Heat pump efficiency and ground/soil conditions

I have a query about the efficiency of heat pumps and the soil
conditions most suitable for ground source horizontal collectors.

I have been told by a very experienced heating systems expert
that there is a rather large dependency on the efficiency of
a heat pump used with ground source horizontal heat collector
on the type of soil and thermal conducitive properties of that
soil. He told me that peaty/boggy soil as opposed to gravelly
soil is by far the best type of soil to be used in conjunction
with a heat pump. He even shyed away from installing a system
in a house where the land just didn't have those spongy/boggy
characteristics.

Can anyone add weight to this argument ?
I am looking at booking a company to do gshp & ufh and I have
heard that a lot of these companies will do a very clean &
professional job of installing and commissioning the system but
they will most likely put little emphasis on giving the customer
a realistic assessment of the limiting aspects of ground conditions
on the sort of COP/efficiency of the heat pump.
My concern is that we put down a horizontal collector in "good"
(non peaty/boggy) soil and the pump has to work harder
than it would do if it was laid in something approximating
bog and

1. I end up with a higher ESB bill on account of the lower effective COP
2. The pump lifetime is statistically shortened because of the harder
work it has to do per annum to yield the required heat for the
house.

Is this a valid concern or am I fixating on the negative ?
I really do have a concern that companies see a 25-30k contract
and they will install even though ground conditions are a good
deal less than optimal !

~ipl

Re: Heat pump efficience and ground/soil conditions

Hi,

Have you had a read thru the key post Geothermal heating system/Ground source heat pump which might give some general advice.

I could be wrong but we may have had an enquiry similar to this some time ago so if you run the search option you may well find something relevant.

Re: Heat pump efficience and ground/soil conditions

Thanks SueEllen !
I had read some of that detailed thread/key post a while back
and some of the soil conductivity/water table issues had
gone over my head at the time. Having looked over the posts
again it has clarified to a fair extent my understanding of how
soil conditions roughly affect the effectiveness of the ground
source loop/heat pump system.

Our land (1.3 acres) according to the guy who did the site
clearance is good land and I was up there after a good deal
of rain and it was good and firm to walk on. The site clearance
man who is a friend of ours said that in one of the trial holes
he dug for us he wasn't hitting water at all even at 6ft depth.

He figures worst case the water table is about 3ft down in
some areas of the land. What is a bit of a mystery though
is that the council deemed that we need to raise percolation
beds drainage pipes at least 1 meters off the existing ground
level. Our engineer maintains this was because the percolation
test success was very marginal and a consequence was
the stipulation of the raised perc beds. This seems to fly
in the face of what our site clearance friend says about his
take on the drainage quality of the land. I wonder are
we entitled to see the results from the perc test for the
land under FOI or just normal informative purposes via
our engineer ?

It seems drainage and ability of water to move rather than
stagnate is a key issue with respect to the efficacy of the
ground source collector system and getting max efficiency/COP
from the heat pump.

Re: Heat pump efficience and ground/soil conditions

You or your agent (Engineer/Architect) provided the T values to the Council, based on which they decided that you needed a raised bed.

So your agent should be able to provide them to you.

Re: Heat pump efficience and ground/soil conditions

It is extremely important that your soil conditions are suitable.

As far as I can remember marl in the soil is bad and also a high water table is bad, as, if the water is stagnat in the soil, when the heat pump has taken the heat from the ground, if the water in the ground is not moving, there will be no new heat available.

I feel that there are a lot of cowboys out there and as you have said, if they see a contract for €25k to €30k, they will of course say that your soil is suitable.

However, I have found that asking the question of "do you do a site visit in advance to ascertain whether the soil is suitable" usually helps to weed them out as most good companies say this is a must and will take the time (free of charge) to do this. Dunstar are one company that do this. I've heard good reports on them, but I do not have any confirmation myself of this.

Your other option is the borehole option.

I hope this helps. Its all a bit of a mindfield with loads of questions but not very many answers.

If you do decide on putting it in please let me know how you get on with it. We are still unsure as to whether the initial output is worth it. As they say, if oil goes up so will electricity.

A friend of mine built a new house last year and had the underground electric heating installed. The bill for the first two months was about €450the next two months was €695 approx and the Dec/Jan bill was €980 even though he was away for xmas. The bills have come down since as he has turned down the thermostat and got the night rate set up. He uses gas for cooking I think his last bill in March/Arpil was around €4oo.