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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorgeoheated
    • CommentTimeAug 25th 2017 edited
     
    Not sure if this is the correct section to post...

    I am installing my 6kW Heat Pump next month. Over the winter i renovated the entire house, Insulated, installed underfloor heating on both floors in all rooms with 116mm pipe spacing.

    I have purchased the heat pump and just need to dig the loops in the field.

    Location:
    At the bottom of the field, 50m from the house, its a valley with a steam running through it. During the summer it dries up typically.
    I will run straight pipes parallel with the stream, digging 1.5m deep i can get down to below stream level.
    Its the end of August now, we have had a damp august, stream is dry, but when i dig to 1.5m its wet clay going into sand at the bottom of the trench. See photos. (IMG_3334 is after 48 hours) plenty of water in the ground all year i suspect.

    I purchased a report from the British geological survey. They confirmed we are on sandstone with a thermal conductivity of 2.59W/m.K.
    I spoke to them, as this is the bedrock figure and we wont hit that, they suggested 2.5W/m.K. as i have just hit unconsolidated sand.

    Looking at the MCS lookup tables, wet sand is 2.4W/m.K.. So i have gone with that figure.
    Average ground temp at 1m is 11.6c from their report.
    I measured the ground at 1.5m immediately the excavator dug it and it is currently 16.3C in August.

    Sizing
    I plan to install enough to satisfy the house demand when we hopefully extend in a couple of years! This would mean upgrading to a 8-9kW heat pump. I have run calcs for both initial and proposed sizes. I intend to install enough pipe now to satisfy the larger pump.

    I have a old 3Ton Kubota i plan to spend a few days sitting on to do this work!

    Pipe will be PE100 32mm.
    I dont plan to blind with sand. There is 0% rock in the ground and at 1.5m i am into sand.

    The MCS tables only reference 25mm pipe. Using 32mm i expect a slight increase in thermal absorption ,but i wont attempt to factor that into the design

    My Question Is:

    The tables talk about 750mm pipe spacing.

    I plan to dig a 400mm wide trench (its the bucket i have). At 400mm it seems reasonable to lay 2 pipes in the trench at 400mm centres.

    MCS calculations and table suggest 182m of pipe for my 6kW.
    Then 335m when i upgrade to 9kW.

    Would it seem unreasonable to lay 400m of pipe in 2x 200m trench, pipes spaced at 400mm in the trenches, with 1.5m between trenches. With the plan to be well oversized initially, and suitable for when i put the bigger pump in.

    I read somewhere that 300mm is good practice to avoid thermal interference between pipes.
    But as MCS tables reference 750mm, does my extra 65m (20%) cover the fact im 400mm centres.....?

    Initial Dig Conditions. Clay into Sand(Grey).
    https://postimg.org/image/epei37y3n/

    Ground Temp measurement during dig.
    https://postimg.org/image/93s2z5xer/

    After 48 Hours.
    https://postimg.org/image/f5zppnlur/

    Sizing calcs for current 6kW HP.
    https://postimg.org/image/6p07eqh5v/

    Future 8kW HP Calcs.
    https://postimg.org/image/f9y4jngpv/

    Location.
    https://postimg.org/image/ebhrhd3dv/
    • CommentAuthortony
    • CommentTimeAug 25th 2017 edited
     
    One pipe per trench, suggest narrower trenches and bigger area deeper the better
    •  
      CommentAuthordjh
    • CommentTimeAug 25th 2017
     
    Posted By: geoheatedThe tables talk about 750mm pipe spacing.

    I plan to dig a 400mm wide trench (its the bucket i have). At 400mm it seems reasonable to lay 2 pipes in the trench at 400mm centres.

    MCS calculations and table suggest 182m of pipe for my 6kW.
    Then 335m when i upgrade to 9kW.

    Would it seem unreasonable to lay 400m of pipe in 2x 200m trench, pipes spaced at 400mm in the trenches, with 1.5m between trenches. With the plan to be well oversized initially, and suitable for when i put the bigger pump in.

