Green Building Forum - Air-source vs ground-source heat pumps - 2023 edition

Air-source vs ground-source heat pumps - 2023 edition

lineweight — Mon, 26 Jun 2023 11:03:26 +0100

At this moment in time is there any kind of broad consensus on when a ground-source heat pump should be considered for domestic houses?

My understanding is that some years ago, there was a quite big gap between the efficiency of AS and GS, but that this has closed considerably in the meantime.

Meanwhile, GS remains a fair bit more expensive to install.

So, is GS something that is only worth even thinking about on larger new-build projects, ones where a garden area is available, and ones where the budget is fairly generous?

Or are there other scenarios where it's worth consideration?

Air-source vs ground-source heat pumps - 2023 edition

Dominic Cooney — Mon, 26 Jun 2023 14:05:26 +0100

I put GSHP in our Barn Conversion and dug the 2 x trenches for the slinkies mysefl (with a 10T digger) to keep the cost down. RHi was a big incentive though. No mains gas here.
Not sure what to do for the next building conversion, if we can get near passive then a GSHP is probably overkill and the money better spent on PV instead, with immersion for DHW.

Air-source vs ground-source heat pumps - 2023 edition

Dominic Cooney — Mon, 26 Jun 2023 14:08:49 +0100

Still contemplating GSHP for the main farmhouse to ditch the oil, but slowly working our way round upgrading the insulation. I am making sure to put a suitable elec cable in from Consumer Unit to Plant Room (currently boiler room) and also larger mains water pipe from stopcock to plant room. (while the floor is up in the bathroom upstairs)

Air-source vs ground-source heat pumps - 2023 edition

lineweight — Mon, 26 Jun 2023 15:59:13 +0100

Posted By: Dominic CooneyStill contemplating GSHP for the main farmhouse to ditch the oil,

But why GSHP rather than ASHP?

Air-source vs ground-source heat pumps - 2023 edition

RobL — Mon, 26 Jun 2023 16:51:31 +0100

We have GSHP, but I think it is a niche technology.
Advantages:
No outdoor fan unit
No planning req'd
The compressor can be put inside or out, and can be acoustically insulated to make it silent as it doesn't need a huge airflow.

Disadvantages
Cost (if pro install)

Efficiency:
Usually similar. GSHP has a "physics advantage", but ASHP have the advantage of higher sales hence R&D budgets, so more sophisticated inverter designs.

We have a GSHP as I built and installed the unit to avoid the whole planning and MCS thing. It's a simple 2.5kW unit, as we have lots of insulation.

Air-source vs ground-source heat pumps - 2023 edition

tony — Mon, 26 Jun 2023 18:10:19 +0100

In other countries GSHP performs better than ASHP 4.5 compared to 3.5 CoP for ashp here GSHP seems not to perform very well but it should. I suspect scrimping on depth of heat field or boreholes

Air-source vs ground-source heat pumps - 2023 edition

Beau — Tue, 27 Jun 2023 08:57:43 +0100

GSHP user here. Not sure why there is such a difference in price other than the cost of a ground loop. We installed both the heat pump and the ground loop. The loop was very easy just hiring a digger for a few days.

Presumably, the better insulated your home is DHW becomes a bigger proportion of your energy use, and I can see ASHP may be on parity on this due to still needing hot water in warm summer conditions when COP should be better with ASHP than GSHP.

Air-source vs ground-source heat pumps - 2023 edition

Dominic Cooney — Tue, 27 Jun 2023 10:18:46 +0100

As RobL says
No outdoor fan unit. Plus we already have 'boiler room' where the heat pump would go and plenty of room for unvented cylinder. Although I guess it would be feasible to connect to an outdoor ASHP unit, the boiler room isn't on an outside wall.

I already have PP as I put in for the barn and the house at the same time (the slinkies are in the field, not in the curtilage, so technically needs PP)

As Tony says, I have been very impressed with the CoP of the Kensa GSHP installed in the barn. We did make the trenches a few metres longer than specified and looped the extra pipe to make about 7 more loops in each of the two trenches. there's also about 40m from the barn to the manifold in the field which will gain a bit of heat from the ground along the way.

As with all things, insulating the house better makes the whole thing cheaper and easier when it come to the heat pump, Air or Ground source.

Air-source vs ground-source heat pumps - 2023 edition

lineweight — Tue, 27 Jun 2023 10:37:37 +0100

The cost of a ground loop presumably is very dependent on location. In a rural setting maybe it's just a case of digging some trenches. But in an urban location space might be limited and much more to worry about in terms of disturbing other services below ground.

