ASHP Sizing - Specification in our quotes seem excessive

[deancatherine09]

Hi all,

 

I've had a little look through some similar posts, and then done some calculations to try and back up my thinking!

 

We are building using MBCs Passive Twin-wall system, with passive foundation and Triple-glazed Velfac windows.

 

We had a SAP calculation done for us, and sent this out when requesting ASHP quotes.

We got quotes back from multiple companies, and all seem to specify 15/16kW ASHPs, but we can't shift the feeling that this is massively oversized.

 

My calculation was as follows:

 

(Data from SAP report)

 

8. Space heating requirement

Space heating requirement for each month, kWh/month

December = 1702.24kWh

 

9a. Energy requirements – Individual heating systems including micro-CHP)

Water heating

Fuel for water heating, kWh/month

December = 82.86kWh

 

So taking the calculation from another thread,

 

December needs 1785.1kWh, which is 57.6kwH per day, or average of 2.4kW (24/7)

 

And assuming it is running for half the time, 4.8kW requirement from the ASHP?

 

 

 

Am I missing something obvious in the SAP report that means we actually need a 15kW ASHP? - as several companies have told us we need such a huge one?

 

Revised SAP Documents - Burbank.pdf

[jack]

26 minutes ago, deancatherine09 said:

December needs 1785.1kWh, which is 57.6kwH per day, or average of 2.4kW (24/7)

 

And assuming it is running for half the time, 4.8kW requirement from the ASHP?

 

It's a combination of ignorance and laziness imo.

 

We had this with our plumber, who had never worked on a low energy house before. He started by proposing a 16 kW unit for our 289 m² MBC twinwall construction house, then countered with a 12 kW unit when we told him how much energy would be needed. He was genuinely concerned about how much the internal temperature of the house would drop if it took 2+ hours to heat the DHW tank. Nothing we could say would convince him otherwise.

 

In the end we went with a 5 kW Panasonic Aquarea and it's been perfectly fine.

 

That said, if I were doing it again, I'd potentially consider a higher capacity unit. With time of use tariffs becoming increasingly common, the potential to push some or perhaps even all of your heating and DHW into the cheap times is appealing. It's harder to do this with a smaller ASHP. For example, we've been on the Octopus GO tariff for a couple of years. We only get four hours at the cheap rate. It can take up to two hours to reheat the DHW tank, leaving only two hours to pump heat into the slab. A more powerful ASHP would help reduce the time spent heating the DHW leaving more time for slab heating.

 

Alternatively, or in addition, I'd also consider adding a buffer tank for the UFH so that can be heated during the cheap time to act as a reservoir for the UFH for the rest of the day.

 

It's possibly less of an issue if you're considering batteries. They allow more flexible interaction with time of use tariffs.

[ProDave]

My heat loss calculations showed worst case heat loss was 2.2kW  and my house is being heated quite comfortably with a 5kW ASHP So you should be fine with a 6kW ASHP 

 

As others have said, a lot of contractors just cannot grasp the concept of a low energy house, and want to specify something bigger so they have no liability if a small ASHP proves inadequate.

 

 

[JohnMo]

Your heat usage is similar to yours, which slightly lower by month, but by  hourly average, pretty similar. I am in the process of installing a 6kW machine. What you need to do is ensure at the lowest design temperature, the capacity the chosen ASHP can meet and exceed your max heating needs, so it leaves capacity for DHW heating.

 

Previous experience and detailed monitoring of our gas consumption shows the unit to be a little over sized, but the next size down unit to be a tight fit heating capacity wise.

 

So I would say they units mentioned by your suppliers are massively oversized.

[JamesPa]

+1 on the thread title.

 

The latest 'mcs compliant' quote I have received is recommending 2 midea 12kW units for a price of £25,086 less the 5k government grant. 

 

Their own estimate of house consumption is just short of 17kW, my estimate using mcs assumptions but the correct u values for walls etc is 11kW, and the actual measured consumption during an extended period when temperatures both day and night were -2C (pretty much the recommend design temp here in the south east), is 7.5kW.  Oh and they propose to run the system at a flow temp of 55C.

 

I can get a 'discount' of 1750 on the price if I accept within 30 days, entailing a 5K deposit.

 

Need I say more?

[sharpener]

Do they think you came up the Clyde on the last banana boat or what?

[eandg]

53 minutes ago, sharpener said:

Do they think you came up the Clyde on the last banana boat or what?

 

Long time since I heard that though unfortunately not that long since I've experienced some chancers in this area - some of the spec/quotes for solar and ASHP from Scottish suppliers has been wild. We've ended up with a 7kW system but one guy specced 13.2kW and wanted 16 grand for it. 

[Conor]

Your house is basically same U value spec as ours but our house is a bit bigger... Our 9kW heat pump coped admirably with the cold snap in December. In theory we could have got away with a 5kW unit, but glad of the larger one as we only run it 7 hours a day on Economy 7. For a 5kW unit, it would have been running 24x7.

 

Get it modelled in PHPP and use the figures it spits out. The SAP is shite.

Edited April 17, 2023 by Conor

[scottishjohn]

if needing a larger ASHP,due to size of house -- is there any savings in  costs going for a 3phase unit over a single phase unit 

as iwill have 3 phase supply

[CalvinHobbes]

On 20/04/2023 at 23:44, scottishjohn said:

if needing a larger ASHP,due to size of house -- is there any savings in  costs going for a 3phase unit over a single phase unit 

as iwill have 3 phase supply

I thought you cant use economy 7/off peak on 3 phase?