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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?
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