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    • CommentAuthorchanters
    • CommentTimeFeb 26th 2010

    Just a quick question! We have a GSHP with a horizontal loop going up the land. We live in the Highlands and have since mid December had permanent snow and then severe ground frost. A couple of weeks ago, the temperature rose and the ice melted...apart from the area where the ground loop is. Could this indicate a possible reason why the GSHP isn't working efficiently for us? Could the loop not have been buried deep enough or is this a normal occurrence? We've been battling with the company (3 years +) who provided us with the GSHP to try and find out why this system is costing us so much but they don't want to know.

    Any ideas/info would be greatly received.
    Well seeing as I recall hearing on a TV programme that GSH was derived from the Sun
    And I have also observed in various photographs that the pipe is perhaps only buried a metre deep
    and the Missis says minimum TWO metres ( & her a Physist)
    So yes you are probably correct.
    Are you aware what depth your pipe is at?
    was it installed by a subcontractor?, In consience the specifer may not be entirely at fault
    • CommentAuthorjon
    • CommentTimeFeb 26th 2010
    It sounds as if your GSHP has extracted all the available energy and is now be at the limit of workability? (hence the cold spot that' remains where your GSHP is)
    • CommentAuthortony
    • CommentTimeFeb 26th 2010
    Sounds to me like insufficient ground loop area and depth. They should know this so you may be able to go after them. The system will cool the ground and so it would be normal for the snow and ice to stay longer above where the pipe field is. It is not normal to have high running costs. It sounds like it is doing its best to work. The colder the return temperature the worse the COP gets.
    • CommentAuthorchanters
    • CommentTimeFeb 26th 2010
    Thanks for the replies!

    The spec was for the pipes to be buried to a depth of 2 metres and the work was carried out by a subcontractor. As we weren't in the area at the time of the build, we didn't get to see how deep the subcontractors had buried the loop.
    • CommentAuthortony
    • CommentTimeFeb 26th 2010
    It is not relevant that a subcontractor was employed to bury the pipes you still have a claim against the main contractor.

    it would seem that they are very likely to have been buried too shallow

    were they "slinkies? what area do they cover and what is the power input of your heat pump?
    • CommentAuthorchanters
    • CommentTimeFeb 26th 2010
    Hi Tony,

    There were 2 x 200 metres of straight pipe laid 2 metres below the surface and the power input of the heat pump is 12kw (this has been changed from the original pump which was 8kw).

    We were given a calculation of £320 per annum (for space heating and domestic hot water) by the heat pump company. In reality it's costing us £2,000 per annum.
    • CommentAuthortony
    • CommentTimeFeb 26th 2010
    May be they should be paying the rest then

    400m sounds like not much to me -- we need Paul in Montreal -- where are you guy?
    • CommentAuthormike7
    • CommentTimeFeb 26th 2010 edited
    This looks like one for Paul in Montreal if he's watching - ha - just seen your post, Tony. Meanwhile, have a look at this ground loop design program (courtesy of P i M): http://canmetenergy-canmetenergie.nrcan-rncan.gc.ca/eng/software_tools/gs2000.html. If like me you're not so hot at getting it to work, it will at least show you the factors involved. Such as..

    Are these 2x200m pipes side by side in the same trench, and if so, how far apart? What sort of ground are the pipes in? What inlet temperature does your heating system need? What flow rate? Etc.....
    • CommentAuthorchuckey
    • CommentTimeFeb 26th 2010
    Rule of thumb is 1kW extracted for every 10m of buried loop. So with 400 m buried available heat should be 40kW. This assumes the loop is not buried in a permafrost area!:bigsmile: If your CH is dragging 50kw you are up a gum tree.
    • CommentAuthorCWatters
    • CommentTimeFeb 26th 2010
    Have you spoken to a solicitor? The projected and actual running costs are so far out of step it sounds like there might be a case for claming the system "isn't fit for purpose". Likewise if they quoted to design the system as well as supply and install it there might be a case to argue that they are in breech of contract for not designing it properly.

    Can someone find that other thread where someone got experts (the BRE?) to write a report on their system?
    • CommentTimeFeb 26th 2010 edited
    Posted By: orangemannotWell seeing as I recall hearing on a TV programme that GSH was derived from the Sun
    Generally, the earth's surface loses more heat to space than it absorbs from the sun - all that core heat to dissipate, otherwise we'd have long since vapourised.

