Ground loop length

[Solair]

Hello.
I have a relative installing a GSHP system at the moment and the company involved usually use bore wells.
This one is a horizontal collector and is their second install. Its in on the edge of Monagahan in Ireland.
Three loops 240m in length. 40mm dia pipe.
Does this sound OK. ?
The heat loss for the building is around 20 KW.
I have read that the ground will give about 15-20W/m2 for damp sandy soil and the refrigerating effect is divided by this to give the ground area needed.

Is it safe to assume that each meter run of pipe in the above configuration will extract 20W/m2.??

Thanks.

[Peter_1]

IGSHPA gives350 to 500 feet/ton, burried 4 to 6 feet deep for 1 pipe in a trench (= 100 ->166 m for 3.2 kW)

In 1 trench at 4 feet and the other at 6 feet gives 210 to 300 feet of trench/ton or 420 to 600 feet of pipe/ton.

The calculation in the IGSHPA manual is more then 20 pages long.

So a total of 720 m for 20 kW is just on the edge of allowable. Ask once what it should cost to install a fourth run of 240 m.

[NoNickName]

The exact figure for moist soil is 20 w/m not for m2.

[Peter_1]

Solair, you see, better suggest a fourth loop.

[Solair]

Thanks for the replys.
The excavation work was completed a few days ago.
Pipes backfilled and excavator gone. Would be a costly exercise to have the machine back at this stage.

[Abby Normal]

With two pipes in a trench, 6 feet deep I was running about 600 feet of pipe per ton in northwestern Ontario aka a cold part of Canada.

Soil there is very wet, and normally there is good snow cover which limited the depth of frost.

[Solair]

So a ton is about 3.2 kw.
And 600 feet of pipe per ton is based on the refrigeration effect of the compressor.
Thats the difference between the heat output and the power consumption of the compressor.

So if we had a compressor with a 20kw heating output and 5 kw power input thats 15kw of refrigerating effect. ?
So 15000/20= 750m of piping.

Is this not close enough guys or am I missing something ?.

[Abby Normal]

while a ton is 12,000 Btu/hr, the closed loop systems I was doing in Canada had a temperature source near the freezing point of water to draw heat from.

Based on a temperature source around zero C, the heat output 'per ton' including the heat of rejection, would be approximately 10,000 Btu/hr.

So perhaps 2/3 of that heat came from the ground in through that 600 feet of pipe.

With warmer temperature sources the equipment can put out more heat. These systems relied heavily on the latent heat in the soil and snow cover.

[Abby Normal]

You also need the flow through those pipes, a mixture of water and anti-freeze to be slightly turbulent in nature to get all the fluid contacting the inside pipe wall.

If the flow is too turbulent or the pipes too long, it is too difficult to pump. Care should be taken with parallel flow paths to ensure the flow is slightly turbulent. Laminar flow results in poor heat transfer.