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Ground source heat coil under slab?

| Posted in Energy, Heating & Insulation on January 16, 2005 01:01am

Whilst pouring a  32ft x 55 ft basement slab, would it be practical to install a 300ft loop of 1″ polypropylene tubing to function as a  ground source heat recovery coil beneath the slab?  The excavation for the slab will be about 4 ft below grade, in  native clay.

There will be quite a bit of ground water passing through the foundation’s drainage system but I’m not expecting that the clay beneath the poured slab will have much ground water passing through it. The site location is in the moderate coastal climate of the Southern Gulf Islands of British Columbia. 

Any advice is appreciated, thanks. 

 

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  1. OldGuy | Jan 16, 2005 02:51pm | #1

    'Many years ago I attended a class discussing geothermal systems provided by New York Electric and Gas. My question at that time was why not lay the circuits in the open trench around the foundation of new construction. The answer given to me the system would pull enough heat from the surrounding ground that the walls would freeze. Actually that was the last time I posed that question and the person responding may have been in error.

  2. DanH | Jan 16, 2005 05:41pm | #2

    I think what the OldGuy said. You'd be pulling heat from under the slab, making it colder, requiring more heat to warm the slab, pulling more heat from under it, making it colder . . .

    If you did it you would want to make sure the slab was very well insulated.

  3. moltenmetal | Jan 16, 2005 08:00pm | #3

    It would be a pretty simple matter to ensure that the heat recovery loop didn't freeze the earth- you're planning to bury polypropylene tubing so you'll be recirculating water or glycol/water not refrigerant, and you can put a limit on the temperature of that recirculating fluid to keep it from freezing.  No point in trying to increase the temperature driving force available by pushing the temperature of the recirculating fluid below freezing anyway, even if you weren't worried about heaving your slab.  

    But you'd be far better off removing heat from earth that was a fair bit farther away from the place you want to keep warm.  Even with significant insulation under the slab you'll still be worse off than if you didn't remove heat from under the slab.  And as you've guessed, you'd be better off if your heat recovery tubing were actually in the permanently saturated zone of the soil, where the thermal conductivity is higher.

    The area of 300' of 1" tubing isn't much either.  I think you'd need a lot more to get a significant, reliable heat gain.  Remember that the driving force for heat recovery is not all that high, and I doubt you're planning on using the unit for heat rejection in summer- not needed in your area.  The earth in your area 4-5' below grade isn't going to be much warmer than 10 C, and the coldest you can get your glycol is 0 C.  Neither the PP tubing nor the unsaturated earth under your slab is all that thermally conductive, and if you configure the tubing in a single loop you'll be limited to what flow you can push through there to keep the driving force as high as possible.  My guess is that you'd need a much larger area of tubing (i.e. more loops in additional trenches farther away from the house) to make the recovered heat worthwhile relative to going with an air-sourced heat pump.  Your mean winter temperatures are pretty warm and you can push all the air you like for very little electical cost.

    1. JPMorgan | Jan 17, 2005 02:40am | #4

      Thanks for the thoughtful replies on this abstract question.

       In just picturing that nice deep hole for the slab to be dug into Mother Earth's hide, I couldn't help but imagine the possibility of mining some "free" energy to support the future house to be built on the lot- especially with  the cost of energy from the grid in the Vancouver Island region being on the rise due to need for upgrading the aging supply system which is  trying to keep up with the rapid growth in energy demands.

      Power outages from winter storms are more frequent now in the islands- those pesky trees that we came here to admire and live amongst have a habit of falling on the power lines during winter storms.      

      I'm a newbie to the idea of "green energy" and would dearly love to get  a recovery loop  into the ground while I have the equipment on-site for the excavation. In hindsight, I should have thought about burying a proper slinky recovery loop when we were putting the septic field in- I could have easily run a loop around the perimeter of the field and beyond. Most likely, that would have recovered enough heat to make it worth while.        

       I hadn't intended to heat the basement, my thoughts were to insulate that slab as well as possible and use the ground source  heat on the main floor of the house and also use it to help in heating domestic hot water. Solar energy is still in the plan for that contribution though. 

      I'd thought of running the poly tubing around an extensive grid of concrete bricks placed within the area that the slab was to be poured on so that some ground water could be influenced to circulate round the poly pipe but that might lead to cracking of the slab eventually- perhaps it's better to keep the ground water in the foundation drains where it's supposed to be.  The suggestion of an air to  air heat recovery system is interesting although I suspect that the overall operating costs may be higher than that of the 300w pump that I'd figured as being sufficient to circulate  fluid in a recovery loop.   

      As I sit here and envision the extra complexity and expense of the system I had in mind,  I realize that my plans to save significantly on heating energy costs in this moderate climate  may well be defeated by the  initial cost of installing the system as we are unlikely to be more than recreational users of the house for some years to come yet.

       

        

      1. moltenmetal | Jan 17, 2005 06:08pm | #5

        Yup- you missed the opportunity when you installed the septic leach field. 

        Air source is often frowned upon because it was applied in climates a lot less moderate than yours where the efficiencies are poorer, especially on the coldest days.  The thing to consider is that at worst, the heat pump is better than the best electric resistance heater.  In your case, the moderation of the climate will definitely work in your favour. 

        300 W for the pump is ~ 1/2 hp, which is a pretty big fan when you think of it that way.  You can push a significant mass of ~ 0 C air with 1/2 hp of fan because your frictional resistance to flow in pushing the air past your heat recovery coil is much lower than it would be pushing water through a single 300' loop of tubing.  The heat pick-up from the (warmer) earth will be limited by the thermal conductivity of the soil and the tubing, whereas the air exchanger is extensively finned to extend its surface area without adding a lot of cost or resistance to flow for the refrigerant or the air.  But one difference between the two systems is that (most of) the energy from your pump would end up heating your house too, but the fan will be mounted outside and most of its energy will be wasted.  But it's still only ~ 300 watts.

        As a pro to be sure, but if you're stuck on electric heat your best bet will probably be an air-source heat pump- unless you can bear the cost of digging more trenches farther away from the house to install yet more tubing for a proper ground-source unit.

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