Long ASHP explainer with questions

floors, windows, detailling around window openings?

I suspect there is little to add their other than replacing current DG with newer or adding triple glazing which we won’t do. Solid floors on ground, again which we won’t disturb. TBF their survey was very thorough.

Micks graph shows this. I’m impressed with his winter COP. Things have clearly come on in the 13 years since I worked around ASHP. Unless thats the seasonally adjusted COP?

The issue is that at the coldest, the efficiency is the least good. You used to see winter COPs of less than 1 in a hard winter. So it was possible to put say, 8Kw in and only get 7 kw back. At a time when your buildings heat demand could be double what the rule of thumb calc shows above, its not hard to see how a HP that looks okay on paper could not deliver.the goods when you need it the most.

Still, If we can expect COPs of 2 at minus 10°C, those days might be behind us.

I’m sure the COP graph is flattering / optimised, but my electric bill would suggest it isn’t ridiculously wrong.

If you drive it like an old gas boiler (i.e. turn it on or up when you are cold, turn it off for a number of hours etc) then I’m sure it will cost a fortune. But you don’t drive it like that.

You run it with your house already up to a stable operating temperature days before it gets cold, and ideally you have a decent bit of thermal mass within the insulated envelope to help smooth things out. Your outside air temperature feedback increases the target flowrate as soon as the outside temperature drops (hours before you see a big drop inside) so it never really has to run it’s nuts off even when it is really cold. Previous winter we had a number of weeks continuous around freezing with a low of -12C and it didn’t struggle and there were no scary bills – and we’re paying monthly for what we use so it isn’t being hidden or averaged across the year.

Surfer – it still might be worth looking under the plaster and windowsill on one of your openings. Our previous late ’60s bungalow was hopelessly detailed in this area despite cavity wall insulation. And solid floors can be a massive heat sink – one of the things I like most about our modern house is the massively insulated floor slab. On the old house I did the miserable job of crawling in the tiny dirty floorspace to insulate under the floor but you haven’t that option.

@Surfer

The ASHP and Gas boiler as doing different work, for the same result.

35kw burning gas to heat water and central heating. Heat comes from burning gas.

ASHP 8kw to run pump, fan and compressor. The heat comes from the air, the heat pump concentrates it.

So 3 times more efficient relating to energy used to get desired result (warm house and hot water).

as others have said, adding more insulation to a fairly well insulated loft didn’t sound like a great idea. It might make a heat loss spreadsheet work but in reality won’t be money well spent and might not make the difference you need. The design should be to MCS standards, and if done properly will have a reasonable margin for error by over sizing anyway. To make the heat loss calcs more accurate you could get a door blower test done to check how leaky the house is. The MCS designer can then update the heat loss calcs with a measured rather than guessed figure. That might bring you safely into the 8 kw heat pump realm. The upside also being you can find where all the leaks are in your house.

I have an alternative solution for you. Check with your installer about using a propane based heat pump rather than an R32 based heat pump. That should be able to deliver water at the same temperature as your gas boiler at a reasonable CoP, so no need to change your radiators. Spend the money saved on a larger rated heat pump.

Based on your age and size of house, I’d say the 8kW unit is too small though. 12.5kW sounds .ore like it.

I am also looking in to this.

I was reading  the intalation manual for a heat pump using propane gas as its refrigerant, and it has minimum distances from window oppenings and doors.   But it does not say anything about air bricks that are  all around the ouside of my house.    Has anyone discussed this with an installer ?

so no need to change your radiators. Spend the money saved on a larger rated heat pump

Changing the radiators is fine as they are very old and likely a job I would have done myself at some point and the company are happy to install the larger HP for the same price as the smaller, although in a less convenient location then the smaller unit. We can live with the new location however and my main concern (and lots of food for thought about it on this thread) is just getting the most appropriate size. Too small and too large bring their own concerns.

“Check with your installer about using a propane based heat pump rather than an R32 based heat pump. That should be able to deliver water at the same temperature as your gas boiler at a reasonable CoP, so no need to change your radiators. Spend the money saved on a larger rated heat pump.”

The trouble is with running at a higher temperature is the extra electric used. Propane is slightly less efficient than older refrigerants, it’s only being used for its greenhouse gas compliance, older refrigerants are far more efficient, but they cannot be used now. A by product of it is that propane can be used to run at higher temperatures. This could be a good thing for some people, but it will mean more elecricity use.(also, as Timf says, siting of the outdoor unit needs to comply in case of a gas leak)
Each degree rise in temperature equates to roughly 2.5% more electricity used, thats why the way forward is low temperature (<40 deg C) flow. A propane refrigerant HP could be a straight swap for a gas boiler, but, it will cost more in the longer run in electric costs if ran at higher temperatures.
One of the HP makers is advertising that they can run at 70 deg. C. Building Regs now say the flow rate (for all heating) should not go above 55 degrees, so the advertising is misleading, the higher temperature can only be used to heat the hot water tank, not run the heating, which should be 55 or lower.

