Underheated concrete floor

[Andy]

I'm building a small block power station to house my batteries and inverters. I have a question for the concrete geeks. I have a 400w cable to embed in the concrete floor which I am pouring over 100mm of celotex left over from some house renovations. From everything I've been reading they seem to talk about dry screed or liquid. I won't do liquid as its expensive. Can I not just do a regular wet mix of concrete 4:1 with fibres in it and drop that on the cable? Is there a reason dry screed is used? It takes a long time to set vs normal concrete. A least a wet mix would be good to go in a week. Is it an issue with wet concrete shrinking away from the cables?

[Oldgreybeard]

No problem with embedding either armoured cable or hituff cable directly in concrete. Better to run a length of duct or conduit in and put the cable in that, though, in case it ever needs replacing, IMHO.

I have cables running in our concrete floor, and in the concrete floor of the garage. All I did was put length of 25mm plastic conduit in the concrete. In the case of one cable, that runs to the island in our kitchen, I put a run of 25mm plastic conduit in with gentle bends at either end poking up, one pokes alongside the wall, the other pokes up under the island (bends done with a bending spring over my knee) and then put the cables for the hob and power points through afterwards. Good idea to put a bit of rope through when placing the conduit, helps to be able to pull cables through afterwards.

[nowty]

No problem with embedding either armoured cable or hituff cable directly in concrete. Better to run a length of duct or conduit in and put the cable in that, though, in case it ever needs replacing, IMHO.

I have cables running in our concrete floor, and in the concrete floor of the garage. All I did was put length of 25mm plastic conduit in the concrete. In the case of one cable, that runs to the island in our kitchen, I put a run of 25mm plastic conduit in with gentle bends at either end poking up, one pokes alongside the wall, the other pokes up under the island (bends done with a bending spring over my knee) and then put the cables for the hob and power points through afterwards. Good idea to put a bit of rope through when placing the conduit, helps to be able to pull cables through afterwards.

Andy's talking about embedding a heating cable within the concrete as a heated floor so he cannot sheath it.

Only issue I see is the sharp stones in the concrete, maybe lay a thin layer of mortar (sand and cement) over the cable, then next day dump the rest of the concrete over the top.

[Andy]

Ok, thanks guys. I was wondering if the stones would be an issue. I'll crack on with a bed of mortar first then just to be on the safe side. Its a race against time at the moment. The system is installed in the middle of next month and my free days have been eaten away by getting absolutely floored from a bout of covid. Just lifting a few blocks breaks me out in a sweat at the moment
Cheers.

[Oldgreybeard]

Andy's talking about embedding a heating cable within the concrete as a heated floor so he cannot sheath it.

Only issue I see is the sharp stones in the concrete, maybe lay a thin layer of mortar (sand and cement) over the cable, then next day dump the rest of the concrete over the top.

Ahh, I missed that! I did lay the electric underfloor heating cable under the bathroom tiles in our last house by just bedding it in the tile cement, as per the instructions, though. The tile cement looked to be very like fine mortar to me, even down to being light grey. I'm pretty sure it was really just a sand and cement powder sold as tile cement but little different to mortar in reality. That worked OK for years, I think I put it in around 10 to 12 years before we moved and it was still working fine when we left.

[Andy]

Hehe I missed that you said armoured cable OGB. I was wondering how I'd pull through a snaking conduit

[nowty]

Your thread is an excuse to show you my electrically heated kitchen floor with a thermal imaging camera Octopus Energy lent me. Its over a suspended timber floor so I added 50mm of celotex to the underside when I put it in. Before last winter I added another 50mm to get it up to 100mm, I did not think I would notice the difference, but I did, a lot of difference.

[Oldgreybeard]

Your thread is an excuse to show you my electrically heated kitchen floor with a thermal imaging camera Octopus Energy lent me. Its over a suspended timber floor so I added 50mm of celotex to the underside when I put it in. Before last winter I added another 50mm to get it up to 100mm, I did not think I would notice the difference, but I did, a lot of difference.

I put 300mm of EPS foam under our floor . . .

Still loses about 8% of the heat from the UFH through the insulation to the ground beneath, though. Definitely needs good insulation if using UFH to keep the losses down. Surprised me when I did the sums as to just how much heat was lost down through the floor to the ground beneath. I thought I'd done a good job with 300mm, but it could clearly still do with a bit more. I put a couple of temperature sensors in the soil under our foundations. They read around 8°C to 9°C all year around, so fairly cool.

[Andy]

I like the photos. I didn't think the strips would have been so pronounced. I am still keeping an eye for a bargain on a camera like Joe got a few years ago. Nothing yet.

The insulation round the perimeter of the floor makes a huge difference as well. If that is missing then its just cold bridging out the side.

[Oldgreybeard]

The insulation round the perimeter of the floor makes a huge difference as well. If that is missing then its just cold bridging out the side.

Very true, we have 200mm of EPS around the edges as well, that connects directly (via a DPM) to the 300mm of insulation we have in the walls (the load bearing part of the walls bears on the 100mm at the edge of the floor slab).