UKC

I'm current in search for my next home, and spending way too much time trying to decide what I would do with a house I'm interested in before even getting offers accepted. It's a problem.

One area of obsession is heating.

As I'm looking at places with some outdoor space, I assumed a GSHP would be ideal. I'd dig the trenches myself to keep costs down. Everyone's happy.

But the numbers seem to suggest that GSHP offer nothing over ASHP.

Take Valliant (picked randomly)

Their 12KW ASHP has a SCoP of 4.21 @ 45degrees. Unit for £6k

https://www.theheatpumpwarehouse.co.uk/shop/heat-pumps/air-source-heat-pump...

Now their equivalent GSHP is £7k (just for the unit, so more expensive, even though the unit should be simpler + cost of ground pipes on top of that). SCoP at 45 is 3.73 (11% worse).

https://www.theheatpumpwarehouse.co.uk/shop/heat-pumps/ground-source-heat-p...

This across the board it seems.

With ASHP there's also loads of manufacturer choice with a variety of prices you can pay, whilst very little with GSHP and all seem to be expensive.

Am I missing something, or is the idea that GSHP being superior due to the stable ground temps a bit of a red herring? 

Or is the SCoP for air source misleading?

The COP depends on the source temperature as well as the heating loop temperature.

You need to take that in to account; I’d be surprised if an ASHP in air at -10C beats a GSHP.  To really know you’ll herd climate data for your area and a source temperature model of the COP as well as an understanding of the geology to understand source temperature for the GSHP.

I’ll likely go for GSHP once we’re insulated enough as it won’t have the ******** noise of the outdoor compressor unit on an ASHP which invariably gets worse with bearing wear in the big fan.

I also like the idea of rejecting heat from a fluid loop on the next set of solar panels to go in and from a ducted A/C system in to the ground source loop to do seasonal heat storage.  The viability of that depends heavily on the local geology and so far I don’t have a clue on that.  It would also need a borehole not a low depth array which adds to the price…

The SCoP is supposed to take seasonal temperature variations into account. How well they do this is where my main uncertainty is.

That’s such an overly absurd generalisation as to be meaningless - where in the country do you live?  What’s your winter heating pattern like (temperature by day, evening and night)?  Whats your insulation like?  Do you use DHW or electric shower for bathing?

For a house that’s unheated for 8 months of the year and uses an electric shower the COP is going to be way worse than a house that’s heated to 24C for 8 months of the year and uses DHW for baths.   A difference of 1-2 in time averaged COP I’d expect 

Sounds like absurd marketing maths.

Apparently the SCoP should be calculated inline with Ecodesign Directive EN 14825, and the only locations provided for the calcs are:

• Average (Strasbourg)
• Warmer (Athens)
• Colder (Helsinki)

The UK typically uses Strasbourg I believe which isn't a great start for a comparable location. If it was Helsinki then I'd be pretty comfortable with the values.

This is a useful read on the topic

https://www.chiltrix.com/documents/App-B-Teknical-SCOP-NEF%20-EN14825.pdf

Noise is one big thing GSHP have over ASHP, if you hate fan/bearing noise and you can't site it somewhere away from where you'll be spending time that's worth considering.

I suspect the quality of the ground source itself has some bearing, a deep borehole into flowing groundwater isn't going to behave the same as shallow surface trenches or pipes in a lake.

By contrast, there's almost always a bit of a breeze to carry away the stale cool air from an ASHP and feed it fresh ambient air, even if it is at times cooler than the sub-surface.

It may also be that for commercial/subsidy reasons ASHP have jumped a model ahead of GSP in the development cycle benefiting from improved refrigerants etc.

jk

I think it's simply that ASHP is easier to install - just plonk the condenser down the side or back of the house and drill through the wall. GSHP means digging up the garden or having a drilling rig in the drive.

I built a house over the past year and I went for ASHP. I did a similar deep dive and came up with a similar conclusion: GSHP is just too expensive to ever make it worthwhile.

It might be that GSHP has financially viable applications if it was a single system to service several properties, but on a one property basis, it's just too expensive.