Air-source vs ground-source heat pumps - 2023 edition

djh — Tue, 27 Jun 2023 10:52:14 +0100

Posted By: Dominic CooneyI am making sure to put a suitable elec cable in from Consumer Unit to Plant Room (currently boiler room) and also larger mains water pipe from stopcock to plant room. (while the floor is up in the bathroom upstairs)

It's an interesting question what extra infrastructure to build into a house against possible future developments. I put several extra cables from the CU to the wet rooms and living room to serve possible heaters (I've subsequently fitted heaters to two of the five) and extra cables from the wet rooms to the plant room for possible humidity sensors for the MVHR (none fitted so far). I also fitted a spare duct between the house and garage so as to be able to run comms cables, but the end in the house is behind a plaster wall so in reality I used first a wifi connection and latterly a powerline system instead :( When I contemplate fitting a split ASHP I wish I'd run some more ducts for the refrigerant pipes and the power, but hey-ho. Hindsight is a wonderful thing.

Air-source vs ground-source heat pumps - 2023 edition

owlman — Tue, 27 Jun 2023 11:23:36 +0100

Ah! Regret with hindsight, I've been there also, all too often.

Air-source vs ground-source heat pumps - 2023 edition

gustyturbine — Tue, 27 Jun 2023 11:50:56 +0100

Lineweight, I'm in a similar position to yourself. I have currently got a GSHP (Worcester Bosch) and it's been very reliable. I was fortunate and got the RHI for this installation. Building again I don't have that luxury so I'm also considering GSHP, ASHP, oil as I'm off gas network. My concern with the ASHP is that I live close to the coast. I'm sure in 5-10 years I'd have a rusty ASHP looking worse for wear with noisy bearing spinning away. GSHP's are expensive though and I bored last time. I'm also considering 10kw PV for power and DHW in summer and using oil for winter months. I don't like fossil fuels but my GSHP does cost around Â£500 a month with some EV charging thrown in but not much. Oil would be more economical imo but I like renewables if the grid is green.

Air-source vs ground-source heat pumps - 2023 edition

lineweight — Tue, 27 Jun 2023 12:06:46 +0100

How much is noise an issue, in the end?

I don't feel like I hear much about it being a complaint but maybe that's just because they aren't currently widely in use.

There are plenty of urban sites where there might be 3 or 4 neighbours within say 10m of a window or an outdoors sitting area. If I try to imagine 3 or 4 ASHP units whirring away at that sort of distance, I can imagine it being a bit of an intrusion.

Air-source vs ground-source heat pumps - 2023 edition

revor — Tue, 27 Jun 2023 13:22:56 +0100

Posted By: tonyPresumably, the better insulated your home is DHW becomes a bigger proportion of your energy use

This is the dilemma I had as my previous abode the space heating was greatly in excess of the DHW one did not need to bother. When doing our current abode we had difficulty working out what we needed so put in solar thermal which negates any use of "fuel" from March to October it then takes the "chill out of the water in the thermal store in the winter which feeds the ufh.

Posted By: gustyturbineI don't like fossil fuels but my GSHP does cost around Â£500 a month with some EV charging thrown in but not much. Oil would be more economical imo but I like renewables if the grid is green.

If you do a very good job of the fabric 1st approach you do not need a sophisticated source of heating. We spend less than Â£300 pa on bottled gas which we started off with to see if we could mange without a propane bulk tank, As we do not need much the Â£100 "maintenance fee" would make bulk gas dearer than our bottled. We have 6kw solar pv + battery and we are more than cost neutral on electricity. The export last year covered the cost of the gas. House by the way is 340 sq M but lots of solar gain and thermal mass which makes a big difference. A wood burning stove in the lounge augments the heating.

Air-source vs ground-source heat pumps - 2023 edition

gustyturbine — Tue, 27 Jun 2023 17:11:48 +0100

revor - Many thanks. Can I ask how many people live in your house. I am a family of 5 (3 girls).

Air-source vs ground-source heat pumps - 2023 edition

Mike1 — Tue, 27 Jun 2023 18:33:35 +0100

Posted By: lineweightis GS something that is only worth even thinking about... where a garden area is available, and ones where the budget is fairly generous?

A garden isn't necessary - I've seen one in a London house with vertical boreholes through the floor, but that wouldn't have been cheap.

Air-source vs ground-source heat pumps - 2023 edition

revor — Wed, 28 Jun 2023 07:57:56 +0100

Posted By: gustyturbinerevor - Many thanks. Can I ask how many people live in your house. I am a family of 5 (3 girls).

