ASHP vs GSHP

[SteamyTea]

29 minutes ago, pstunt said:

know how to apportion the limited budget.

Get the airtightness sorted.

All the rest becomes a smaller problem.

[Omnibuswoman]

46 minutes ago, SteamyTea said:

Get the airtightness sorted.

All the rest becomes a smaller problem.

 
I second this. We are building a near passive house and the key to this is air tightness. 

[SteamyTea]

36 minutes ago, Omnibuswoman said:

 
I second this. We are building a near passive house and the key to this is air tightness. 

Are you going to get an interim airtightness test before you close everything up?

[binary]

Hi This may help, if you install yourself you may be best with a monobloc version

 

air to water heating and cooling .pdf

[saveasteading]

On the principles of ASHP and GSHP.

 

I have spent a huge amount of time researching GSHP. Firstly for business as I needed to know what is best, and sustainable, for clients. Secondly because some 'affordable' housing near me had GSHP that did not work, and I was helping the shivering occupants. Included some 'off-the -record' chats with specialists.

In doing this I established as follows. I had best say 'in my opinion'.

 

ASHP works in small and large buildings. 

 

GSHP by slinky also works. It is very simple and understandable. However it needs lots of open ground. What has only recently been admitted is that it is not really ground heat. It is summer sun and air heat absorbed by the top surface, and harvested in the winter. Therefore it needs warmth on the area used. It is possible to 'run out' of heat. Reversing the flow in the summer can store heat for the winter.

 

GSHP by borehole only works in the right ground conditions, which are rare. This was not admitted say 15 years ago, but is now. Not remotely sensible in heavy clay, or dense cold  UK rock, as the surface is not rewarmed by conduction or local water movement. Boreholes freeze and need to be warmed by electric heating. It has become standard that the ground has to be reheated in the summer, for which there is a running cost.

You only have to look at the numbers of contractors then and now. In a lot of cases it was installed on the basis of the grant available, rather than on sensible assessment.

On the 'affordable' housing, many of the occupants have had the boreholes disconnected, and air-source installed, by either the housing association or contractor...I don't know which as I am told they are all sworn to secrecy, in case word gets out.

 

The downside of ASHP is noise. It is possible to build a timber surround that reduces noise but allows enough air-flow.

 

 

 

 

 

 

[Bonner]

1 hour ago, binary said:

Hi This may help, if you install yourself you may be best with a monobloc version

 

air to water heating and cooling .pdf 138.38 kB · 8 downloads

 

Looks very similar to these on eBay, with health mark up!

 

[SteamyTea]

6 minutes ago, saveasteading said:

The downside of ASHP is noise. It is possible to build a timber surround that reduces noise but allows enough air-flow.

Does not have to be.  That is usually a symptom of an undersized unit, or one that has had the kids bikes thrown against them too often.

[ProDave]

4 minutes ago, saveasteading said:

The downside of ASHP is noise. It is possible to build a timber surround that reduces noise but allows enough air-flow.

 

Noise compared to what?

 

I keep saying, My ASHP probably makes about the same noise as the roar of an oil boiler burner, except the ASHP noise is outside so does not bother me.  The only time it might bother me is sitting in the garden in the summer, but if it's warm enough to sit outside, it would not be heating the house, just the hot water, and you could go and turn it off and let it heat the water at a different time if that bothered you.

[binary]

Just a note on the heat pump information i included: the cost of the heat pump can be reduce if you are willing to locate a different delivery option the cost is also in US dollar so your pound will go further, as for Monobloc version you won't need a engineer to install under Part F gas as this type of unit is an enclosed unit different to a split version which needs gas charging   

[Dan F]

Another ASHP here:

- The efficiency of a GSHP isn't much better than an ASHP. 

- Significnalty cheaper than GSHP.  With a highly insulated/airitght house the payback time on extra cost of GSHP for better COP is a very very long time.

- Better RHI payments on GSHP aren't worth much if you have a highly insulated/airitght house that doesn't use much to heat it to start with.

- Noise on a good unit isn't much more than a fridge and it'll be down the side of the house. Prefer this to noise inside the house.

- GSHP requires AFAIK more room inside the house.

 

That said, if was building in a much colder climate then I would definitly reconsider this and GSHP may make a lot more sense in this case.

