ᐅ Connection shaft for district heating: precast concrete element or cast in place?

Hello everyone,

Sometimes I feel like I'm the only person in the country building a house when I call about various construction-related issues—there seems to be a complete lack of knowledge on these topics...

I suspect I have a special problem, but I still hope you can give me some advice.

For the connection to the district heating system, I need to install a 1 x 1 x 1.2 meter (3.3 x 3.3 x 3.9 feet) deep connection shaft into the foundation slab, either

a) as a precast element, or

b) cast it during or after the foundation slab work (probably the more expensive option).

For option a), I imagine the shaft would be placed by the excavation contractor during groundwork, with gravel compacted around it for the foundation slab, then the slab would be poured, and that would be it. Although I will likely need to insulate the shaft separately to avoid thermal bridging.

With the second option, I am a bit confused about how it would work. Do you pour the foundation slab first and leave an opening for the shaft? What happens to the compacted gravel next to it—wouldn’t it get disturbed?

Maybe some of you have had to do something similar?
Which option would you recommend in terms of watertightness and reliability?
Where can one even source such shafts?

Thanks in advance for your tips!


Regards,
Robert

In our case, the shaft was essentially created as a recess in the floor slab. The gravel can still be compacted – the shaft walls are made of formwork blocks.

Ok, so basically option c). The shaft was not poured, but built with masonry. Were the concrete blocks also insulated on the outside and sealed with bitumen membranes?

Most houses here are built with frost skirts made of shuttering blocks. The shaft is then constructed together with the frost skirt, so it basically costs nothing.

Once the pipes were installed, the shaft was filled up to the level of the slab. On top of that came the bitumen membrane, insulation, screed, and tiles. At that point, the screed was otherwise already finished. It is important that when welding the slab, a perimeter edge remains all around, which can later be used for reworking when closing the shaft.