Air-source vs ground-source heat pumps - 2023 edition

lineweight — Wed, 28 Jun 2023 09:07:19 +0100

Posted By: Mike1
Posted By: lineweightis GS something that is only worth even thinking about... where a garden area is available, and ones where the budget is fairly generous?
A garden isn't necessary - I've seen one in a London house with vertical boreholes through the floor, but that wouldn't have been cheap.

Not just expensive but very disruptive I imagine.

Air-source vs ground-source heat pumps - 2023 edition

djh — Wed, 28 Jun 2023 11:08:17 +0100

Posted By: lineweightNot just expensive but very disruptive I imagine.

Depends when it's done I expect. If done before the slab is laid, I'd expect it to be less disruptive than laying slinkies.

Air-source vs ground-source heat pumps - 2023 edition

Simon Still — Wed, 28 Jun 2023 11:17:54 +0100

Re Ground Source piles in London - piled foundations are often necessary in any case.

This build claims to combine the two -
"with a range of features including â€˜energy pilesâ€™ |(heating coils inserted within ten of the foundation piles) utilising a solar-assisted ground-source heat pump to create a thermal store beneath the building; PVTs (solar thermal panels);"

They're a neighbour. The house has changed hands a couple of times and I know there were some issues with the heating early on (the second owners replaced the GSHP with a larger unit I think) but, without knowing any of the tech details it *seems* like a good solution - combine structural piles with GS piles. IIRC they were combined Solar Thermal/PV panels with the idea being the thermal cools the PV increasing the efficiency and the excess hot water is pumped into the piles to create a ground thermal store.

I seem to remember reading (on here?) that that type of panel was not so great but would be interesting to have a proper tech analysis of the system.

https://www.archdaily.com/300175/slip-house-carl-turner-architects/50b7da21b3fc4b239a00011e-slip-house-carl-turner-architects-section?next_project=no

What I don't understand is this comment from the architects own site -
https://turner.works/thoughts/post/slip-house-in-details/
"The house is preforming brilliantlyâ€“once the heating is turned off it takes a good 12 hours for the temperature to drop by a degree or two. However you have to get used to not opening the doors unless itâ€™s 22 degrees outside"

All the floors (and ceilings) are precast concrete and it's described as having a high level of insulation - it should have huge thermal mass. Our own house only has a concrete ground floor (and Fermacell internal walls which must give a significant bump to mass over plasterboard) - you can open doors wide in really cold weather and the retained heat of the structure very quickly warms the internal air. So that seems odd.

Air-source vs ground-source heat pumps - 2023 edition

WillInAberdeen — Wed, 28 Jun 2023 11:59:42 +0100

Isn't the idea of a ground loop that solar heat falls on the ground in the summer and warms it up by a few degrees, storing heat for the ground loop to draw down in the winter.

How would that work in such a city situation where there's no bare ground for the solar heat to fall on to, just buildings and their shadows?

By drilling the ground loop downwards in a pile, you access solar heat that was stored some years ago and has permeated deep into the soil, so guess it would work for some years without solar recharge each summer, does it eventually 'run out'?

Air-source vs ground-source heat pumps - 2023 edition

Dominic Cooney — Wed, 28 Jun 2023 12:23:46 +0100

Maybe it takes the excess heat from the London Underground transport system which I seem to recall something on here about it increasing year on year in the surrounding soil.

Air-source vs ground-source heat pumps - 2023 edition

fostertom — Wed, 28 Jun 2023 12:33:54 +0100

Heat input to the ground is by contact with warm air as well as by insolation.
Where ground is covered by floor slab, that stays warmer year-round than open ground, whether heated or insulated or neither.
At a certain depth (2-4m usually), stable year-round ground temp is about the mean of year-round air temp (i.e. excl, or perhaps balancing, allowances for both summer insolation and winter clear-sky radiation loss) - say 10C.
That should be minimally depressed when heat is being extracted by a properly proportioned installation (GSHP vs slinky). Inasmuch as temp is depressed, heat is drawn in/replenished from surrounding areas.

Air-source vs ground-source heat pumps - 2023 edition

lineweight — Wed, 28 Jun 2023 14:31:16 +0100

Posted By: Simon StillRe Ground Source piles in London - piled foundations are often necessary in any case.

