Horizontal Vs vertical for geothermal heating

Discussion in 'Homes and gardens' started by pat_os, Jul 26, 2006.

  1. pat_os

    pat_os Registered User

    I'm looking at putting in a gshp in a new build but am wondering whether it would be better to go with the vertical or horizontal collection method. The vertical is presumably more efficient but more expensive to install but I'm wondering how much more expensive it is. If the horizontal pipes are going to require a digger and driver at ~€30 an hour for a week and the grant for the vertical is 2k more than for the horizontal then would it be worth my while going for the vertical
  2. Sue Ellen

    Sue Ellen Moderator.

  3. ipxl

    ipxl Frequent Poster


    I was originally all set to go with a horizontal collector when I just
    by chance happened to ask a chap locally about his house build and
    heat system choice. He told me that his plumbing expert advised
    against GSHP for his site on account of the type of soil (gravelly,marl,
    etc). I invested a couple of weeks checking the issues and what
    I have found is that amongst the most reliable folks in the know
    there seems to be a consensus that horizontal collector is very
    sensitive to the soil conditions and because of that one needs to
    be wary of installers/suppliers who de-emphasise site survey to
    advise on ground conditions before giving the choice of horizontal
    versus borehole.

    The horizontal collector relies on a sort of replenishing conditioning
    of the ground by rainwater/moisture and the heating of the upper
    layers (within the first meter to meter & half) of soil.
    The "marly" soil it seems loses the moisture conditioning too fast
    to boost thermal conductivity of the soil and the other extreme
    is if you have very poor drainage as a result of stagnant water.
    The latter scenario is very bad news as you have risk of ground
    freezing up around collector area.

    It seems there are ways to counteract the first case .. eg,... importing sand
    and creating drainage channels.. but when you add the cost
    of excavation, sand import, drainage channels, refill for such a
    wide area of ground you might be well worth exploring the borehole
    option now that the grant is available.

    My current plans are borehole .. and if the drilling isn't feasible
    I'm going to look at wood pellets ... or a condensing oil boiler
    (option 3).

  4. pat_os

    pat_os Registered User

    Thanks for that. There is a lot of fill on the site so the soil quality isn't great which was what was making me think about the borehole originally. Have you gotten any quotes on how much the bore hole will be?
  5. ipxl

    ipxl Frequent Poster

    At the moment the keenest quote (from a reliable/well recommended
    supplier/installer) is approx 19.2k (including VAT - without SEI
    grant allowance). The installers are grant approved so actual
    outlay would be approx 12.9k inclusive of drilling & borehole collector
    installation and heat pump commissioning. I've to get a separate
    quote for the UFH controls & installation in the house itself.

    The borehole drilling is subject to hitting bedrock at 6metres and
    other small print which I have to read over.
    Someone in very recent days mentioned to me that I should also
    investigate if there might be a well somewhere on the site because
    a water source heatpump can have the most efficient coefficient
    of performance (COP) in terms of geothermal.
  6. Jolly Man

    Jolly Man Frequent Poster

    Hi there IPXL, i will be starting my house in the coming months and will be drilling a well, would this be able to double up with the ground source heat pump? ie could i give the lads drilling the well a few extra euros to keep drillling? Or your last comment about the well?
  7. ipxl

    ipxl Frequent Poster

    Hi JollyMan ;)

    Interesting that you picked up on my comment regarding a wet well.
    The plot thickens. Someone yesterday mentioned to me that our
    site might be positioned in a place where there is a high likelihood
    of a well. This wasn't the heat pump supply/installer I'm looking at
    but a plumber who has associations with another heat pump supplier.
    Nice guy but the amount of "multiple fingers in multiple alternative
    energy pies" which plumbers seem to engage can be more than
    a bit confusing and disconcerting at times.

    Anyway - I did take his point. It would be worth checking if there
    might be well water source on site. How did you go about this ?
    Was there a costly survey ? He mentioned a diviner. Last time I
    saw a diviner was about 20 years ago when I was a teenager
    watching the Late Late Show and there was a balding country
    man going around fields with a willow stick or something similar.

    We should compare notes on well water sourced heat pumps.
    I'm an ordinary punter and have no affiliation with plumbing
    or heat pump suppliers. I'm just interested in getting a decent
    COP with the heat pump if we are going down that route.
    The horizontal collector principle looked way too marginal and
    delicately balanced to be capable of giving a decent long term
    return on investment or confidence of paying for itself for
    my liking.

