I live in Michigan and am considering a closed-loop 4-ton system and want opinions on slinky pipe in trenches vs horizontal drilled straight pipes with a u-bend at end.  I have plenty of space and mostly clay soils.  I am willing to pay extra to avoid the mess of trenches and to have the lines drilled to the house (I don't want to tear up my landscaping). 

The directionally-drilled systems seem to mostly use one-loop of pipe per ton, with each extending 220 ft out and then connecting with a U-bend and returning in a pipe right next to the pipe going out.  Thus, unless one of the two pipes is smaller diameter so coolant is running faster one way than the other, it seems that the fluid coming back would be warmed or cooled by the fluid flowing out.  Just to be clear here--the coolant travels out in winter and is gradually warmed by the earth around it as it flows to the furthest point out (220 ft), but then as it returns to the house it gets cooled back down by the outflow.  So the warmest soil in winter is the very end of the 220 ft and then the pipe comes back through the coldest soil.  This countercurrent flow system is how a bird's legs work and it is an efficient system if you want the extremities to be at a different temperature than the core. 

It seems like it would be more efficient to run the pipes out, even as double lines, and then connect them to return lines that come back separately and several feet away from the outflow lines.

Can someone comment on this?

Agreed, if they will separate the two pipes for the same cost, then that is better. Otherwise you need to use software to compare thermal performance vs price.

Good question. I am not sure if it makes much difference, since you chill down the soil then much further down around the 2 outgoing, so I think it will be a wash

Your question is asked a lot and has been debated since the first ubend was installed.

Your assumption is correct. If you did double the drilling, then you would solve the problem, but the extra cost of doing the double drilling will never be recouped.

There are probably close to a million ubends installed around the world and all of those systems work just fine.

Thanks for quick responses.

Regarding comment about weighing thermal benefit vs costs for separating the lines: I am talking about same amount of drilling but connecting the bits furthest from heat pump, so that the two lines in one bore are both going out and then the two lines in another bore are both coming back.  I don't think it would have to cost much more.

Regarding comment that outgoing soil would be more chilled:  By the time the coolant comes back to the heat pump, it would be warmer, right?  Seems to me that the temp coming back is all that matters, not the temp along the outflow pipes.

Regarding comment that U-bends are widely used and seem to perform well: Has anyone modeled soil temp at various locations along the pipes?  Is there much difference between the soil closest to heat pump from that farthest away?  Does this even matter? 

Geo doesn't care which loops you use, just that you have enough.

The price of each must be weighed against benefit.

"enough pipe" is determined by load, soil, depth etc.

If every other trench is made colder to make every other trench warmer, I doubt there is a significant net gain.

I do have a do it yourself customer who has done what you describe. I'm hoping to get measuring equipment on it one day.

You are right that connecting the pipes as you describe should not cost more.  I tend to think that since the pipes are in contact with the same amount of soil either way, that the outcome would be the same.

It would be interesting to see some real world testing.

Let's say that piping it your way raised the  EWT ( Temperature of the loop water as it enters the heat pump)   2° higher than it would otherwise be,  will this make a difference in the performance of the heat  pump?   I can't put a number to it but the increase in heat pump performance would be miniscule at best

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It's called "thermal crosstalk". It's a real effect but, as others have said, it's small.

Posted By mikevh59 on 24 Aug 2012 09:29 AM
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Regarding comment that outgoing soil would be more chilled:  By the time the coolant comes back to the heat pump, it would be warmer, right?  Seems to me that the temp coming back is all that matters, not the temp along the outflow pipes.
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No the point is that since the outgoing soil is colder is does heat up the water as much. That is compensated by the ground being warmer on the return pipe, but it equals each other out.