This build claims to combine the two -
"with a range of features including â€˜energy pilesâ€™ |(heating coils inserted within ten of the foundation piles) utilising a solar-assisted ground-source heat pump to create a thermal store beneath the building; PVTs (solar thermal panels);"

They're a neighbour. The house has changed hands a couple of times and I know there were some issues with the heating early on (the second owners replaced the GSHP with a larger unit I think) but, without knowing any of the tech details it *seems* like a good solution - combine structural piles with GS piles. IIRC they were combined Solar Thermal/PV panels with the idea being the thermal cools the PV increasing the efficiency and the excess hot water is pumped into the piles to create a ground thermal store.

I seem to remember reading (on here?) that that type of panel was not so great but would be interesting to have a proper tech analysis of the system.

https://www.archdaily.com/300175/slip-house-carl-turner-architects/50b7da21b3fc4b239a00011e-slip-house-carl-turner-architects-section?next_project=no" rel="nofollow" >https://www.archdaily.com/300175/slip-house-carl-turner-architects/50b7da21b3fc4b239a00011e-slip-house-carl-turner-architects-section?next_project=no

What I don't understand is this comment from the architects own site -
https://turner.works/thoughts/post/slip-house-in-details/" rel="nofollow" >https://turner.works/thoughts/post/slip-house-in-details/
"The house is preforming brilliantlyâ€“once the heating is turned off it takes a good 12 hours for the temperature to drop by a degree or two. However you have to get used to not opening the doors unless itâ€™s 22 degrees outside"

All the floors (and ceilings) are precast concrete and it's described as having a high level of insulation - it should have huge thermal mass. Our own house only has a concrete ground floor (and Fermacell internal walls which must give a significant bump to mass over plasterboard) - you can open doors wide in really cold weather and the retained heat of the structure very quickly warms the internal air. So that seems odd.

It may be unfair to judge a 2006 project in 2023... but spot the cold bridges

Air-source vs ground-source heat pumps - 2023 edition

djh — Wed, 28 Jun 2023 21:35:35 +0100

What's a 'Linit'?

Air-source vs ground-source heat pumps - 2023 edition

fostertom — Wed, 28 Jun 2023 21:53:32 +0100

https://linituk.com ? Yeah, pics look like it.

Air-source vs ground-source heat pumps - 2023 edition

lineweight — Tue, 04 Jul 2023 09:51:51 +0100

Interesting to read about this

https://www.salford.ac.uk/news/uks-first-roof-mounted-air-source-heat-pump-installed-eh20

If this proves feasible, it looks to me to solve the main problems with ASHP in tighter urban situations - the fact that they can use up valuable limited outdoor space, and the noise issue.

Air-source vs ground-source heat pumps - 2023 edition

WillInAberdeen — Tue, 04 Jul 2023 10:36:14 +0100

Thanks, interesting link! Heat pumps of the 'air-con' style have been mounted on urban roofs and upstairs walls for many decades. This one is mounted inside the loft with ducts to suck in air, which has been done before with EAHPs and DHW tank heatpumps.

Will be interesting if it takes off for CH units, internal noise and weight are the obvious issues. But its good to see new things being tried out, especially by a gas boiler company that was pushing for gas boilers to be retained in the hope that hydrogen might arrive for them!

Air-source vs ground-source heat pumps - 2023 edition

lineweight — Tue, 04 Jul 2023 10:44:23 +0100

I also came across one where the idea was that it's installed inside a loft space (would have to be an unheated one of course) and takes advantage of waste heat within that space.

Air-source vs ground-source heat pumps - 2023 edition

Simon Still — Tue, 04 Jul 2023 11:46:45 +0100

Posted By: WillInAberdeenThanks, interesting link! Heat pumps of the 'air-con' style have been mounted on urban roofs and upstairs walls for many decades. This one is mounted inside the loft with ducts to suck in air, which has been done before with EAHPs and DHW tank heatpumps.

Will be interesting if it takes off for CH units, internal noise and weight are the obvious issues.

It sounds good in principle but there are a few issues that it will be interesting to see the solutions for -
1) you've now put the supposedly noisy, supposedly vibration prone object within the house structure. I've heard of issues even with loft mounted MVHR units transmitting vibration through the house structure (which magnifies noise).

You're not just going to be dropping these on top of a board on top of ceiling joists

2) ASHP needs to move a lot of air - it's going to need a really big vent hole. And somehow you're going to need to silence that (otherwise any noise from the unit is just going to end up transmitted down the duct and still be audible to neighbours.

But encouraging to see alternative solutions. A lot more likely to work efficeintly than the AirCon units I saw in a French restaurant where they'd stuck them in a small space under a roof.