Never use a slinky in Scotland?

[skyewright]

...250-400m seems way too deep.

Ah, cross-purposes Mr. Desparate.... Nibe are talking about a horizontal collector: 250-400m of pipe along a ~1m trench

Ah. I think you've hit the nail on the head. I didn't spot the possibility of a directional misunderstanding. Knowing the ground loop was horizontal I was thinking Desp's was wondering why the ground loop wasn't even nearer to the surface than 800-1000mm!

[desperate]

...250-400m seems way too deep.

Ah, cross-purposes Mr. Desparate.... Nibe are talking about a horizontal collector: 250-400m of pipe along a ~1m trench. BGS are talking vertical (borehole) temperatures.

how quickly does the heat return after being extracted

depends on the thermal conductivity of the ground around the collector... hard dry rock is less conductive, hence less 'productive' for ground-source, than damp sandstone.

DOOOOHHHH ...........  I must engage brain BEFORE opening mouth
                                                              I must engage brain BEFORE opening mouth...................

Desp

[skyewright]

We are 8 miles from Scotland and we have a slinky (homemade) which has coils about 1 metre diameter

Do the coils overlap, or are they clear of each other?

[djs63]

 SKYewright the coils are divided into 4 slinkies running parallel in trenches about 5 metres apart. Each coil has circles that are fastened to the next one with cable ties at the contact point and to the continuing pipe running to the next coil so that it doesn't unravel. We assembled them alongside a run of sheep netting and temporarily fastened the slinky to the fence as we went along.

OOOOOOO.  It looks a bit like this fastened at the top to the pipe from the next door circle and to the neighbour half way up

Predictive text will not let me type your name properly. Sorry. david

[skyewright]

OOOOOOO.  It looks a bit like this fastened at the top to the pipe from the next door circle and to the neighbour half way up

I get it. That's a better arrangement than some, & it obviously works for you, so that's what really matters (unless you need to jump though hoops [no pun intended] to comply with something like MCS, which probably didn't apply in your case).

Since this topic started the most likely to get the job ground works guy has been for a site visit. Most likely is that he'd be taking a single narrow trench for a 400m 'walk' away from the house, then back & forth across the available land finishing back at the house. I think he said minimum separation would be 1.2m, but that practicalities like  digger size, spoil, etc., would probably mean wider spacing in practice.

Predictive text will not let me type your name properly. Sorry. david

No problem. Nice to know you both noticed & cared. 

[knighty]

I don't think it really matters if you use straight pipes or slinkys... what really matters is how much ground area you cover ?

[skyewright]

I don't think it really matters if you use straight pipes or slinkys... what really matters is how much ground area you cover ?

And for a given length of pipe a slinky "covers" less area because of the overlaps, i.e. if the calcs say you need a 400m run with 1m separation, then a 400m slinky probably doesn't actually provide that.

[dhaslam]

If using the heatpump for summer water heating it would be  best to have  one pipe high up in the trench  and the other one as deep as possible.  It just needs valves to allow changeover.     Most heat pumps will give about 50% more output when  used at their maximum input temperature with only a small increase in input.   Also even a few degrees  at the coldest time of the year will  make quite a difference.   

[Philip R]

As a child I tried my slinky in England, Germany( grandparents) France and Belgium ( Hotels) although in the latter, it slid of the stone staircase treads.

Sorry you are talking about ground source heat pump collectors.

How much heat are your trying to extract from the given ground area.? I have heard between 8 to 11 watts per square meter. Any higher and permafrost can result.

Philip R

[skyewright]

How much heat are your trying to extract from the given ground area.? I have heard between 8 to 11 watts per square meter. Any higher and permafrost can result.

There's a detailed MCS document with tables for maximum power to be extracted per unit length in various ground conditions, for various mean ground temperatures, & for various "FLEQ run hours". There are separate tables for "boreholes heat exchangers", "horizontal ground heat exchangers" (which is what I assume we are calling "straight pipe"), and "slinky ground heat exchangers".

It's available from http://www.microgenerationcertification.org/mcs-standards/installer-standards

I'm interested to note that the for "horizontal ground heat exchangers" the tables refer to "25mm OD SDR 11 pipe", but the quotes we are getting mention "40mm OD SDR 17" Other MSC GSHP pdfs do mention various pipe sizes, including 40mm, so it's not that I don't think 40mm is acceptable, just that I'm not sure how pipe diameter affects heat transfer? A 40mm pipe obviously has a greater surface area than a 25mm one, & a much greater volume.

[JonG]

The MCS look-up tables are a bit limited, we tend to always work with 40mm in horizontal trenches, but you do have to be mindful of the Reynolds no which relates to the turbulence in the pipework (turbulent is better in the active elements), so the 40mm means less pipe/trench but more brine and care as far as the RNo is concerned, but the surface area available is larger and it is more robust to work with at SDR11.

We use NIBE's software to design the array when using their heat pumps, but change the ground lambda value and mean brine temps to err on the side of caution and meet MCS requirements regarding no incoming brine temp lower than 0 for 20 years.

To be honest though this should all be coming from your installer with local knowledge as to what lambda values are appropriate?

The NIBE software can also be changed to reflect different diameters, but 40mm seems to be a norm in Sweden, whereas Austria/Germany favour 25mm.

[skyewright]

Thanks for your input Jon.
Reynolds numbers, eh. Not dealt with those for many a year, since undergraduate days. 
In terms of kit the quotes are coming in identical (same heat pump, same buffer, same ground loops, etc.) so they are probably using the same software and parameters - which is fair enough.

[titan]

Ground arrays have been discussed before with 134 replies   http://www.navitron.org.uk/forum/index.php/topic,15536.msg174039.html#msg174039

[Gavferg]

sadly after much too and frowing with a number of installers and "experts" we have declared gshp as a heat source as notworth the hastle. i just got fed up with the lack of clarity the epic quotes with vague terms andconditions and exclusions. the best was one for 47.000 this is for a traditonal 4 room cottage with a 4x10m modernextension. in reading the quote they quitely mentioned this only include the gshp side. As out southwest facing 1 acre field is above the house. the pupme would be able to cope with the pressure. the alt would have to be bore holes at additional costs!!!! my house sits on whinstone which is one of the hardest stones its been my misrotunt to try to drive through. For fun we asked a local drill company for a quote. Their reply was buy and oil boiler. They know about whinstone and it eats drill heads.

and until i can find a scandinavian air source pump that will work at minus 10 for a week or 2 like we had in 2010 i might be interested.

until this ndustry clears out the snake oil sale men and stops using smoke and mirrors to justify thier claims. time to get rid of all the fits rhi etc and see if this gear will survive.

[rogeriko]

Exactly and after you have spent all your money buying the thing you discover it uses 3 or 4 kilowatts of electricity, in other words 12 pounds per DAY to run.