[Jilly]

2 hours ago, saveasteading said:

On the principles of ASHP and GSHP.

 

I have spent a huge amount of time researching GSHP. Firstly for business as I needed to know what is best, and sustainable, for clients. Secondly because some 'affordable' housing near me had GSHP that did not work, and I was helping the shivering occupants. Included some 'off-the -record' chats with specialists.

In doing this I established as follows. I had best say 'in my opinion'.

 

ASHP works in small and large buildings. 

 

GSHP by slinky also works. It is very simple and understandable. However it needs lots of open ground. What has only recently been admitted is that it is not really ground heat. It is summer sun and air heat absorbed by the top surface, and harvested in the winter. Therefore it needs warmth on the area used. It is possible to 'run out' of heat. Reversing the flow in the summer can store heat for the winter.

 

GSHP by borehole only works in the right ground conditions, which are rare. This was not admitted say 15 years ago, but is now. Not remotely sensible in heavy clay, or dense cold  UK rock, as the surface is not rewarmed by conduction or local water movement. Boreholes freeze and need to be warmed by electric heating. It has become standard that the ground has to be reheated in the summer, for which there is a running cost.

You only have to look at the numbers of contractors then and now. In a lot of cases it was installed on the basis of the grant available, rather than on sensible assessment.

On the 'affordable' housing, many of the occupants have had the boreholes disconnected, and air-source installed, by either the housing association or contractor...I don't know which as I am told they are all sworn to secrecy, in case word gets out.

 

Really interesting. I know a surveyor who told me from his knowledge of geophysics, he couldn't see how they could work and would be liable to 'run out' of heat. I'm guessing we all erroneously intuited that the heat was from proximity to magma ?

[saveasteading]

Magma: I think you are right. this was implied if not stated.

 

I really wanted GSHP principle to work. Went to a Mitsubishi demo at their HQ, and slinkies clearly worked but needed so much space.

 

Went to industry exhibitions where the reps of borehole companies all swore that boreholes worked everywhere.  Give me a quote then.  In Kent???yes.. On clay?   yes...but clay is not suitable!   ok then you might be right. They went quiet and then changed their tune completely  The moral I think was, they would do it anywhere but not target technical people who know it is silly or will test it for value.

An industry expert (really top) told me that there had been lots of cowboys who lost their industry most of its credibility.

I assume that the companies still around are better and honest.

 

 

Also went to lots of 'sustainability' talks and GS was always mentioned as being credible, along with little wind turbines. Unfortunately lots of designers fell for it, and clients paid.

 

The person I mostly helped in the 'affordable' development was in a 3 bed new house, and spent £2,000 a year on electric, with most going to heating the ground to 120m depth I think. The air-source replacement works fine, Just a tiny one.

 

Can you see which I favour?

 

Other comments are very helpful, thanks all. I will be back for advice as I have only worked on commercial and sports buildings, which are very different to domestic.

My  fence will also keep the bikes and footballs off the unit.

 

 

 

 

 

 

[ProDave]

Our village hall is heated by a GSHP from a borehole and it does not stop working in winter. 

[JamesP]

2 hours ago, ProDave said:

Noise compared to what?

 

I keep saying, My ASHP probably makes about the same noise as the roar of an oil boiler burner, except the ASHP noise is outside so does not bother me.  The only time it might bother me is sitting in the garden in the summer, but if it's warm enough to sit outside, it would not be heating the house, just the hot water, and you could go and turn it off and let it heat the water at a different time if that bothered you.

Noise concerned me about the ASHP (Ecodan) and the original plan was to site the unit behind the garage some 8 metres from the house. The installers suggested as close as possible to the UVC and it sits just outside the side door. It is not intrusive probably being in a rural location with no fixed objects in front of it, might be different in an urban garden bouncing off the neighbours fence.

Looked at GSHP (slinky's) and just could not accept the disturbance of digging hundreds of metres of trenches. 

Insulation and airtightness first.

[saveasteading]

10 minutes ago, ProDave said:

Our village hall is heated by a GSHP from a borehole and it does not stop working in winter. 

 

And some of that is probably working in reverse (straight electric heat) to melt the ice for 100m into the earth?

Any idea of the bill?

 

Suggest have a jumble sale for funds to change to Air Source.

