This whole process is going to be fascinating, albeit Im sure distressing and stressful for you.

@jamespa, 😀, and the thing that has pissed me off the most is that this is a direct result of a horribly bodged retrofix. The irony!

But the positives will be huge, as we will be able to showcase all the bad things that get slapped into systems and what to look out for and avoid, with the content being ready for next year's mass installation season that will hopefully save loads of homeowners the heartache.

it begs the question whether relocation is an option ...

I doubt it.

The Flights to Rwanda programme was scrapped by the Government last year.

But it's certainly worthwhile noting that Rwanda has far fewer complaints regarding heat-pump installations than Britain.
Perhaps MCS would consider a fact-finding mission to discover why.

I think we are at a point where Mars doesn't want 25 meters of driveway dug back up

Thats rather easy to understand, although it begs the question whether relocation is an option but Im sure @mars has already considered that!

The rep proposed an 18kW unit (which was the largest they had) but the story gets worse. The only document he provided calculated our actual heat loss at, wait for it, 50.6 kW.

Wow,  What planet was he on (obviously one with a much colder climate!)

it's going to be hugely insightful to see what unit we end up with.

Our next step is to get a correct heat loss calc,

Im sure you don't need me to remind you about my experience with heat loss surveys, GIGO definitely applied as I have said on many occasions. 

Have you got any data which might allow you to sense check.  Given the problems with pipe size etc this is going to be a more important parameter even than 'normal', you need to be as certain as is humanly possible.

This whole process is going to be fascinating, albeit Im sure distressing and stressful for you.

Mars currently has a GES Caernarfon 18kW ASHP.

I presume he needs that much!

We've never had a proper heat loss calculation, James. Our only one was seven years ago with a rep from GES, back when we were new to heat pumps.

The rep proposed an 18kW unit (which was the largest they had) but the story gets worse. The only document he provided calculated our actual heat loss at, wait for it, 50.6 kW.

To be clear, our heat loss is absolutely not over 50kW. This huge discrepancy perfectly encapsulates the state of the industry at the time, and I'm not convinced it's improved much since given what we regularly seem on the forums.

Our next step is to get a correct heat loss calc, and we're arranging for someone to visit us, and to get the process started properly. 

And our heat pump is not even close to be being 18kW at sub-zero, so it's going to be hugely insightful to see what unit we end up with.

Mars currently has a GES Caernarfon 18kW ASHP.

I presume he needs that much!

We are yet to do a heat loss calculation, but I'm currently 'assuming' the house will need this and possibly a bit more.

we don't have many options here

At the risk of over-simplifying the issue...

... please explain why running TWO pipes in parallel isn't a possible solution.

I think we are at a point where Mars doesn't want 25 meters of driveway dug back up. Its going to be bad enough digging up each end to fix the terminations (where the pipework in the ground hasn't been fitted/insulated properly). So we are kinda stuck with DN32 (26.5mm internal bore) plastic pipework. My inital calculations are suggesting we can get 10kW down it happily enough. Anything more than this and we are going outside of good hydronic design practices, which I am not willing to do.

As for doubling up pipework, it might give you more flow and reduce presure drops, but you are just adding to copper cost, wall space and visually it would look really poor.

Mars currently has a GES Caernarfon 18kW ASHP.

I presume he needs that much!

we don't have many options here

At the risk of over-simplifying the issue...

... please explain why running TWO pipes in parallel isn't a possible solution.

@bontwoody, not big enough (an excellent spot) 😉

Indeed. If the primaries are not big enough to carry the heat into the property, heat loss due to poor insulation is the last of your worries as there may be insufficient heat flowing in the first place.

Are you going to replace with 28mm or 35mm copper, and properly insulate?

Mars currently has a GES Caernarfon 18kW ASHP.

The data sheet (Caernarfon) shows approximately 14kW of heat energy at multiple points, including -4°C. At DT5, this would result in a mass flow rate of 2400 litres per hour. A 28mm John Guest plastic pipe can't handle that flow rate without exceeding 300 Pa/m. Neither can the DN32 plastic underground pipework that the plastic pipe is fitted to.

So, we have a bit of a conundrum regarding the best plan of attack here. I think we will be designing a hybrid system and some clever DT work internally to get this all operating as it should. Not ideal, however, we don't have many options here.