Heat pump and ground/soil conditions

iplogger1

I was talking with a local chap who did self-build about 1 year ago.
We were chatting about roof slates and I happened to ask him
about his central heating source and whether he opted for oil,
heat pump or pellet boiler.

He told me that he was all set to go with heat pump when his
heating/plumbing man (a very reputable one) looked at the
ground conditions and soil quality and he made the conclusion
that the ground was not nearly efficient enough for heat
collection required to get a good COP efficiency with the
ground source heat pump. I spoke to this plumber today by
phone and it is his honest opinion that boggy/peaty soil
(ironically poor soil for agriculture,etc) is the best ground
for the horizontal collector version of heat pump systems.
I'm going to check with my subcontractor who did the site
clearance/levelling about general soil conditions where we
are building but my instinct is that our garden area is of
a more gravelly consistency rather than spongy bog.

I am now a bit concerned that the company I'm looking
at to install the heat pump will just eagerly engage the
job and tell me what I want to hear without doing at
least a superficial ground survey and giving me the honest
truth about the soil conditions versus potential COP of
the heat pump.

Has anyone here got an insight on ground conditions
best suitable for GSHP ? The system will work regardless
of the soil conditions but the key issue is that it would
seem that soil conditions will be a key determining factor
on pump efficiency and the amount of electricity required
to drive the pump.

~ipl

tribesman

I don't really know for sure but I'll tell you my experience.

iplogger1 wrote:

I spoke to this plumber today by
phone and it is his honest opinion that boggy/peaty soil
(ironically poor soil for agriculture,etc) is the best ground
for the horizontal collector version of heat pump systems.

We were told the opposite. Our supplier said that a boggy area is not ideal because the water is stagnant and once you extract the energy from it it takes some time to be reheated. Maybe because water is a poor conductor of heat.

He reckoned that a soil with good drainage was best. Our soil is blue/grey gravelly stuff with only a very thin layer of top-soil. We find it works well. We have a 17kW heatpump as the sole source of heat for a 2500 sqft house.

Naux

I have a similar soil conditions query with regards to GSHP.

However the site that I am thinking of putting it down on is wooded so does this effectivly rule out GSHP?(because of tree roots etc)

I don't mean to hijack iploggers thread so any info while answering his query would be great!!

sunnysoutheast

we've been (informally) told that well drained sites, i.e. with good drainage/moving water, are a pre-requisite for GSHP/ground collector. If the soil is very shaley/gravelly it's much less efficient which can cause "ground freeze" in very cold conditions.

tribesman - who's your pump from?

tribesman

sunnysoutheast wrote:

we've been (informally) told that well drained sites, i.e. with good drainage/moving water, are a pre-requisite for GSHP/ground collector. If the soil is very shaley/gravelly it's much less efficient which can cause "ground freeze" in very cold conditions.

tribesman - who's your pump from?

www.shamrocksolar.com

iplogger1

sunnysoutheast wrote:

we've been (informally) told that well drained sites, i.e. with good drainage/moving water, are a pre-requisite for GSHP/ground collector. If the soil is very shaley/gravelly it's much less efficient which can cause "ground freeze" in very cold conditions.

SunnySE - thanks for that. I've checked with a very reliable and
knowledgeable source this afternoon and the advice you got above
is more or less exactly what he gave, ie. good drainage and moving
water were ideal. Worst thing was soil which stagnated the flow
of water. He did say that peaty soil was good assuming there
was ability of water to flow freely through it. It was also said
that locating near the area near treatment plant for domestic
sewage filtering might be to my advantage as you would be
in a region where the water is already draining off and would
encourage local soil near to it to be mobile also.

In any case what I have learnt is that I need to have a better
clue myself (and also involve the proposed ground loop/heat pump
installation company) about the soil drainage properties.

I know that our planning was conditional on having raised percolation
beds above what is normally needed. This concerns me as it would
suggest that drainage in the soil isn't great (even though
others have remarked on how good the land we are building on is!?)

iplogger1

I've found out (from site engineer regarding the percolation T-test) that
the drainage on our site is not great at all. This explains the requirement
from the CoCo that we put in raised percolation beds as a
planning requirement.

From the vast majority of those I've consulted it would seem that
poor drainage soil isn't ideal at all for horizontal collector/GSHP.
I went back to the plumber whose original comments sparked my
whole interest in whether my system might be less than efficient
based on the ground conditions.

That plumber still maintains that peaty soil is better than gravelly
because in his words "gravelly soil has *too* much drainage".
hmm. Anyway I am glad I found out more about the ideal soil
conditions for GSHP and I accept that poor drainage is probably
a showstopper for horizontal collector ground loop/GSHP.

I am now quite reluctant to go ahead with a heat pump solution
with horizontal collector given what I know about the drainage
properties of the soil.

Anyone else able to add fuel to my angst or to put in words of
wisdom. Regrettably I think the installers avoid this question if
they can and my gut instinct is that most will install regardless
of the ground conditions. Sorry if that seems cynical but that
seems to be borne out by reality.

~ipl