[puntloos]

Wow tons of discussion, thank you all. Some thoughts:

 

Budget> in my area, and assuming I can get RHI, the raw price difference isn't huge. Interesting point about refreshing the brine and maybe antifreeze. Need to look into running cost somewhat

Running out of heat to draw> Strange! I'm somewhat surprised that drawing heat out will be a problem, wouldn't "balancing it out" in the summer not work? I've certainly heard that the london underground slowly heating up so ineed the idea in principle doesn't sound crazy. 

Slinky> One installer I spoke about said the slinky wasn't great but 'strings' were better. I don't know what the difference is really. Also, how deep do these slinkies lie? I have a small kid, if he can dig it up I'm sure one day we'll be out of heat. But also I guess what about trees that over the years start to crowd out the string?

Gas> eh, I don't think it's a good enough option to really care about. Yes it's cheaper but it costs the environment much more so doesn't seem good to install anymore (if you have viable other options, of course)
Self Install> I know this is buildhub but not something I personally dare attempt. Perhaps I can manage (no) but even so I wouldn't want to be responsible if anything goes wrong or is poorly done.

Noise> It all depends. From the Nibe manual it hovers between 32 and 47dB where eg a random fridge I found is 44dB so 'eh', and the outdoor units of ASHPs are definitely somewhat worse. (first random one I found was 40-60dB, so I guess it depends where you can stow it. 

Size/Location> Again depends on where you have the space you care about, but given my cramped garden and largeish house I am tending towards effectively a standing fridge device you can put anywhere, rather than a huge outdoor device that needs decent clearance from all sides etc etc.

 

 

8 hours ago, SteamyTea said:

What puts most people off then comparing is the "known unknowns". Or the weather and build quality. Weather can be calculated pretty well within small errors, build quality can't. Why it amazes me that people don't get an interim air test done while it is still possible to easily fix things.

£250 for a test, people spend more on a tap.

 

What does an interim test do? Any details? Between when and when would you run it?

[pdf27]

1 hour ago, saveasteading said:

The person I mostly helped in the 'affordable' development was in a 3 bed new house, and spent £2,000 a year on electric, with most going to heating the ground to 120m depth I think. The air-source replacement works fine, Just a tiny one.

One of the critical factors here is that with an air-source heat pump if you need a bit more energy you just turn the fan up. With ground source the heat has to come to the collector - either through moving groundwater or thermal conduction through the earth. Conduction will always work - it's pretty much a statement of the zeroth law of thermodynamics that energy will flow from hot to cold - but this requires a temperature gradient and as a result if you pull too much heat out for a given length of ground loop this temperature differential will get very big and you can easily freeze it up. That's very likely to be what happened here - the ground loop was too small and froze the ground up. Rather than spending the electricity to try and thaw out the ground, the GSHP was working really hard to extract heat from a source at -10°C - while the new ASHP can pull the same amount of heat from air at +5°C. That requires half the work, and hence gives a much better COP.

 

6 minutes ago, puntloos said:

Wow tons of discussion, thank you all. Some thoughts:

 

Budget> in my area, and assuming I can get RHI, the raw price difference isn't huge. Interesting point about refreshing the brine and maybe antifreeze. Need to look into running cost somewhat

Running out of heat to draw> Strange! I'm somewhat surprised that drawing heat out will be a problem, wouldn't "balancing it out" in the summer not work? I've certainly heard that the london underground slowly heating up so ineed the idea in principle doesn't sound crazy. 

Slinky> One installer I spoke about said the slinky wasn't great but 'strings' were better. I don't know what the difference is really. Also, how deep do these slinkies lie? I have a small kid, if he can dig it up I'm sure one day we'll be out of heat. But also I guess what about trees that over the years start to crowd out the string?

Gas> eh, I don't think it's a good enough option to really care about. Yes it's cheaper but it costs the environment much more so doesn't seem good to install anymore (if you have viable other options, of course)
Self Install> I know this is buildhub but not something I personally dare attempt. Perhaps I can manage (no) but even so I wouldn't want to be responsible if anything goes wrong or is poorly done.

Noise> It all depends. From the Nibe manual it hovers between 32 and 47dB where eg a random fridge I found is 44dB so 'eh', and the outdoor units of ASHPs are definitely somewhat worse. (first random one I found was 40-60dB, so I guess it depends where you can stow it. 