If you have extra money, much better idea is to splash it on covering every vaguely S-facing roof section with solar panels (unless you are totally surrounded by trees or tall buildings ofc). My system isn't even very big - 6kW - and it's still enough that my approx 140m2 detached house a cold and exposed, windy location is predicted to produce more power than it uses. Strongly consider a battery too.

I didn't even go all out with this in mind. It's a standard timber frame place with mostly regulation insulation but slightly thicker in a few locations, Nordan windows and doors and a bit of attention paid to taping so it isn't too air-permeable (don't overdo this or you end up too tight and with building control demanding an HVAC system). But it ticks none of the passive haus standards and there were no efforts to go overkill on anything (for reference, I found forums online where people talked about 300mm or more of PIR underfloor insulation, which I found completely insane).

I moved in 31st March and so far I've not run the heating even once. At the coldest time of the morning the main living area is between 19 and 21 degrees and during the middle of the day and afternoon, I often have to leave the door open to cool the place down.

To date the solar array has produced approximately 200% of the energy I have used.

The house I'm currently negotiating has an old portal frame barn with a south facing roof with enough room for 30KW of solar. It would need a pretty big battery to make use of that but if I get this house I will definitely explore this option. Starting small and building up.

It has been very warm so far this year, so may not be indicative of other years. House I'm looking at is listed with an EPC of G. Solid brick walls. Should be fun. 

This will all be subject to listed build consent but I really want a home with HVAC. It's high on my wish list. I could then go nuts sealing every draft.

This house is very much beyond what a 12KW ASHP can manage, but I like the idea of under sizing the heat pump to keep it 240V, but provide a really big tank capable of supplying the whole house.

That way if we have a particularly cold winder, should just be a batter of lighting a wood boiler to top the tank once a day when the heat pump is struggling. Could equally be done with oil, but would prefer wood. If I'm finding I'm lighting it too often, then I need to find more creative ways to insulate. 

Endless fun.

I helped a  friend of mine has do that last summer and he is very happy with the result. 

I've a few other friends who have ASHP's. Looking at their usage over the last 6 months it appears to be significantly more efficient, especially when you want it to be, during a cold spell. He has installed it in a similar(ish), 5 bed detached house, without cavity wall insulation, that you later describe, as like you he was concerned that an ASHP would be able to cope efficiently.

I'd be tempted myself, but it would involve disrupting Mrs J's raised beds.

That might be tricky. You'll need to apply to your DNO for permission and they want every single detail before giving you permission, including exact panel models, battery model, whether it's ac or dc, etc. etc. I think if you repeated this whole process again and again as you added to the setup, you'd be looking at an incredible amount of paperwork and hassel. Not to mention that all the cabling in the setup depends on the max current it will carry, so adding to an existing setup piecemeal could be problematic and you might struggle to find a solar installer by the 3rd or 4th stage when they are looking at an already complex set of existing work and worrying what unknown problems they might find. If you can get permission to put in 30kW, you'll also be needing to consider things like a split or 3 phase supply and that might mean a new transformer from the DNO and in areas with no 3rd phase, you might be getting an almighty quote for adding a third wire to all the local electrical poles.

Yes and no. The system has been running since the start of December 2024 and there were quite a few times during the winter when the heating was running flat out because there were no thermostats and taping/filling/painting was in progress and needed dried out. And we've really only just started summer.

Your project sounds like a grand design, is the plan to make something like this?

https://www.yorkshirepost.co.uk/webimg/b25lY21zOmNmYzc4ZWE4LTg2YzMtNDE2Ny04...

I have a wood burner in the dining/lounge too. That is the other reason I hope to use less electricity during the winter. But it won't heat the water, just the air.

Noise is a very big issue.

I'm probably about to serve an abatement notice on one, which has MSC certificate....but is causing a Statutory nuisance to the neighbour. 

Basically they'll have to move it or remove/ stop using it.

You don’t need permission and supply upgrades to put in 30 kW but to export 30 kW.

