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Sizing of heat pumps

Mars · 2021-03-05T09:48:26Z

A lot of people that are new to heat pumps have questions about whether a heat pump has been correctly sized for their home. Discuss heat pump sizing questions and issues here.

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Air Source Heat Pumps (ASHPs)

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Mars

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Topic starter 21/04/2021 10:11 pm

@bobbt9866, so do Freedom make their own ASHPs? I was not aware of that if they do.

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batalto

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21/04/2021 10:40 pm

@editor no they are just a supplier of them. Mine is coming from them, via an installer

12kW Midea ASHP - 8.4kw solar - 29kWh batteries
262m2 house in Hampshire
Current weather compensation: 47@-2 and 31@17
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bobbt9866

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22/04/2021 10:10 am

@editor

Freedom are a distributer of Heat Pumps but also add a bit more added value to their support services

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Mars

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Topic starter 24/05/2025 11:00 am

We’ve published a deep dive into heat pump sizing, and we’d love to hear from you. Did your system turn out to be the right size? Does it keep your home warm when temperatures drop?

There’s a quick poll at the end of the article... please take a moment to vote and share your experience:

RENEWABLEHEATINGHUB.CO.UK "https://renewableheatinghub.co.uk/the-heat-pump-sizing-mess-why-are-we-still-getting-this-so-spectacularly-wrong/"

Heat Pump Truths & Sizing Fixes | Renewable Heating Hub

Heat pump sizing in the UK is flawed—discover why proper design is vital for real performance.

Pre-order: The Ultimate Guide to Heat Pumps

Subscribe and follow our Homeowners’ Q&A heat pump podcast

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benson

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31/05/2025 11:48 am

We have a Clivet 6.1 (12 kW). The datasheet provides output figures for OAT of 7, 2 and -7, at set water temperatures.

Is there somewhere where we can find the heating output at specific design temp (-2)/flow temps, or is it a case of querying with the manufacturers? Just wondering how installers decide the right capacity when it isn't overly clear from what I can see, even from the datasheets.

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Old_Scientist

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31/05/2025 1:59 pm

I agree it's not very helpful. Generally, the max heat output at -2C is going to be a little lower (maybe 3-5%) that the figure at -7C. That may sound counter-intuitive, but it's because of the likelihood of defrost cycles occurring at -2C, whereas the air is 'dryer' (less moisture) at -7C so less likely to encounter frosting / defrost cycles which reduce the overall amount of available heat. Anyway, the figures for 2C, -2C and -7C are not going to be a million miles apart, and if they are very close to your heat loss then you probably want to go for a larger heat pump. Most heat loss surveys are going to contain far larger variances (or errors) than the variances between manufacturers output data at 2C, -2C and -7C OATs.

This post was modified 3 months ago by Mars

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.

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ASHP-BOBBA

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09/06/2025 9:13 am

@benson HI, I don't know Clivet that well but if they have the data you will be able to get it straight from the manufacturer. I though most manufactures had more than 4 sets of data around 15,7,0 and -15 operating data.

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cathodeRay

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10/06/2025 12:23 pm

Posted by: @benson

↑

Is there somewhere where we can find the heating output at specific design temp

You might be able to get some idea from the Midea Engineering Data manual which can be found online (and I think has been posted on this forum at some point, but I can't seem to find it now), on the basis the Clivet is as I understand it a re-badged Midea unit. Certainly the controls etc are remarkably similar. The manual I am thinking of has detailed output tables for various models including the 12kW one, you may need to do a bit of interpolation for a specific design OAT, but won't be far out.

Edit: found a link to an online copy:

MICROWELL.SK "https://www.microwell.sk/assets/uploads/matrix/files/downloader/TM_Midea_A_series_Mono_M-Thermal_Heat_Pump_R32_2020513_V1_7.pdf"

see page 41 et seq

This post was modified 3 months ago by cathodeRay

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

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benson

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10/06/2025 2:53 pm

Thanks! That is most useful. I've had a look at the Midea data manual (it is the same heat pump I believe) and that does indeed provide a lot more info.

One of our outstanding queries with our installers is the size of the ASHP they installed- they calculated around 7.8kW heat loss and we have a 12kW unit. They told us that the 10kW only has a heating output of around 7kW when -2 and this is the reason why we needed the 12kW.

From what I can see from the manual, the 10kW at LWT of 45, has an output of around 8.2kW at -5 and 8.9kW at 0.

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