What numbers do I need to size a heat pump?

[JohnMo]

This is ours heated via 3kW immersion. 210L slimline cylinder. Temp messing about 1/3 up from bottom. Basically 1hr or 3kWh to heat.

 

[Nickfromwales]

18 hours ago, Selfbuildsarah said:

🤦‍♀️🤦‍♀️🤦‍♀️

Anyone know a good installer in Cambridgeshire?

@Dreadnaught  may know one?

[Scoops]

20 hours ago, JohnMo said:

Our Haier 4kW will kick out over 6kW, even though the datasheet doesn't state it does. So will easily heat a big cylinder. But we use the immersion.

 

Wow even at low temperatures?

 

I assume the 4kW and the 6kW are essentially the same units with some software limit applied

[Selfbuildsarah]

I’ve finally got around to looking at the heat loss figures and roughly calculated that the suggested size of heat pump by our installers is too large. They have told us 12kw but my calculations said 7kw. I will do a room by room calculation to double-check but I wondered if some kind soul has the time to check for me if I send my SAP report. I simply can’t get on with the online checkers. Would be so grateful.

[MikeSharp01]

43 minutes ago, Selfbuildsarah said:

but I wondered if some kind soul has the time to check for me

Just post the figures here, redact any address details, and I am sure someone will give it a look. 

[Selfbuildsarah]

All 16 pages? 😳

[SimonD]

1 hour ago, Selfbuildsarah said:

I’ve finally got around to looking at the heat loss figures and roughly calculated that the suggested size of heat pump by our installers is too large. They have told us 12kw but my calculations said 7kw. I will do a room by room calculation to double-check but I wondered if some kind soul has the time to check for me if I send my SAP report. I simply can’t get on with the online checkers. Would be so grateful.

 

Better if you can send a dimensioned copy of the floor plans and elevations together with all the U-values for the elements - windows, doors, internal/external walls, floor, ceiling, roof, etc. - and the design air permeability or test results in q50 and any MVHR specs. Then privately share your postcode. If I have all these details I can run a fully BS EN12831 heat loss calc to current MCS standards which not only provides a heat generator loading, but also the room loading based on calculated air permeability. I can then also provide initial figures for any underfloor heating design and requirements together with flow temps & floor surface temps assuming that's what you're having.

 

I have a tool that will soon be available free for self-builders, but I'm currently working through final bug fixing and hosting, so it's going to take a few weeks until it's online. 

 

Please feel welcome to send me a PM.

[MikeSharp01]

3 hours ago, SimonD said:

I have a tool that will soon be available free for self-builders, but I'm currently working through final bug fixing and hosting, so it's going to take a few weeks until it's online. 

Sounds very interesting!

[JohnMo]

5 hours ago, Selfbuildsarah said:

but my calculations said 7kw

If your a new build that is pretty airtight with MVHR and decent insulation, unless the house is 400m² I doubt 7kW is correct, especially near Cambridge.

[SimonD]

On 06/04/2026 at 18:47, MikeSharp01 said:

Sounds very interesting!

 

I don't want to hijack this thread, but here are some screen shots. The app will be open source with repo on GitHub of course. Problem is I keep on thinking about additional functionality and adding that after customer questions so I need to be more disciplined and release a version 1 I can then build on rather than continuously develop!

 

[marshian]

Blimey that looks really good - looking forward to trying that out.

 

PS in my day job I'm often testing functionality of new software

 

[SimonD]

25 minutes ago, marshian said:

Blimey that looks really good - looking forward to trying that out.

 

PS in my day job I'm often testing functionality of new software

 

 

Oh, nice, hopefully you can give me lots of feedback!