Also thinking about going ASHP, anyone help with two more questions?

Apart from cost, is there a downside to having a larger HP than strictly needed?

Currently on oil, so will be getting rid of a large plastic tank. Can we put the HP where the tank is? It’s about 5m from the house. I guess the pipes would need to be insulated (and underground).

Thanks.

I think if the pump is too large it constantly “cycles” (turns itself off and on as oppose to running near constantly) which may cause premature wear to components.

Using the 2900 rule my 1930’s externally insulated 3 bed semi, 110m square needed 5.5kW. Using heatpunk, it came in at just over 5kW. Got a property survey done by Heat Geek approved and it came in at 5.5kW. The estimate then proceeded to size up to 7kW and add buffer storage. Post grant was £8k. At least £3k of that is heat geek guarantee as far as I can see. That cost isn’t justified imo. As touched on above the quality of installers is not great yet, the good ones are hide to find. There’s also a degree of pocketing some of the grant money.

I’m choosing to go multi split air to air, half cost to install, run costs similar, and I prefer the flexibility of temperatures across the rooms. Bonus I can install myself relatively simply.

Using the 2900 rule my 1930’s externally insulated 3 bed semi, 110m square needed 5.5kW.

Using Heatpunk I am finding that the recommended unit size is >8kW and an 8kW unit does not provide enough heat for my house. I need to spend more time playing with it and making sure my numbers are right although at the moment it seems to bear out my concern, that it is near the limit.

Interestingly I was running with a group yesterday and one of the guys is senior in grid renewables and lives near me (his house isn’t comparable, a large Victorian with high ceilings and large rooms) and he went for a 16kW ASHP and is very pleased with the performance.

I’m choosing to go multi split air to air, half cost to install, run costs similar, and I prefer the flexibility of temperatures across the rooms. Bonus I can install myself relatively simply

My brother, architect in NZ, has been specifying this way for the last decade in NZ, albeit in a warmer climate.

He’s just built the base for his own pump, which sparky is due to install this week. 60 year old 100m2 Aukland home with new windows, new insulation throughout and MVHR he self installed.

And as a comparison of how expensive the UK is on heatpumps – Daikin 5.5kw multi split pump at £1550 / NZ$3000 installed, on the base he prepared. UK cost is around £1800+vat unit alone, £2000 as multi split, and at least double that installed apparently…

He also has solar and battery due to be fitted in a month – looking at again half what the UK cost is…

Update in case anyone else finds this useful:

Decided to follow the installers guidance (they provide a minimum heating guarantee) and go for the 8kW unit. I used the Heatpunks website and that appeared to show 8kW was underpowered but I wasn’t able to populate it fully with things like our specific cavity wall insulation type, DG type etc and I suspect it was over estimating our heat loss as a result. Other less sophisticated models were indicating a heat pump size of around 5kW would suffice. Had a long conversation with the planner yesterday and he came across as extremely knowledgable and answered every question first time. He explained that the heat loss calcs after their detailed survey show the 8 is appropriate, after we increase our loft insulation as per their recommendation.  I asked to see the loss calcs and unsurprisingly he said he would send them over once we had signed the order. Made a number of mods to radiators types and sizes all of which they were happy to do at material cost. Just need to set an install date and have the house ripped apart for a week!!

Update:

install went ahead mid June and leaving aside a few minor issues the work went ahead as expected. They made an error with their calcs and had to do more pipework which set them back 2 days, but no grumbles from them.

Up until the last couple of weeks we havent needed to draw any heat from the pump and only used it for hot water, 3 of us at home and we have a 250ltr tank so that energy was all drawn either from solar or battery charge at one of the lower Octopus Cosy “windows” I know its pretty meaningless to estimate any savings over such a short period with so many variables but I cant help myself… Love Octopus but their billing is a nightmare (not helped as we change tariffs through the year and feed back which buggers things further)

So from 22/7 to 22/8 2023 (with our old boiler) vs 22/7 to 22/8 2024 (ASHP) we seem to have made a net saving of £52

Of course the winter months will be the killer but of our annual gas bill of £1608 in 2023, £904 of it was Dec, Jan, Feb and March so as they are the worst for solar, it will be interesting to see how high our electricity bills are then. I have an 8.7kW battery so may boost that by another 4kW to allow us to bridge between the 3 low cost Cosy windows.

In terms of the ASHP itself it is not silent but I have to be outside to hear it, and the only indication I have that it is heating water is my power usage spikes on my Home assistant. I know it is heating now as the outside temp drops but the house is comfortable (at the moment) at 18C and it seems to hold that rock steady hardly using any power.

Anyway though a bit of feedback may be useful