Size/Location> Again depends on where you have the space you care about, but given my cramped garden and largeish house I am tending towards effectively a standing fridge device you can put anywhere, rather than a huge outdoor device that needs decent clearance from all sides etc etc.

• Assuming you can get RHI would make me nervous - if for any reason you can't it's a hell of a bill to deal with, and getting it is reliant on the whim of the current government.
• Essentially ground source heat pumps work on the fact that there is a lot of thermal mass in the soil. That's why the earth is at ~10°C at depth - it's the average air temperature for the UK over the course of a year. Depending on your geology and how much heat you have to dump, recharging a borehole from waste heat **may** work: it works very well for the underground as they've got a load of heat from passengers & trains plus a load of cold water to get rid of, but unless you live in a triple-glazed greenhouse then you're probably going to be relying on heat coming from elsewhere. The normal route is a mixture of rain and flowing groundwater.
• Basically, a slinky is coiled pipe laid in a trench, "strings" will be straight pipe. To get the same performance, the slinky will need more pipe and the straight pipe will need more trenching. I'm looking at straight pipe because the loops are a bit more flexible (so easier to fit under my lawn), pumping losses will be slightly lower and it'll use less antifreeze. Again, system size is critical - my calculated heating load is about 2kW, and that means I can get away with 3 x 15 m trenches with a loop of pipe in each and still be heavily oversized.

[Marvin]

14 minutes ago, pdf27 said:

To get the same performance, the slinky will need more pipe and the straight pipe will need more trenching.

I assume you will be spacing the trenches for a slinky wider apart....?

[Marvin]

16 minutes ago, pdf27 said:

Again, system size is critical - my calculated heating load is about 2kW, and that means I can get away with 3 x 15 m trenches with a loop of pipe in each and still be heavily oversized.

I assume you realise that the surface area of the ground and the depth of the pipe/slinky in that ground determine the efficiency effectiveness of a GSHP?  

[pdf27]

25 minutes ago, Marvin said:

I assume you will be spacing the trenches for a slinky wider apart....?

The MCS sizing calculation gives a single 30m slinky or 85m of straight pipe at 1m spacing for my predicted heat load and ground conditions. Garden is 40m long, so no reason to consider more than one slinky. Trenches for the straight pipe would be bucket width and 5m apart which should be good enough.

Just noted an error in my earlier statement - 3 x 14m gives me the pipe length requirement from the MCS calculation which is probably rather pessimistic as nearly half my calculated load is for hot water and I'd use it for summer cooling as well - the calculation basically assumes heating only. I'm actually assuming 3 x 20m trenches which is the longest I can go without some trees starting to be a nuisance for the digger. Still nearly 50% oversized however.

 

22 minutes ago, Marvin said:

I assume you realise that the surface area of the ground and the depth of the pipe/slinky in that ground determine the efficiency effectiveness of a GSHP?  

Yes, although in practice depth is less critical than you'd think - the difference between 1m and 2m is pretty minimal for instance. Surface area is critical, but for a low energy house you really don't need very much of it.

[IanR]

45 minutes ago, pdf27 said:

my calculated heating load is about 2kW,

 

Is that your calculated energy loss? So you need 2kW to maintain say 21 internal at -10 external?

 

If so, I wasn't comfortable sizing my HP on this value, I felt a bit of head room was needed, so that in the same conditions the heating was sufficient to raise the internal temp from a lower value (in the case there'd been a heating failure), as well as heating a tank of water up in a reasonable amount of time.

 

Or does the 2kW already factor that in, in which case it is a very low value, could you not just go with electric duct heater in your MVHR,

 

 

[SteamyTea]

1 hour ago, puntloos said:

Wow tons of discussion, thank you all. Some thoughts:

 

Budget> in my area, and assuming I can get RHI, the raw price difference isn't huge. Interesting point about refreshing the brine and maybe antifreeze. Need to look into running cost somewhat

Running out of heat to draw> Strange! I'm somewhat surprised that drawing heat out will be a problem, wouldn't "balancing it out" in the summer not work? I've certainly heard that the london underground slowly heating up so ineed the idea in principle doesn't sound crazy. 