    So the sun does not naturally replenish heat into the ground, especially in winter, when ground temp is way above air temp. It may do so somewhat in summer, when at least daytime air temp is above ground temp - but at night air temp drops below ground temp again, so any shallow-depth solar gain is lost again.

    The only way the sun can unnaturally replenish heat extracted by GSHP, of whatever depth, is by creation of an unnatural downward temp gradient - in other words, by chilling the ground! There's no 'correct' way of installing the ground loop that avoids that fact. GSHP chills the ground around or near the house, which in turn cools the house's general environment. Do they tell you that?
    • CommentAuthorDantenz
    • CommentTimeFeb 26th 2010 edited
    Posted By: chantersHi Tony,

    There were 2 x 200 metres of straight pipe laid 2 metres below the surface and the power input of the heat pump is 12kw (this has been changed from the original pump which was 8kw).

    We were given a calculation of £320 per annum (for space heating and domestic hot water) by the heat pump company. In reality it's costing us £2,000 per annum.

    400 metres of collector might have been somewhere near correct for an 8kW heat pump but I can tell you it isn't sufficient for a 12kW machine - needs at least half as much again (55 m per kW output of heat pump- ish) . If the peak heating load of your property is 12 kW or more then the 400 m of collector will not be able to keep up with the energy extraction rate over a prolonged period ie during the winter. For every 1'C less temperature on the collector side will reduce efficiency by 3%; so if the brine temp in to your heat pump is -5'C rather than a more normal +2 'C your heat pump will be 21% less efficient.
    • CommentAuthorwelshboy
    • CommentTimeFeb 26th 2010
    We laid 600m of 25mm mdpe and a flow/return of 50m 32mm from the manifolds.Heat pump is 9.5kw.
    Return temp to the groundloop is 4c from the heatpump.
    • CommentAuthorMegacycles
    • CommentTimeFeb 26th 2010
    Many factors affect the specific heat collection capacity of the buried loop.

    I would attempt to find the depth of installation once the ground has thawed. It won't be easy as you can't manually dig much below 1.2m but warning strips or tapes may have been laid which will help. Obviously if you find pipe within this depth you may have found the problem, in the highlands I would say the deeper the better so 1.8m minimum.

    Although the total length of pipe is important you need to find the total area collector, ie the spacing. Hopefully you have a photographic record of the installation but measuring the frozen area should provide the spacing.

    Also important is the type of soil , saturated sand/soil yields 4x more energy than loose dry soil, so if you go digging for the pipe check what your digging up. Also important is how the trench was backfilled, the pipe should be protected by a shell of sand which will provide good thermal conductivity.

    Hydrogeology is also important as a source of heat replenishment so active groundwater flow will help collection capacity. Tree shading of the collection area will be detrimental in summer if the loop is shallow.

    I would expect the ground to possibly freeze toward the end of the heating season especially with this winter but the design should permit complete replenishment before the start of the next. With more information you should be able to make a reasonable calculation of what the loop will yield but I get the feeling a lot of 'fudge factors' are used by the professionals to cope with the wide range of condtions encountered.
    Great thread, very interesting info, especially your input Megacyles,
    you state:
    "protected by a shell of sand which will provide good thermal conductivity"

    its funny, I have read the exact opposite to this, that the sand covering acts to cut the conductivity the surrounding soil.

    chanters, do you not have the original spec sheet from the company that designed the installation detailing the cals used to specify the design? I would have thought if they did not provide it they should be able to give you a copy or reproduce it, to show how they calculated that your energy needs would be met with the system as installed.
    • CommentAuthorblacksmith
    • CommentTimeFeb 26th 2010

    It could useful be to ask John Cantor if he could give some feedback or advise - I know John and he has been designing, installing and experimenting with heat pumps for for many years. He is no nonsense and honest and I have found his vast experience of heat pumps to be invaluable.


    ps I have no connection at with his business other than pick his brains - and my mother-in-law plays in the same wind band.
    • CommentTimeFeb 26th 2010
    I will go along with what Megacycle has said, it is not just a matter of depth and pipe run.

