Air Source Heat Pump - 12kw vs 16kw

[sokoma]

Hi,

 

 

 

We currently have a house which is poorly insulated and has an heat output of 11kw (lots of single glazing). After double glazing, the heat loss is 8.9kw.

 

We are planning to do a bug extension and loft at some point, adding c.150sqm to the existing footprint.

 

We have been offered a 12kw heatpump. Should we push for a 16kw heatpump instead? To me, it seems like an obvious yes but I wanted to check.

 

Thanks!

[MikeSharp01]

If you build the extension to a high insulation / air tightness / glazing spec 12kW should still be OK but it depends on the % increase in size and where it is going in relation to the rest of the building. 

 

 

1 hour ago, sokoma said:

 

We are planning to do a bug extension

How many bugs are you intending to house.

[sokoma]

A big bug indeed :).

 

increase would be c.75 pts of the existing floorplan (from 170 to 300). it would be back and double storey side extension + loft

[JohnMo]

29 minutes ago, sokoma said:

A big bug indeed :).

 

increase would be c.75 pts of the existing floorplan (from 170 to 300). it would be back and double storey side extension + loft

Why would you bother, I would just move, if we needed house circa twice as big. Especially if it leaked heat like a seize.

[nod]

We have a 11 kw servicing 400 m2 and four bathrooms It’s more than adequate 

Have you thought of buying a better insulated home 

[sokoma]

Thanks all. We bought a house in need of renovation because we like a project and it is the only way to afford this. 

[Andehh]

Sounds like 16kw, unless insulation is the absolute focus for your project!

 

We have a well insulated 270sqm bungalow (very very well insulated outside of build hub scales), albeit it a very inefficient design, and 12kw suits us nicely.

 

 

Check it still runs on single phase, and obvs inevitably higher Elec usage etc.

[Alan Ambrose]

The other thing to check is the modulation i.e. how low output it can be run. If it can’t be ‘modulated’ down to your typical heat load then it’ll cycle on and off - which is inefficient.

[ReedRichards]

For some heat pump manufacturers, their 12 kW model is just a software limited version of their 16 kW unit.  In such instances there is no disadvantage in opting for the larger output one - except that they may charge you more for it!  

[Russell griffiths]

Sort your insulation and airtightness first, don’t just fit a bigger heat source, or you will be one of the thousands that say they didn’t realise heat pumps cost so much to run. 

[Iceverge]

On 09/03/2025 at 06:29, sokoma said:

We bought a house in need of renovation because we like a project and it is the only way to afford this. 

 

Congratulations on your new house. 

 

Perhaps more details of the current state of it might help us guage what kind of heating system is most likely to be most comfortable and efficient in the long run. 

 

Some houses are terminally unsuitable for an ASHP but this depends on,

 

1. Airtighess 

2. Insulation levels

3. Window spec

4. Footprint

5. Occupancy patterns. 

 

Perhaps you could give us an idea of some of these. 

 

At the moment it sounds like a gas boiler might be best if that's what you currently have installed? 

[sokoma]

Hi,

 

Thanks a lot for your message.

 

See some details / context below:

- Detached house built in 1940’s

-c.150sqm of habitable space, to be extended to 300 sqm (extension + loft conversion of 50 sqm).

-walls: cavity walls (50 cm cavity, to be insulated); additional 150 sqm will be extended with new build spec (potentially SIPs)
-Loft 300mm loft insulation

-Windows - currently single glazed but will be double / triple glazed

-WFH a decent amount of time

 

To give an idea:

-150 sqm house with single glazing: total heat loss of 11kw (79 w/sqm)

-150 sqm house with double glazing: total heat loss of 9kw (65 w/sqm)

 

We currently have an oil boiler - the house is eligible for free insulation / ASHP hence why we are checking if we can upsize the ASHP

 

[JohnMo]

MCS rules will not allow you size heat pump based on something you MAY do on future. Until it's done, it isn't done, so may only get done.

[Andehh]

Im not sure ASHP Is best suited for your place, I'd be tempted to max the insulation, over size the rads and stick with the Oil burner and re evaluate once you've lived with it all for a few years. Free ASHP might prove costly to then operate.

Edited March 11 by Andehh

[ReedRichards]

18 minutes ago, Andehh said:

I'd be tempted to max the insulation, over size the rads and stick with the Oil burner

 

I switched from oil to a heat pump and up-sized the radiators at the same time.  Almost all oil boilers are just on/off so if you just oversize the rads but stick with an oil burner you will heat the house more rapidly and increase the frequency at which your oil burner cycles.  That's neither good for efficiency nor good for the longevity of your oil burner.   

[Dillsue]

5 hours ago, sokoma said:

To give an idea:

-150 sqm house with single glazing: total heat loss of 11kw (79 w/sqm)

-150 sqm house with double glazing: total heat loss of 9kw (65 w/sqm)

The heat geek Web site suggests a range of 40-65 w/m2 for a renovated house with CWI and DG so your 9kw loss is based on their worse case. Using 40 w/m2 gives you a 6kw heat loss. If you allow half that figure for your new build extension then you're up to 9kw so a 12kw heat pump should be more than adequate and likely oversized.

 

The more I read about heat pumps the more I beleive they should be undersized rather than oversized for max year round efficiency. If there's concern about being cold for the few days of the year it's freezing then keep a £20 fan heater handy

[JohnMo]

1 minute ago, Dillsue said:

The more I read about heat pumps the more I beleive they should be undersized rather than oversized

It really depends there are many variables. Heating method is one, depth of modulation another. Thick screed UFH is way different to radiators. The difference in heat capacity of air 'v' concrete is huge. So a radiator has very little place to dump heat and the air is soon up to temperature, a thick screed UFH system has a mammoth buffer store built in by the nature of way its built.

 

Agree with a radiator system you need a heat pump capacity close to the max heat loss AND a good volume of water. The ideal for a radiator system is 20 to 25L per kW at minimum load output of the heat pump. This ensures a good run time and good control of modulation.

 

Our heat pump 2x oversized for the heat loss, but only really heats a thick screed slab (57 Tonnes of concrete), so the heat pump will chug away for as long as you want without cycling. Then when it does go off, it's off for the rest of the day. 

[Dillsue]

I'm guessing a HP heating 57 tons of concrete isn't gonna be found in your typical British home??

[JohnMo]

6 minutes ago, Dillsue said:

I'm guessing a HP heating 57 tons of concrete isn't gonna be found in your typical British home??

But the OP is talking about adding 150m2 to the existing house so could easily have that much (or more) concrete in the slab if he wanted.

[Dillsue]

Would need at least a 300mm slab over 75m2 to gobble up 57 ton so not your average build

[JohnMo]

On 08/03/2025 at 19:37, sokoma said:

c.150sqm to the existing footprint.

 

1 hour ago, Dillsue said:

Would need at least a 300mm slab over 75m2 to gobble up 57 ton so not your average build

Not sure where the 75m² comes from they said 150m² footprint. Plus they are doing up the existing house so who knows the final footprint available. No one said average build (except yourself) I am just applying stuff I know to the OP questions.

 

 

[Dillsue]

On 08/03/2025 at 21:26, sokoma said:

increase would be c.75 pts of the existing floorplan (from 170 to 300). it would be back and double storey side extension + loft

Possibly less than 75m2 with the loft.

[sokoma]

Thank you very much for the help all. We will get a 12kw