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Advice on ASHP choice needed

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Air Source Heat Pumps (ASHPs)

Last Post by Majordennisbloodnok 2 years ago

60 Posts

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Pie_Eater

(@pie_eater)

Trusted Member Member

Joined: 2 years ago

Posts: 38

Topic starter 10/07/2024 9:16 pm [#1614]

Hi all

first time poster . I’m really getting very confused about my ASHP choice.

I live in 2016 build EPC B detached house , quite sizeable 220m2, just north of London.

i went through the very long journey of getting an octopus quote , they did 2 surveys and recommended a 8kw daikin. Heat losses c 6.4kw based on 2 surveys. Various people warned me about them (too small heat pump mainly, inexperienced installers) so got cold feet.

I then switched to Aira - heat lossss also 6.2kw , they recommended their 12kw unit which looked great and seemed to fit the bill. Long survey but 30 mins into the install has to be aborted as various things missed in the survey (like changing internal pipework).

Now have a local recommended installer coming up with much higher heat losses (11kw) and therefore speccing and LG therma v monobloc 12kw. Really ugly unit double fans.

I am really confused as to how the heat losses can differ so much - the last guy said ocotpus will be overestimating my U values and working the machine too hard. Not sure what to think of that.

My main 2 questions are : should I be concerned that the LG 12kw monobloc will be too large ? as I’ve read stories of oversizing being expensive to run if top big.

And 2 is there any other way to benchmark my heat losses ? For instance my total kWh gas usage in 2023 was 11300 kWh, does that help?

thanks!

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Mike H

(@mike-h)

Reputable Member Member

Joined: 4 years ago

Posts: 200

10/07/2024 10:55 pm

Heat loss estimates can vary considerably, depending on who is doing them. Have a look at this for a variety of methods including using your gas usage.

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bontwoody

(@bontwoody)

Noble Member Contributor

Joined: 4 years ago

Posts: 1009

10/07/2024 11:02 pm

The short answer is yes, there are other ways to check your heat loss estimate. I will post a link tomorrow when I’m at my pc. I would not be surprised if your 6.2-6.4 kW wasn’t far off. The 11kW is the outlier and sounds large based on the type of house.

House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60

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HCas

(@hcas)

Estimable Member Member

Joined: 2 years ago

Posts: 132

11/07/2024 9:15 am

Would recommend asking another opinion. The 11 kW heat loss seems high indeed if 2 others came up almost half of that.
I'm also surprised Aira would install a 12 kW heat pump in a property with 6.2 kW heat loss. That's way oversized.

On Octopus, you mention a concern about too small heat pump. An 8 kW Daikin for 6.2 kW heat loss is not too small so that shouldn't be a worry.

CEO and co-founder at HavenWise

👍

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Pie_Eater

(@pie_eater)

Trusted Member Member

Joined: 2 years ago

Posts: 38

Topic starter 11/07/2024 9:26 am

@hcas well spotted - they were going to install a 8KW .

👍

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4516

11/07/2024 9:31 am

@pie_eater welcome to the forums. If you have any Heat Geeks near you, it might be worth them coming out to do a survey too.

Get a copy of The Ultimate Guide to Heat Pumps

Subscribe and follow our YouTube channel!

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Pie_Eater

(@pie_eater)

Trusted Member Member

Joined: 2 years ago

Posts: 38

Topic starter 11/07/2024 9:33 am

I did a review of the assumptions of the heat losses and a decent part of it is down to design temp, for Octopus and Aira they use 19 for hallways and bedrooms and 21 bathooms whereas the local company uses 21 everywhere and 22 for bathrooms.

My main question is, how inefficient would the LG 12KW be if heat losses turned out to be 6-7kw ? Would it cost me a lot extra per year or just marginal?

The installers seems to really like the LG 12KW, perhaps because of low noise. Not entirely sure why but he rattled of a range of unsuitable alternatives.

He tells me not to worry it will modulate down as its "inverter driven"

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Anonymous 5011

(@Anonymous 5011)

Joined: 3 years ago

Posts: 562

11/07/2024 9:39 am

The trouble with heat pumps manufacturer use different criteria for naming the heat pump size. So for actual sizing you need to look at running curves and data.

We spend a very small fraction of the heating season at or below the design heat loss, and a lot at about 6-7 degs which is generally half the heat load. So your 6kW is really nearly 3kW most the year, plus DHW heating. I would allow 3 hrs for DHW heating.

So take you design heat loss of 6.4 and multiply by 24 that becomes 154kWh. Now due to DHW heating you only 21 hrs for house heating, 154 / 21 becomes 7.3kW. So you need a heat pump that can do 7.3kW at -3. But also turn down (min output) ideally to 3.2kW at 7 degs.

My house is a similar size but EPC A, our heat load is 3kW so with your B circa 6kW is way more realistic than 11kW.

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bontwoody

(@bontwoody)

Noble Member Contributor

Joined: 4 years ago

Posts: 1009

11/07/2024 9:59 am

@pie_eater

Here is the link I mentioned, it contains various methods of calculating heat loss. It depends how accurate you want to go.

https://energy-stats.uk/what-size-heat-pump/

Using two very simple examples given:

The Heat Geek cheat sheet estimates your heat loss to be between 4.4 and 8.8 kW

Michael Podesta's quick and dirty method gives it as 4.7 kW (-3C OT to 21C IT) using your annual gas figure (assuming that is all heating and not hot water, it would be less if you use gas to heat you hot water).

I certainly would not consider a heat pump of more than 8kW for your property and might well be looking at one in the 5-6 kW range depending on how more detailed calculations come in.

As mentioned before, its important to double check the badged output on heat pumps as that can be misleading and also check how low they can modulate down, Im currently using about 2kW to heat my home (of similar heat loss) and Im starting to cycle.

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Pie_Eater

(@pie_eater)

Trusted Member Member

Joined: 2 years ago

Posts: 38

Topic starter 11/07/2024 10:21 am

@bontwoody

ooh that’s a great one thanks. Lots of geeky things to dig into

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bontwoody

(@bontwoody)

Noble Member Contributor

Joined: 4 years ago

Posts: 1009

11/07/2024 10:22 am

@pie_eater LOL, you are going to love this forum then! 🤣

👍

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Judith

(@judith)

Prominent Member Member

Joined: 2 years ago

Posts: 519

11/07/2024 5:49 pm

Each installer/designer can have their own idiocies

one wouldnt count our cavity wall insulation because we couldn’t prove it was still working after 40 years (of course not!) so said we needed a12kW Midea NO!

Another used default air leakage despite us having a pressure door test result (said MCS insisted) so recommended a 12kW Daikin NO

I’ve used both heatpunk https://heatpunk.co.uk/loggedout and PHPP and my own spreadsheet and it comes out as ~7kW HP needed (yep nerd!)

That corresponds with Podesta rule of thumb well (see link from energy stats above) with our gas usage.

If you ran your gas continuously at the lowest possible temperature you could be sure the gas usage would correspond well.

A heat geek guy did agree that 7kW is what we needed, but even he had to play around with the software since our MVHR system could only be included if the house was built after 2006. Wierd! So that came out as the 7kW Vaillant, so guess which installer we’re going with?

2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP 4.7) open system operating on WC

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