I’m planning to expand our solar capacity to around 20 kW.  We’ll use a current sensor in the meter cupboard and appropriate inverters to limit the export to 4 kW max for the regs.  Internally the wiring is sufficient for all 20 kW to be used this side of the meter cabinet.

https://www.sma-uk.com/solar-systems/export-limitation

… and the units are only just starting to age to the point of noise inducing wear.

A predictable issue that purchasers weren’t warned about - I wonder if there’s going to be a low grade mis-selling scandal and Lawyers4U over the likely long term maintenance costs not being advertised.  It should still be cheaper than boiler maintenance and replacement cycles mind.

No idea what it’s like for ASHPs but my long running experience with our site’s HVAC support engineers is that by the time things get faulty, they’re more more interested in trying to sell a new system than to fix the old one, and spare parts for the old one are invariably hard to find…   

If your inverter is over 3.68KW then you can use a G100 compliant export controller which the DNO can control to limit your export. You could even have export limited to zero if that's what the DNO requires.

In which case I would have no choice but to build a heated swimming pool, which cools the house down at the same time. No point wasting those electrons.

Do you meant the portal farm building? I've no idea what I would do with that yet. It currently has no walls. There's also an overage on them so I would want to do anything habitable with them.

The house itself is a Georgian farm house.

The OP didn't mention being off grid, so it seems reasonable to assume they are not. If you have a grid connection and are putting 30kW of capacity on your roof, I would say that the best financial decision would be to try and get approval of the max size of export you can. There are a lot of details to consider but fundamentally if you have a comparably small battery bank then you want to be able to export a relatively heavy load onto the grid during a sunny summer afternoon when the batteries are full and you aren't really using any of it and, conversely, if you have a big battery bank, you still want to be able to dump a heavy load onto the grid so you can export all your stored stuff during the short peak period in the late afternoon where you get by far the best payment for it.

3.68kW

I was chatting to someone 2 months ago about their choices for ASHP, GSHP or gas combi.

They immediately assumed GSHP was the way to go. But .. they looked at their garden and it was steep and rocky and sized as a generous garden. But the sheer size of ground source needed was immense - bigger than any normal suburban garden and unsuitable for their rocky land anyway. The borehole needed to be 40m deep and would cost thousands, I didn't hear exact figure but I gather it was an enormous cost. Do some maths into the sizes and costs of ground source and you might find it a "WTF????" moment

Who said anything about being off grid?  I certainly didn’t.  My post was very clearly referring to a grid connected setup.  I don’t know why you have mentioned this?

As you went to great lengths to point out, this could be quite costly if the wiring - and even local transformer - is not up to spec.

To re-quote you:

Obviously if the OP can export all their power it makes sense, I was simply pointing out that if - for the reasons you give - it doesn’t make financial sense to export it all - that contrary to your post they can still go ahead by using export limit control.

I’m in that boat - 3-phase power doesn’t come within a mile of us.  We’ll go large in capacity and export limit it.  Still works out financially beneficial and gives us a meaningful level of power in the winter.  

It's quite possible you'll be able to get a G100 certificate to allow export of all 30kW.  It seems to depend largely on what existing solar there is in the area feeding back into the local transformer.  I had no problem getting one to export 6kW at home.  I put just shy of 100kW on the roof at work - we already had a three phase supply, but there was no issue with getting permission to export all of it.

On the storage side, give thought to thermal as well as battery storage.  At home I have a 9.5kWh battery, but also an 800L thermal store.  Big tanks of water are cheaper than more and more batteries.  They are quite a lot bigger, though...

I'd never get 30KW of invertor power to export. Way too pricey. 10kw seems like a useful reasonable limit if not a bit overkill perhaps. The only reason to get 30kW of solar is to try and spec the system so it's not useless in winter.

About half a mile away they are building a massive solar farm. So either it's a drop in the ocean and they don't care, or they consider it at capacity.

new installation in this case

Ah, I read "The house I'm currently negotiating is an old portal frame barn" not "The house I'm currently negotiating has an old portal frame barn", which makes quite a difference, doh!

Yes because of the input temperatures can vary massively (as can the feed/return dor the heating system). Plenty of the claimed CoP seem to use "interesting" values.

