Underground flow and return

[HughF]

I've spent some time this weekend figuring out where I can site my ASHP on my retrofit project. The cylinder cupboard is upstairs in the middle of the house and the flow/return runs in the upstairs floor void, front to back with 15mm drops in the corner of each room, nice 🤦‍♂️

 

We are end of terrace and are planning a rear extension under PD, and we want to site the ASHP on the narrow end wall of this extension,  facing west (up the garden, pointing away from the neighbours)... I could bring the flow and return through the wall, then pipe it up inside the extension but it would then have to cross above two doorways before it meets the original back wall of the house, where it can go up the wall and punch through some cored holes, to pick up the run in the upstairs floor void. I'd need to box this in and I think it would look a bit rubbish to be honest.

 

I'm thinking of how I can bring the flow and return in insulated (the proper, twin-pex-in-a-duct-with-insulation stuff) pipe, under/through the trench fill , through the hardcore layer under the PIR, then bring it up vertically out of the slab where it meets the wall. I want to have UFH in this new extension so don't want to trench across the slab/screed because it would give us an unheated area.

 

Does this sound like a terrible idea? It's almost making me want to put in a suspended timber floor, to match the rest of the house, this would give me a nice void to work in where I could just snake the ducted pipe across, before bringing it up through the floor.

 

Would a better idea just be to form a trench around the perimeter of the slab, deep enough to take a pair of 32mm MLCP runs (or 28mm copper, although I don't really like playing with copper anymore), this trench could then be covered over with a ply cover board. The neighbours did this when they moved their combi to the back of the new rear extension, they just formed the trench with some reverse shuttering when they poured their slab, then tiled over the top.

 

I understand that the twin ducted stuff has a terrible bend radius though, so I assume I'm going to need to dig it pretty deep if I went with this option?