Slinky Loop improved heat pickup

[buffalobillpatrick]

http://beopt.nrel.gov/sites/beopt.nr...nd%20Bath2.pdf

[SDMCF]

Interesting document, but it seems to me there are too many negatives to running a field loop around the footings of a structure. This "could have an adverse effect on an earth-coupled basement and draw heat from the structure and the earth under the structure". Obviously not desirable. To prevent that would require additional insulation (= additional cost). Also it would mean the field loop could only draw heat from the side of the trench away from the structure, and so to get the same heat transfer the field loop would have to be longer (= additional cost). A moisture barrier is required between the structure and hydrated bed; again this is additional cost and also an additional point of possible failure. I don't see enough potential benefit to offset these downsides.

However I have considered something similar that might be a better option. My house currently has no gutters and no drains to remove rainwater. I have been considering adding gutters and French drains, and running a heat pump field loop in the drain. Effectively the same as the hydrated sand bath described in the document. This would give a second benefit to installing the drain, without the problems of running the loop around the building. I discarded this idea because during our winter there would be no benefit anyway, as there is would be nothing flowing from the gutters until the spring thaw. Perhaps my French drain approach could be a good option in a milder climate?

[Ormston]

Not totally convinced about the idea of compact ground loops close to the building, this goes against everything i was told on the Worcester Bosch GSHP installers course.

They are against the idea of slinky ground loops, we were told the loops effectively short themselves out and greatly reduce to performance compared to straight runs.
They also claim that the slinky coils rub against each other with ground movement and are prone to leaking.

The return of the ground loop should be at least 1m from the flow or insulated to prevent collected heat leaking back to the colder flow.

Any part of the ground loop within 2m of the foundations of a building should be insulated so it can,t freeze and crack the foundations.

The Worcester Bosch heat pumps are rebadged IVT ones and this info is mostly also in IVT's manuals.
IVT are a Swedish company that have been selling GSHP for 30+ years so I would imagine they know what works by now.

Steve

[SDMCF]

All of that makes sense to me.

Our installation uses straight pipes, with the return about 1.5 to 2m from the flow. I can see that straight pipes need a longer trench but that wasn't a major issue for us. We have the space for it and the cost of just getting a digger here was high enough that the length of the trench it dug whilst it was here was a relatively minor consideration.

One thing we did was to install 2 pipes instead of one so we have double the required length. The main reason for doing this was so we could easily add a second heat pump if needed, since the one we have is theoretically borderline for the size of the house (but has coped fine). Double pipes has the side effect that flow rate is reduced which should be more efficient for both thermal transfer and pumping. It also means if we ever have a leak we can re-plumb to use just one of the loops and avoid digging up the field again. For the comparatively small extra cost of a second pipe I think this was a good investment. Anyway, we made sure the 2 pipes were separated and never crossed, thus avoiding some of the issues you raise re coiled pipes.

[AC_Hacker]

Count me in also as being someone who is not in favor of a loopfield being close to a foundation. There is a possibility of freeze-cracking of a foundation.

A GSHP will monitor the evaporator HX to prevent it from freezing, so it should have a similar effect on the loopfield.

However, in the event of a severe prolonged cold snap, if a loop field is close to a foundation, and it has had heat withdrawn due to a heat pump... then there is an elevated danger of totally frozen ground, and ground heaving, and foundation cracking. All bad, all expensive to repair, not good.

I would say that it would be best to have the loopfield no closer than 16 feet from the foundation, for all the reasons that previous posts suggest.

* * *

There is some great info here regarding how water content can increase thermal transfer. Very good info, very useful charts.

* * *

An additional aspect that the document/powerpoint neglected is that, in the very same way that the heat pump condenser HX gets hot because of condensation, rain falling from the sky is carrying heat with it, because it is also a product of condensation, and it is releasing back to the earth, the same energy that was required to create water vapor from liquid water.

So, whenever possible, rain should definitely be captured and directed to the loopfield, because it aids heat transfer, and it is also, itself, a source of heat.

I know this to be true because I did a two month test of my little heat pump and I watched my heat pump's output increase after a heavy rain. Of course, there was a lag of about two days between rain event, and the heat output increase event.

-AC_Hacker

[Ormston]

Originally Posted by AC_Hacker

A GSHP will monitor the evaporator HX to prevent it from freezing, so it should have a similar effect on the loopfield.

-AC_Hacker

GSHP manufacturers usually require glycol in the ground loop to protect against freezing the evaporator HX (controller usually shuts down at -5 to -10C), it's quite normal for the water being returned to the ground loop to be sub zero.

Steve

[AC_Hacker]

Originally Posted by Ormston

GSHP manufacturers usually require glycol in the ground loop to protect against freezing the evaporator HX (controller usually shuts down at -5 to -10C), it's quite normal for the water being returned to the ground loop to be sub zero.

Steve

But that does mean that you are freezing the ground. Of course there will be a significant amount of heat at that point because of the release of change-of-state heat.

But this will impede the ingress of ground water and all the lovely effects that the paper outlines.

ALSO... If you are circulating sub-zero water around your foundation, are you not freezing it, and exposing it to ground heaves, and cracking??

To say nothing of the foundation becoming a 'super sponge', thermally speaking.

-AC

[Ormston]

This is the very reason for the ground loop to be insulated whenever it's withing 2m of foundations(and as it passes through them).

Whilst it's not desirable to run the ground loop that cold it does happen.
A new install we did around 3 years ago which had 800m of 40mm HDPE in 4 loops, within hours of switching it on the return to the ground loop was at -1.5C.
This was an 11kw GSHP as below.

https://www.google.co.uk/url?sa=t&rc...76247554,d.d2s

Steve

[jeff5may]

There is a big difference between pushing the envelope and pushing your luck. It is in the architect's hands to tell the difference. Pushing and stacking the odds with a device made to last 50 years or so sounds rather unwise to me. Redefining the envelope doesn't mean what you're doing is wise. A prudent judgement must be made by a competent professional as to whether or not a specific site is suitable for the conditions at hand.