Slinky> One installer I spoke about said the slinky wasn't great but 'strings' were better. I don't know what the difference is really. Also, how deep do these slinkies lie? I have a small kid, if he can dig it up I'm sure one day we'll be out of heat. But also I guess what about trees that over the years start to crowd out the string?

Gas> eh, I don't think it's a good enough option to really care about. Yes it's cheaper but it costs the environment much more so doesn't seem good to install anymore (if you have viable other options, of course)
Self Install> I know this is buildhub but not something I personally dare attempt. Perhaps I can manage (no) but even so I wouldn't want to be responsible if anything goes wrong or is poorly done.

Noise> It all depends. From the Nibe manual it hovers between 32 and 47dB where eg a random fridge I found is 44dB so 'eh', and the outdoor units of ASHPs are definitely somewhat worse. (first random one I found was 40-60dB, so I guess it depends where you can stow it. 

Size/Location> Again depends on where you have the space you care about, but given my cramped garden and largeish house I am tending towards effectively a standing fridge device you can put anywhere, rather than a huge outdoor device that needs decent clearance from all sides etc etc.

 

 

 

What does an interim test do? Any details? Between when and when would you run it?

It is most useful in ICF and TF construction  You do it as soon as the exterior walls, roof (can be tricky with a cold roof), doors and windows are in, but in the case of TF, before insulation is in.

The idea is that you can seal any gaps in the external wall before it is insulated (that just gets in the way) and boarded out (which has the VCL behind it).

 

There is probably no 'one time suits all builds', the idea is to test before everything is closed up, as as I explained earlier, the VCL is not the airtight layer, it is on the warm side of the building.

Edited May 18, 2021 by SteamyTea

[pdf27]

1 minute ago, IanR said:

Is that your calculated energy loss? So you need 2kW to maintain say 21 internal at -10 external?

 

If so, I wasn't comfortable sizing my HP on this value, I felt a bit of head room was needed, so that in the same conditions the heating was sufficient to raise the internal temp from a lower value (in the case there'd been a heating failure), as well as heating a tank of water up in a reasonable amount of time.

 

Or does the 2kW already factor that in, in which case it is a very low value, could you not just go with electric duct heater in your MVHR,

180m² house, planning to go the full Passivhaus route which means 10W/m² at the design low temperature condition. It's worth noting that the ground array sizing is based on the annual heating requirements in any case, rather than the peak ones: soil has a hell of a lot of thermal mass, and the limiting factor is thermal conductivity within the soil so a cold snap really doesn't come into the sizing calculations. The ground loop sizing calculations are based on 15kWh/m²/year for heating and 10 for hot water as the total heat demand to feed into the MCS calculator.

 

The reality is that the smallest heat pump (air or ground to water) I can find out there is 3kW, and in extreme cold conditions should you need headroom you could always use an immersion heater. That means I'm at least 50% oversized whether I want to or not, and can provide all the required heating and hot water from the heat pump down to the design cold condition without needing additional heat. Planning for warming the house up again after a heating failure during a cold snap is overkill for me - I'd just grab a couple of fan heaters and run them until it was over, and it's worth noting that the house would have a very long time constant against cooling down in that case anyway. It would probably be possible to go smaller with an air conditioner, but that would annoy me a lot.

 

An MVHR duct heater would be a very cheap solution, but some form of active cooling is non-negotiable for my wife: she grew up in the US where they all have air conditioning and bitches about the lack of it every year. I'm not even going to try convincing her that a well-insulated house really doesn't need it. The Kensa Shoebox 3kW can be built with a passive cooling module while the Nilan Compact P Geo 3 can also do cooling. If either comes out too expensive we're likely to switch to an ASHP - but again a very small one.

[Dan F]

7 minutes ago, pdf27 said:

If either comes out too expensive we're likely to switch to an ASHP - but again a very small one.

GSHP for heating + ASHP for cooling, or did I misunderstand?

[SteamyTea]

5 minutes ago, pdf27 said:

soil has a hell of a lot of thermal mass

No it don't, it has a reasonably high specific heat capacity when wet.  About a third of what water has (as long a sit is not frozen, or steam)

From here:

https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html

 

Units are J.kg^-1.K^-1

 

Soil, dry	800
Soil, wet	1480