Fingers crossed the system will be up and running in June.
Once I have more info on the system Iâll document the journey in a new thread and ask all sorts of stupid questions once installation has started. 
I do want to add my ASHP to the monitoring platform.
Thanks all.
From your usage it doesnât look to me like your heat loss is as big as that. From your 6 feb measurement your heat loss at -3 oat should be (with 3C average OAT, 20C indoor temp and 90% boiler efficiency):
(20C-(-3C))/(20C-3C)204kwh0.9/24=10.35 kWh
Not suggesting youâd base your design on that one data point! That said in my case heat loss based on usage and oat gives me the best correlation with what my heatpump is outputting and Iâd definitely use it to verify any design.
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I just wrote up this experiment I conducted to try and find a simple way to double check heat-loss results.
Essentially, it is just getting an electric radiator in one room, then turning off the radiators in that room and monitoring the energy required to maintain the temperature.
The idea being that if the house is of similar structure throughout then you can adjust your heat-loss calculations to your empirical data. It or a variant of it might work for you.
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I agree with Andre_K. I am also not in the UK but the reaction was similar to my own challenge on sizing. My installer would not take responsibility for a smaller installed heat pump below the 6kW he recommended, albeit he agreed it was above his own calculations. He simply did not want a call out in a cold winter day for a poorly performing pump. He offered no opinion on running costs. I ultimately issued an instruction to buy a smaller 4kW machine. In my case, I had no ÂŁ7500 grant, so it was not linked to hard cash.
This may be a radical thought, but maybe the ÂŁ7500 grant towards equipment cost is part of the problem. Could the UK grant be tailored to both capital and running costs after a year of operation. Maybe 50% grant on the initial capital cost and 50% on healthy annual running costs (SCOP>3.0).
This is an excellent method. When our gas boiler was removed and our heatpump installed, I had to heat two days using electric heaters only. The average power draw was almost exactly what I now get from monitoring heat input of the heat pump.
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Hi all.
Just to update this excellent thread.
I had a 2nd heat loss survey carried out by my local mitsubishi installer. Their figure was 21.2 kw. A much more realistic figure. Iâve calculated it to be 15kw with gas and including my wood burning stove.
My EPC has been update to a B. Best on the road. Took many years to get that. House was a E when we purchased it 11 years ago.
So, a 22kw Mtsubishi cascade Ecodan system is being installed on Tuesday.
DNO is happy for the work to commence but they are upgrading my main 100amp fuse to newer version. They are also stripping back the core so it will fit the new cut out.
Its been a long journey getting to this point. Fingers crossed it runs as advertised. It does feel like an expensive house experiment though.
Iâll be fitting phils monitoring to my system so I can look at those wonderful graphics on home assistant.
Thanks all for your input. It does give me confidence thats itâs the right decision.
Thatâs a big one 
Indeed it will be.
Fingers crossed the install wonât show up any surprises. This house does keep you on your toes.
They should just make electric for heat pumps vat free.
Id absolutely love to get a vat rebate from my own solar and batteries