    If you work out the total area that the MCS figures imply and make sure you cover that area, I can't see how that would be worse. So they want 750 x 335000 = 251250000 mm². For every metre of your dual pipe trench you cover 500 x (750+400) = 575000 mm² so I think you need to lay two pipes in 251250000/575000 = 437 m of pipe in 219 m of trench.
  1.  
    Posted By: geoheatedI plan to install enough to satisfy the house demand when we hopefully extend in a couple of years! This would mean upgrading to a 8-9kW heat pump.

    Is it worth installing a 6kw pump now only to upgrade it in a couple of years, when presumably you will then sell the 6 kW pump as second hand with the aggravation of selling and the loss of value new to old?

    If you have the space and you are doing the work yourself for the cost of the extra pipe and some of your time and fuel in your place I would be putting in a good bit more pipe than the calculations suggest.
    • CommentAuthortony
    • CommentTimeAug 26th 2017
     
    +1 and may as well do a bigger hear pump now or insulate better which will be cheaper in the long run.
    • CommentAuthorgeoheated
    • CommentTimeAug 26th 2017
     
    Thanks for your input guys.

    I purchased the 6kW second hand for a really bargain, so its no loss thankfully.

    Il see if i can get the pipe length up as much as possible now.

    If i was to add another 200m of pipe to get 600m total now, it would mean hiring in a bigger machine as i cant dig all that with my 3ton, its too much time on the digger. Hence my plan to do a 2 pipe trench, which i figure still has less pipe density/m2 than a slinky, so should perform, its just knowing what a good length of pipe is with this configuration?, none of the standard calcs consider this approach. Its single pipe or full slinky, i see this somewhere in the middle.
    • CommentAuthortony
    • CommentTimeAug 26th 2017
     
    How deep? And how will you turn the corner at the end of the trench?
    • CommentAuthorRobL
    • CommentTimeJul 4th 2021
     
    I note that you can get trenchers to dig narrow trenches down to 1.2m, and I expect that they would make less mess and be easier to use than a digger. I've operated a digger for a friend, and it didn't half make a mess! Kensa recommend slinkies over straight pipes, but I think a narrow 200mm wide 1200mm deep trench is only really suitable for a straight pipe.
    Has anybody here used one of these to install a GSHP?

    We now use 1.5kW max heating (35kWh max use of gas over any 24hr period last year), so a 2 or 3kW heatpump - I'm still unsure whether air or ground source. For ground, I think that would need ~90m of 25mm HDPE pipe, probably 2 runs up and down the garden would do it, so that would be 2 lengths of 45m pipe in parallel keeping the pressure drop low. Just thinking, that's all.
    • CommentAuthortony
    • CommentTimeJul 4th 2021
     
    2m deep every time for me and never use slinkies - shallow = inefficient too smaller heat field = inefficient and frozen ground
    • CommentAuthorRobL
    • CommentTimeJul 4th 2021
     
    Kensa specifically says 1.2m deep - it's possible that this is for health and safety reasons rather than some optimum depth though. They also say to use slinkies, as it means less digging (they say) - I think that they assume the use of an excavator making a 1.2m deep 1.2m wide hole, 10m per kW, which lends itself to a pro install maybe?
    https://www.kensaheatpumps.com/slinky-pipes/

    In contrast, trencher people say that the result of trenching is neater mounds of loose soil rather than the hard lumps you tend to get from an excavator. The looser spill is easier to backfill afterwards.
  2.  
    Rob, maybe talking about the same thing, but straight water pipes can be laid by a "mole plough" implement behind a tractor, it cuts a slit that closes itself behind the pipe without needing backfill.
    This one is apparently 700mm deep:
    https://youtu.be/C1FXgpZknJc

    (Edit: the gshp manufacturers don't mention using these, presumably for some good reason...)


    New Ashps are about as efficient as gshps nowadays. Given you don't use much heat, would any small advantage in efficiency outweigh the extra installation costs?
    • CommentAuthorRobL
    • CommentTimeJul 5th 2021
     
    Hi Will - I did see the mole idea, they look great! Except generally too shallow, and I'd never get a tractor through our garden gate. It does seem that there must be a better way to put a pipe in the ground, than digging a 1.2m deep 1.2m wide long trench.