    Generally considered that the top 10 metres is just solar energy (does depend where in the world you are). As you are in the Highlands your average temperatures are much lower as is the solar energy up there. Also with this winter being colder than most you have taken a double hit (snow cover will also reflect what tiny amount of light was getting though and the chilled ground above the pipes will unfreeze last). To give you an idea today down here (almost as far south as one can go in mainland UK) the average temp was 8.7C and the W/m^2 has been 241, even here today you would need a lot of land with warm rain.
    • CommentAuthorchanters
    • CommentTimeFeb 28th 2010
    The company who we purchased the system from did not make any visits to our land prior to installation and would not agree to lay the ground loop. Our builder laid the loop by following the installation manual they provided, in all fairness to our builder he had never done this work before, but is a very ruputable builder. I have a copy of the manual and state the requirements.

    The pipe for the ground loop was delivered to our land by the GSHP company:

    No. of trenches/loops 2
    Length of trench(es) 100m
    Length of Loop(s) 200m
    Depth of trench(es) 1m
    Width of trench(es) 1m

    This was based on: Thermia Duo 8 which provides heating and DHW - single phase
    Thermia Mbh220 litre mains pressure hot water tank.

    After 18 months following the installation the heat pump was changed for a 12kw pump. Usage on both the 8 and 12kw heat pumps have proven to be expensive to run.

    The GSHP company took charge of the installation of the Heat Pump and also filled the loop with the anti freeze additive. One thing that I have always questioned is the fact that after the ground loop had been laid and filled, our builder pointed out to the company that on our land there is a slight incline up the land so this would mean that the ground loop would be higher than the heat pump, they then sent their engineer back to cut the loops and inserted an air release valve on both loops. I have often wondered if this would impact on the performance, they say not. I have just taken the following readings from the heat pump.
    Temperature Menu
    Outdoor 3
    Room 19
    Supplyline 31(30)c
    Returnline 27(55)C
    Integral 52c
    Brine in -1
    Brine Out -6

    Heat Curve Menu
    Curve 36
    Min 18
    Max 55
    Curve 5 0
    Curve 0 0
    Curve -5 0
    Heatstop 16

    Is there any other information that you may need, thank you so much for all your help everyone.
    • CommentAuthorGavin_A
    • CommentTimeFeb 28th 2010
    when they swapped the 8kw for the 12 kw heat pump did they add an extra section of ground loop?
    • CommentAuthormike7
    • CommentTimeFeb 28th 2010
    This manual includes explanations of Chanter's data:-
    • CommentAuthorchanters
    • CommentTimeMar 1st 2010
    Hi Gavin

    No they didn't add any further amount of pipework.
    • CommentAuthortony
    • CommentTimeMar 1st 2010
    The return from the ground loops is too cold --- sounds like you might need a bigger pipe field needed or a bore hole

    can you tell us the flow rate in the ground loops? there might be an air lock or restriction to flow --- what type of air valves were fitted and what is the pressure in the ground loop pipes?
    • CommentAuthorJoatex
    • CommentTimeMar 1st 2010
    fostertom's comment is interesting. Would it have been better to have a vertical borehole, particularly with granite as the substrata. The thermal conductivity of granite (2.1) is much better than soil (dry, 0.14) and a little boost too from the radioactive nature of granite. With such conductivity the thermal conduction would reach further and hence supply heat longer than soil.
    Time for me to chime-in. A good GSHP installation needs to have a proper design/analysis performed of the ground collector. Sand is fine to embedded the pipes if the ground is wet - its purpose here is to avoid air spaces next to the pipes which vastly reduce the collector effectiveness. Spacing and length of pipe are related as well - too close and you need greater length. "Short looping" is a common problem as there is always a trade-off between the cost of the ground collector versus time spent using "auxiliary" heating. Making the ground collector (vertical or horizontal, it's the same trade-off calculation) sufficient for the worst case conditions can mean that extra cost is involved compared to what would be spent on auxiliary energy. A real calculation has to be done. In my specific case, the system is undersized by about 20% but a bin-analysis of the typical winter heating conditions show that this will actually satisfy 98% of the heating demand in an "average" winter. The cost delta to size to 100% (both in equipment and ground collector) would take 80 years or so to payback at our current auxiliary energy rate.

    In this particular case in this thread, it does look like either the system is short looped (entering and returning water temperatures are very low for a UK climate - so the COP _and_ capacity will suffer) or there is a thermal contact problem with the ground collector (either the pipes themselves or an airlock or insufficient velocity of circulation). You need to know the flow rate and can then calculate the extracted heat via some simple mathematics. Low flow can be bad as there may not be enough turbulence in the pipes to get good heat transfer.

