Saturday, October 9, 2010

Defrosting the Earth in shallow ground loops

9 Oct '10: There's been a discussion on the N-tron forum
http://www.navitron.org.uk/forum/index.php/topic,11137.0.html
about whether shallow ground loops can cause subsidence or ground heave through changing the moisture content of the soil, or freezing it. It started with a discussion of problems in the US where some reverse cycle HPs will dump heat below as a method of cooling, and can shrink the soil by drying it out. If the loop is under the building this could be very serious.
 (By the way, my system cannot shrink the soil as the depth goes down to 48 metres, the water content in 8,000 tons of soil will not be changed by shifting the base temperature from 12 to 14 during the summer, and moisture cannot escape so high.... The upper surface is tarmac and houses, and the house is not directly over the boreholes anyway.).

Coming back to the UK where the soil is used for drawing heat out, the main risk is of poor installation leading to ground freezing. This can be that the boreholes or slinkies were undersized, that the HP is trying to warm an oversized and poorly insulated house, or the owners think that because the heat from the soil is 'free', they can use it for a heated swimming pool and the like.

So, My advice on the forum was: 
If the ground loop is too small and shallow, then soil will be getting down to temperatures below freezing. If you continue to use the GSHP through into the spring, there is not enough heat to be drawn from the earth, so you will have some risks,

1. Frequent panics by the GSHP leading to it using its 1:1 immersion heater which triples the electricity consumption immediately. (this happened to me last March for one day, and average daily consumption went from about 23 kWh a day to 69 kWh in one day).
2. Frost heave in the area where the ground loop is planted if the soil around the pipes get below Minus Four degs C.

I advised him to read the thread entitled 'Mirrors' in the 'Ideas and Inventions' area. Or read my blog re chargingtheearth.
With a ground loop that is horizontal you cannot really "Charge the Earth" with heat, but you can use Solar-Air panels to provide a daily Defrost that will reduce the risk of ground heave.

For summer use, there's no way that the panels will shrink or over heat the ground in summer for two reasons:
1. The amount of soil relative to the input of the solar-air panels is small. But if the ground loop is small:
2. You have a simple OFF switch, so DO NOT pump heat into the ground loop during a prolonged hot summer if you know it won't stay there till winter.

If you know that the Ground loop is small,
1. Digging it up and laying another is virtually out of the question due to the high cost, and perhaps you don't have enough land to extend it.
2. Having a deep borehole drilled to replace it if you can get a drilling rig on to your site and have about 5 grand is probably also too expensive.
3. For a far lower cost than either, use a solar-air panel to supplement the ground loop and to defrost it in winter, and provide most of your HW needs in summer. The plumbing loop could be designed with a slightly different valving arrangement so that in summer, the GSHP only uses the sunboxes directly, if the Thermostat permits.

All of these are cheaper than repair to your house if it suffers subsidence or heave - even if the cause of the subsidence or heave is cured, the house continues to show the scars of the repair (uneven pointing, wobbly looking windows etc) that will for ever affect its resale value.

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