    And yes, you are right, ashp is likely the simplest way forward. Supposedly they either need a pro mcs install, or strictly speaking planning permission I understand. As a keen diy-er, it persuades me to look around for other options.
    • CommentAuthorphiledge
    • CommentTimeJul 5th 2021
     
    I think youve already mentioned it but you can get 1200mm deep pedestrian controlled trenchers that cut a slot 100-150 wide. See Ditchwitch.co.uk

    I dont think you need a MCS installer unless youre going for one of the subsidy schemes. As I understand it you only need an F gas fitter if your install involoves breaking into the refrigerant system and recharging.

    I appreciate ASHPs efficiency may have improved but I cant see them being as efficient as an equivalent age GSHP when the temp drops and you need heating the most.
    • CommentAuthorEd Davies
    • CommentTimeJul 6th 2021
     
    Posted By: philedge: “I dont think you need a MCS installer unless youre going for one of the subsidy schemes.”

    Indeed, you don't need a full MCS certified install. However for permitted development (at least in England and, IIRC, Scotland; dunno about the other UK areas) it does have to comply with the MCS 020 Planning Standards:

    https://www.planningportal.co.uk/info/200130/common_projects/27/heat_pumps/2

    I haven't looked into what that implies.
  3.  
    MCS020 requires that both the ASHP and the installer have been certified by MCS.

    It also includes a procedure for calculating noise levels to check acceptability under permitted development.

    This implies that DIY ASHP installs are not permitted development under this provision. I suppose you could put in a planning application to show that it wouldn't cause a nuisance, or you could chance it.

    Of course there's legal issues about DIY installs of gas/oil/electric heating too, and electrical hookup to a HP

    Phil, you can check on the certified SCOP values for heatpumps at

    https://mcscertified.com/product-directory/

    For example:

    Kensa 3kW GSHP SCOP= 3.52
    Mitsubishi ecodan 5kW ASHP SCOP = 4.57

    Both at 35degC CH water flow temperature

    (SCOP = coefficient of performance, averaged over a standardised set of weather conditions to represent a heating season)
  4.  
    RobL, did you use the MCS tables to calculate the length of the ground loop?

    Possibly suggests a longer loop than you mentioned? But depending where you are and what soil you have.

    For us it would suggest about 20kWh per year per m of horizontal pipe.

    https://www.gshp.org.uk/pdf/MIS_3005_Ground_loop_sizing_tables.pdf
  5.  
    I have heard that the 1.2m depth is the level below which you should have a trench box, so only digging the trench this deep can be done without one. I think deeper is better but maybe diminishing returns comes in to play? (At some points I went deeper than this, and at one point slightly shallower due to a mahoosive rock that wouldn't budge).

    We used slinkies (Kensa system) and I dug the trench about 1500mm wide using a 900mm riddle bucket (overlapping each pass), because it proved easier to clear out the bucket on our heavy clay. There was also a 1500mm flat bucket that I occasionally used to scrape out the trench base where necessary.

    I believe that a straight pipe can be put in each side of a single trench but they should be 1m apart, so you would have to use a 1m bucket. We have a section approx 45m long from the manifold (at the edge of the field) to the barn where there is one larger pipe laid down each side of a single 900mm trench (the width of the bucket that I had).

    I looked into hiring a trencher for a narrow trench and using straight pipe (as opposed to slinkies) but in hindsight it would have covered a larger area of the field so would have cut through more land drains, and there are a few random larger rocks which I think it would have struggled with. Plus you need to dig out to turn around and change direction so you need a digger anyway as well. I think this might have resulted in a neater job and a field that recovered quicker.

    The design spec was for 2 x 40m slinkies but I dug the trenches to 50m as there was enough space, and we cable-tied a few more slinky loops in to use up the spare header length on the pipe. So it should end up slightly oversized, without spending any more money.

    It took me 2 days to dig the trenches and 4 days to backfill (carefully) with a 10 tonne JCB360 machine.
    Trenches are on 5m centres.

    Rolling out and cable-tying the slinkies was an "experience", in a rush, in the p***ing rain, trying not to fall out with SWMBO. I can see why companies prefer straight pipe.
  6.  
    With regard to laying GSHP collector pipes - no one has yet mentioned moleing the pipes in. That is 2 trenches as deep as you want as far apart as needed and then a moleing machine to put the pipes between the two. Is this ever done?