  8. Jolly Man

    Jolly Man Frequent Poster

    Its exactly as you said i got a local diviner with the willow branch, she also had a steel chain! If you are drilling a well i know the subcontractor who carrys out the job will survey it for water in the process and is all included in the price! Do you have any indication on the price of the ground water heat pump? I have no idea how all this works so please excuse me if some of this comes across as being clueless!
  9. ipxl

    ipxl Frequent Poster

    No price indications as of yet.
    I'll send you a PM.

  10. extopia

    extopia Frequent Poster

    Your man should be able to dig out the area in half a day easily. Another half a day to fill it back in. Who quoted you for a week?

    AFAIK vertical grids are generally used where there isn't enough room in the garden for the horizontal version. Costlier, as you've discovered.

    We went with horizontal, and our soil is of the heavy, wet type. No problems last winter (which was very cold up our way).
  11. ipxl

    ipxl Frequent Poster

    Extopia - You characterize your soil as the wet type. How did things
    go in terms of your T-test/percolation test for septic tank/&-or treatment
    plant ? We were told we needed raised percolation field which was
    to address the poor or non-ideal drainage quality of the soil area
    on our land.

    I'm not great at understanding the various types of soil morphology
    that exists around the countryside but from what I can gather our
    subsoil is gravelly/marly. It seems it might dry out quick. I think
    the horizontal arrays work best if the rainwater gets a chance to
    condition the soil (water being a relatively good conductor of heat)
    and also there needs to be ability for water to freely move (stagnant
    bog would be bad). We don't have stagnancy issue but it seems to
    be the marly subsoil which would probably lower the effective
    thermal conductivity of the soil. This is what is pushing me to
    investigate either dry borehole or well. We have 1.3 acres of space.
    Some installers seem to de-emphasise the soil conditions issue
    when it comes to installing horizontal collectors. That concerns
    me greatly !

  12. extopia

    extopia Frequent Poster

    Well perhaps that is because it's not really an issue? I've come across the theory myself (elsewhere on this site, actually) but I'm not convinced. Geothermal heat pumps extract only a small amount of heat from the soil, and the theory is that ground temperatures are pretty uniform below a certain depth, regardless of the composition of the ground.

    I'm no expert, but our system seems to be working well. I would describe our ground conditions as "heavy."

    Re waste treatment - our ground does not percolate well. However ours was a renovation, not a new build, and we opted to continue with the existing septic tank, in the absence of any obvious problems. I would expect to have to install a treatment plant in the future, however.
  13. pat_os

    pat_os Registered User

    I got the timeline of a week to dig the trenches from another post on aam. I spoke to one supplier at one of the ideal homes exhibitions and he also suggested that if I could source a well on the site that this would be more efficient to take the heat directly I must say though that I ws a bit sceptical of this. Even with research done I had never heard of it before. I have no idea of what is involved in digging a well, is it just a matter of getting below the water table? Prior to all the fill that went in to the site there was a number of small springs there but nothing major and the soil type below the fill is peaty.

    Something else I had conidered is locating the collection pipes near to the percolation area of the septic tank. My percolation test results weren't great but passed and maybe a boreholoe nearby would prevent percolation problems and also give some way of collecting the warmer water leaving the septic tank.
  14. extopia

    extopia Frequent Poster

    A vertical collector does not have to be in a well. You can go straight into the ground, water or no water.

    However what you were told is correct - siting the collector in a well or a pond or river, if available, is indeed more efficient as the temperatures will always be that little bit higher. As far as I know, it's not necessary for a good system, but more efficient if available on your site.

    A week to dig the trenches is a little over the top, unless you have a huge collector area. Mine was pretty standard, about 20m x 20 m (not trenches - we dug out the entire 400 sq metre area). Like I say, a total of about a day to dig and then fill back in.

    How would a borehole prevent percolation problems? If your ground doesn't percolate the borehole won't make much difference.

    Warmer water leaving the tank? Maybe - but remember the whole idea of ground source heat pump is to collect the 8 degrees (average winter soil temperature at 1m deep), and multiply it using a compressor, then circulate it through your indoor heating system (preferably an underfloor system).