    Hope this helps.

    Paul in Montreal

    p.s. despite what fostertom says, GSHPs use solar energy (and a tiny bit of core heat, but solar vastly dominates)
    • CommentAuthorGavin_A
    • CommentTimeMar 1st 2010
    Posted By: Joatexfostertom's comment is interesting. Would it have been better to have a vertical borehole, particularly with granite as the substrata. The thermal conductivity of granite (2.1) is much better than soil (dry, 0.14) and a little boost too from the radioactive nature of granite. With such conductivity the thermal conduction would reach further and hence supply heat longer than soil.

    fostertom's comment may be interesting, but it's also wrong.

    if it weren't then the ground temperature below tarmacced car parks / roads would be cooler than under grass as the black road surface would emit more heat than the grassed area which is lighter and offers some protection from wind etc. In reality the reverse is true, and the ground temperature below tarmacced surfaces is generally significantly higher than below grass because the dark surface absorbs more energy from the sun.
    • CommentTimeMar 1st 2010
    Posted By: Gavin_Athe ground temperature below tarmacced surfaces is generally significantly higher than below grass because the dark surface absorbs more energy from the sun.
    True, to 150mm (the max depth of diurnal penetration, as any horticulturist will confirm) while the sun shines, but by the same token cools, emitting slightly more, when the sun goes in.
    True, daytime, but at night tarmac cools more than grass, because it radiates most efficiently to the night sky, a little more than it gained daytime.
    True, in summer, but likely to be the reverse in winter.
    True, in the tropics, but not true at all near the poles - dissipating tropical nett solar gain to the nett-loss poles is what creates weather.
    In each of these gain/loss cycles - hourly, daily, seasonal and tropic vs pole, the loss always averages to a bit more than the gain - it must, otherwise the earth's core heat would not dissipate into space, but accumulate.
    • CommentTimeMar 1st 2010
    Posted By: Paul in Montrealdespite what fostertom says, GSHPs use solar energy
    but only because, when and if the coils have unnaturally cooled or even frozen the ground, winter-time, so the building sits on a colder environment than it would without GSHP. Unless your coils happen to lie in moving ground water, in which case the cooling that precedes solar replenishment is inflicted on somewhere else.
    • CommentAuthorGavin_A
    • CommentTimeMar 2nd 2010

    With respect, you appear to fundamentally misunderstand the nature of the earth's energy balance.

    put simply, the average annual energy input per square meter of the UK from the sun is around 900-950kwh, whereas even if you averaged the total heat emitted from the earth across it's entire surface area, the average annual energy input per m2 from geothermal would be significantly less than 0.4kwh per annum unless we're talking about an area with hot springs or other such geological features. In reality most of this heat is transmitted into the oceans, or via hotspots such as Iceland, due to the earths crust being much thinner at these points, so the input into the UK's ground temperature would be even lower than I suggest.

    essentially the heat of the earth's mantle below the uk is insulated by tens of miles of the earth's crust, meaning that only a very small fraction of that heat reaches the surface at any time.

    The reason the earth doesn't get hotter and hotter is simply that it's reached a point of dynamic equilibrium, where the combined heat input of the total solar radiation and geothermal energy is roughly in balance with the total heat output to space. Similarly the ground reaches a dynamic equilibrium point usually somewhere around 10-15 metres down in the UK (IIRC) where the temperature is stable year round, and the annual heat input from the sun, and temperature losses to the air are balanced with the geothermal heat input from below.

    In thinking about this sort of stuff, it's worth remembering that the temperature of space is around -270 degrees c, and if their was no solar input the earths surface temperature wouldn't be much higher than this, with permafrost stretching probably several kilometres down.*

    or you could just look at the attached diagram of monthly variations in soil temperature at various depths, which clearly shows that the annual recharge at 2 metres comes almost entirely from the sun.

    *not that the earth would be likely to exist at all without the sun of course, but........
    • CommentAuthorCWatters
    • CommentTimeMar 2nd 2010
    There have been a few posts on this forum from people with heat pump problems both ASHP and GSHP. These seem to drag on and on typically taking years to resolve. Sounds like there is need for some regulation in the industry with a mechanisim for preventing and resolving such problems. Otherwise as more and more reports surface on the web people will be put off installing heat pumps.
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