I have no direct knowledge of GSHP ground loops but it feels counter intuitive to greatly oversize the ground loop. Might you not hit the same issues as with an oversized ASHP with short cycling and efficiency decrease?

It just seems wrong not to match the heat requirement of the house to the size of the collector regardless of it being ground source and not air. 

Many thanks for the comments! they are much appreciated. I think there would be an efficiency benefit but the question is how significant and is it worth it! An extra loop would increase the surface area of the loops in contact with the soil, but the pump pushing more water round the loops would be doing more work. An independent consultant I've been in touch with has suggested it probably isn't worth it, so we're going to put that money into Solar PV. Best wishes, Graham  

@heacol is one of the best people to offer some thoughts and advice on this one, if he's around.

Posted by: @thewiseoldbird

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Hello everyone,

 

We're in the process of installing the horizontal coils for our GSHP. There is enough space in the field we are using to dig an extra trench, and the manifold has the capacity to have an extra coil.  Obviously, there's an extra cost for the slinky, etc, but we do have the people on site to do this, and they are getting faster as we do more in the field (especially now we've found a water pipe running across our land to another property) but is it worth doing? My gut feeling is that it will improve the efficiency of the system but it's only once it's all in and we've done a couple of years that we'll know if it was worth doing. GSHP is new for us and our builder who is nervous to do anything other than follow what Alto (the supplier) is specifying. We're on the top of the Chilterns and temperatures are always 2 deg C lower than the surrounding areas. Any suggestions would be very gratefully received! TIA  

I would deffo install an additional slinky run, especially as each run is being separately terminated into a manifold.

We don't know how many runs you were already installing
   nor their pipe-length,
      nor the trench-depth

but it is essential to ensure that they won't freeze the ground in winter.

I happen to live in an area of the country with hills and high rainfall.

Slinkys installed on a slope with groundwater slowly running past aren't likely to freeze.
The water is effectively bathing the slinky collectors with a continuous supply of fresh heat to absorb.

The benefit you have with separate manifold connections is that your system could be quickly restored in the unlikely event of a failed underground pipe run.
You could afford to isolate the faulty pipe at the manifold, and refill the underground system to the required pressure, using just the remaining slinkys in the meantime

Anything which results in greater system resilience is worthwhile having.

A correct sized collector is all you need. Saying this, there is a lot in that statement to make sure it’s a correct size. 

Adding extra pipe and antifreeze only adds to extra cost. Also higher load/resistance on your brine pump

We have 1000’s of customers with geo collectors for 20 years plus in Ireland and I’ve never know one to add more collector at the start over than what’s required. 

typically 600-900 meters or pipe, to a manifold covers “most builds” 

Only need for an extra loop in my mind is if one fails, then you have spare capacity. I have had customers in the past go from 3 loops and isolate 1 that’s leaking them down to 2 loop.. 

hope this helps. 
go geo is the main thing.