    Although whilst I have no experience of heat pumps from what I have read it seems that the advances in ASHP are such that GSHP seem to have little advantages considering the extra cost and disruption of installation.
    •  
      CommentAuthordjh
    • CommentTimeJul 6th 2021
     
    On a quick read of https://www.planningportal.co.uk/info/200130/common_projects/27/heat_pumps/2 I noticed another perhaps interesting condition for an ASHP to be permitted development:

    "Used solely for heating purposes"

    That's annoying. Though how they would tell or enforce it I don't know.
    • CommentAuthorphiledge
    • CommentTimeJul 6th 2021
     
    Posted By: WillInAberdeen

    Phil, you can check on the certified SCOP values for heatpumps at

    https://mcscertified.com/product-directory/" rel="nofollow" >https://mcscertified.com/product-directory/

    For example:

    Kensa 3kW GSHP SCOP= 3.52
    Mitsubishi ecodan 5kW ASHP SCOP = 4.57

    Both at 35degC CH water flow temperature



    Im not sure how up to date that info is but Kensa website says the smallest(7kw) Evo GSHP SCOP is 4.72 at 35 degC.

    I dont know how the COP varies for an ASHP when its freezing but I cant see output or COP changing for GSHP, unless the collector piping was installed by the lawn turfers!
    • CommentAuthorphiledge
    • CommentTimeJul 6th 2021
     
    Posted By: Peter_in_HungaryWith regard to laying GSHP collector pipes - no one has yet mentioned moleing the pipes in. That is 2 trenches as deep as you want as far apart as needed and then a moleing machine to put the pipes between the two. Is this ever done


    Ive no idea if its ever done but there would be limited contact between the ground and piping so I guess there would be limited heat transfer??
  7.  
    Hi Phil,
    Rob mentioned he needs 1.5kW, so I imagine he is looking at their 3kW "Shoebox" range. I think you were maybe looking at their larger "Evo" range, 7-17kW?

    Either way, you can check the definitive SCOP on the MCS register pages. A few years back, GSHPs had a definite advantage, but newer ASHPs seem to have caught up now, so I guess you'd only pay the extra for a GSHP if there was a planning issue such as noise or appearance or a Listed building. The air-air heatpumps seem even cheaper and more efficient!

    The SCOP includes for the effects of freezing weather on the ASHP - it's a weighted average CoP taken over a whole nominal heating season, including some freezing weather and some milder days. In the same way AIUI the SCOP for GSHPs accounts for the ground cooling down round the pipes as heat is extracted over the season.

    I noticed that the manufacturers seem keen to quote the 'ErP SCOP' which is more favourable than the MCS SCOP numbers as it doesn't include things like the power for running the ground loop pump.
    • CommentAuthorRobL
    • CommentTimeJul 7th 2021
     
    Some great insights there people!
    I had not seen that mcs document, and I think it indicates 104m (East Anglia, wet clay, 3MWh/year extracted + 1MWh elec). I notice that the tables in the doc are split into various runtimes, 1200hours, 1800hours, etc, which makes sense. If I used a slinky, that mcs doc suggests a 32m long trench, with 128m of pipe in it.

    Kensa suggest more simplistically 100m of pipe for a 2kW heatpump, either as a slinky in a 1.2x1.2x20m hole, or straight. As they don't offer a 2kW unit, they would suggest a 3kW hp, and 150m of pipe. The figures do end up similar for us, but the mcs doc indicates it's more about the total yearly heat demand than the heatpump power, as Will pointed out.

    Very good point about turning a trencher, I hadn't considered it! I guess I'd have to do a series of parallel straights with top and tail straights, so several 200mm radius bends. I'm expecting bending the pipe to fit those bends would be "annoying". I guess it could be filled with hot water to help, and worst case the pipe would end up riding up on the bends, and not living at the bottom of the trench. Can you get things that help bending it to a 200mm radius?

    I'm guessing the PP ashp for cooling issue is because people go outdoors more in warmer weather, so noise is more likely to cause offence than during the winter. Gshp avoids this issue, and gives much cheaper limited cooling.
  8.  
    Posted By: RobL200mm radius bends


    might need to check that it's okay to bend the relevant sized pipe to that radius?

    EDIT: I'm not paying attention, its radius not diameter, so I'm sure it would be fine. A kink would be a disaster though!
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