Switch to ground source heat pump with RHI

jn6543

I have moved to a cottage in the Forest of Dean which has an oil boiler with oil tank in the garden. I was looking into the possibility of switching to a ground/air source heat pump. I spoke to a local service - Forest Eco Systems - which installs them, and based on my property's EPC they suggest that I could get RHI equal to £25000 over 7 years and the cost of the project would be around £20000. I have the money available, but as it is a big outlay I am looking to get more information about whether this is the best option. I could for example just continue with the old oil boiler for the time being or opt for air source heat pump which I have been quoted for £14000 from the same company (with £11000 RHI over 7 years). Any wisdom on this would be much appreciated. Thanks in advance.

Reed_Richards

I looked into Ground Source Heat Pumps before opting for Air Source.  For ground source you either need a large area of "lawn" where you can lay pipes under the surface conveniently; a paddock would be ideal.  Alternatively you would need to drill boreholes but I presume the £20k quote is for the former option (because borehole drilling is expensive).  GSHPs are supposedly more efficient that ASHPs, although I don't really understand why.

RHI is scheduled to be abolished next March and historically grants for eco-friendly projects have been replaced by something less good.

I have not yet run my ASHP for a full year but it is shaping up to have a running cost similar to the oil boiler it replaced.    

QrizB

Reed_Richards said:

GSHPs are supposedly more efficient that ASHPs, although I don't really understand why.

Soil temperature is a lot less variable than air temperature, so GSHPs don't lose COP when there's a cold snap (which is when you most need heating). I saw a chart once indicating that GSHP COPs fall slowly over the heating season as the ground they are in cools.

My sister has a GSHP, it was commissioned last year and she's happy with it - but she has a smallholding, not just a garden!

Reed_Richards

QrizB said:

Reed_Richards said:

GSHPs are supposedly more efficient that ASHPs, although I don't really understand why.

Soil temperature is a lot less variable than air temperature, so GSHPs don't lose COP when there's a cold snap (which is when you most need heating). I saw a chart once indicating that GSHP COPs fall slowly over the heating season as the ground they are in cools.

I can see how the better efficiency of a GSHP could be true but it is not intuitively obvious.  You might think it would be swings and roundabouts where the ASHP wins out at those times of year where the air temperature is greater than the soil temperature, which must be most of the time.  But the GSHP wins when you need the most heating and that must be what enables it to win-out overall. 

Martyn1981

Reed_Richards said:

QrizB said:

Reed_Richards said:

GSHPs are supposedly more efficient that ASHPs, although I don't really understand why.

Soil temperature is a lot less variable than air temperature, so GSHPs don't lose COP when there's a cold snap (which is when you most need heating). I saw a chart once indicating that GSHP COPs fall slowly over the heating season as the ground they are in cools.

I can see how the better efficiency of a GSHP could be true but it is not intuitively obvious.  You might think it would be swings and roundabouts where the ASHP wins out at those times of year where the air temperature is greater than the soil temperature, which must be most of the time.  But the GSHP wins when you need the most heating and that must be what enables it to win-out overall. 

Not sure about air temps being higher most of the time. The mean annual temp of the soil (down to about 10m) will be roughly equal to the mean annual air temperature for any given location, since both are being heated by the same source, the Sun.
For very deep vertical ground source (over 100m) then you actually start to benefit from geothermal heat. The surface layer down to about 1m tends to have a slightly higher annual average than the local air temp, but like the air, will change faster.

Screwdriva

https://www.youtube.com/watch?v=GhAKMAcmJFg

This cracked me up. And totally stopped me from proceeding with my heat pump solution.

QrizB

So I think the lesson is, don't get your heat pump installed by him! Get whoever fitted RR's, or Martyn's, to fit yours.

Solarchaser

Screwdriva said:

https://www.youtube.com/watch?v=GhAKMAcmJFg

This cracked me up. And totally stopped me from proceeding with my heat pump solution.

That's a real shame.
Generally skill builder is a very good channel, I guess I understand why Robin Clevitt has moved to his own channel now instead of just using skill builder channel.

Great joinery advice on the RC channel

Reed_Richards

It's a complete myth that you cannot use a heat pump with a poorly-insulated building.  You may not be able to claim the RHI before ensuring that your exterior walls and your loft are sufficiently well insulated but that is the government trying not to waste its grant money.  The two important thing that you must do is to ensure the heating capacity of the heat pump matches the requirement of your house and that you have suitable radiators or underfloor heating that give you the heat output that you need.  If you do have a building with a high heat loss it will be expensive to heat, however you do it.