There are plenty of real-life comparison studies out there, the primary takeaway is in real cold places GSHP outperforms air source massively. For the UK there is a study using  OFGEM data from continous monitoring of a substantial number of installations and the main points are GSHP outperforms ASHP by a long way and the forecast performances are wrong.

ASHP median actual CoP 2.65. Forecast SCoP 3.61

GSHP median actual CoP 3.24. Forecast SCoP 3.95

One of my drinking buddies has a business installing the things and for my area it's GSHP all the way (it gets pretty cold in Bavaria) but the installation costs can vary wildly or might not be possible, my neighbour for instance had to bore 30m down to get to deep ground water, he is by a river and the environment agency don't allow them to affect the winter river temperatures. I've measured my groundwater and 2.5m down it manages 6° in the depths of winter and the extra efficiency of going down another 10m would pay off for me (in the long run). The loop system has to go around 1m deep unless you are using brine which for the size I'd need would be some brutal groundwork and the guys that have them say you can see the efficiency drop-off as the winter proceeds.

We have a NIBE GSHP with a 120m borehole supplying heating and hot water to a 150m2 house in the Scottish Borders. Probably overkill in terms of technology for such a relatively small house but it performs fine for us. It's been in 18 months, so two albeit mild winters now. It's been running a ratio of output to input of 4.6 over that period. And if/when I get round to fixing one of our windows it will improve further.

Not cheap to install mind; you could probably buy a stack of ASHP for the same money. But the operating noise levels are pretty good - the pump lives in the utility room and most of the time we can barely hear it, and even during the weekly legionella burn still only makes the same noise as our fridge.

The borehole hit water at about 16m down - we had checked out the scanned handwritten borehole records for the area on the British Geological Survey website. In terms of depth the calculation/guess was the hole would provide 50 watts per meter drilled; the 120 metres gives us a theoretical 6KW of heat which is well within the design for the house.

Nice and quiet for the neighbours too. Wish I could say the same about the powered treatment plant SEPA insisted we install ...

Interesting! The opposite is the case here in Scandinavia; air-air heat pumps are the norm and are everywhere. 60% of houses in Norway have heat pumps and 90% of those are air-air so more than half of all households, despite it being quite nippy

I guess a cop of 3.24 vs 2.65 implies ~22% cheaper bills for gshp; according to google average heating bill in Oslo 30kNok, ghsp installation 170kNOK - 250kNOK, ASHP installation 10kNOk - 30kNOk so would take ~32 years for the gshp to become cheaper than ashp for the average Oslo-ite assuming those scop.

Scop here is probably worse given it gets actually cold but still probably not cost effective for most people if the return horizon is 20+ years, I guess really big houses probably win though. In the UK worse insulation, more expensive electricity, and cheaper installing costs would change the maths...though if scoping for 30 years including climate change impacts might too

Probably the drilling costs are horrific in Norway, I live on kilometer-deep sandy junk!

Ah yes good point, I was thinking of labour costs which are also proportionately high compared to most places but drilling into granite is probably a much bigger factor... Given their general love of tunneling it is a bit surprising they haven't got into drilling up their back gardens despite the cost.

There's another system which I don't know much about but I'll ask my buddy, I only saw a stack in a garden as I drove by. They are spirals of formed blue plastic tube maybe 50mm diameter wound up about 30cm across which are installed using a standard post-hole auger and I guess two or three metres deep like they use for telegraph poles so cheap enough, there were maybe eight of them in  the heap. I shall enquire, they would be ideal for my  place I guess.

Edit. I found them, they are called spiral collectors and usually 2.5-3m deep and ca 50cm dia and depending on local conditions produce around 0.5kW each. One picture shows them being installed by a normal wheeled excavator with a rotary head driving an auger. They can also be laid horizontally in a normal trench.

I looked into ASHP and GSHP for our new house. The costs of installation for a GHSP made it a non-starter. The ASHP is very quiet (but it is new so might become noisier with age). I think GSHP is an option if you have a lot of land, but boreholes seem prohibitive.