How good is underfloor heating in a large room

[marcuswareham]

I have electric underfloor heating in my consventory and bathroom, Both I installed using the heating element wire on a roll rather than matts, but i believe the matts are the same thing just the wire is already nicely spaced for you

The underfloor heating in my bathroom works very well, although the bathroom is a very small room and also upstairs (timber ceiling)

The consventory on the other hand is not so good, I think the reason for this is that underneath the heating wire is concrete floor. I think I am losing heat into the concrete and in hindsight I should have done something to insulate below the element so all of the heat will be going upwards into the slate tiles and into the room rather then heating up some concrete, so might be worth thinking about or looking into insulation etc

The consventory is about 3m x 2.5m and although the floor gets warm so is nicer to walk on, it does little to heat the room

The bathroom can be heated of the underfloor heating, and if you put the thermostat right up the floor can be almost to hot to walk on (something that can never be achieved in the consventory)

I also put in backup heating wire, which is not connected but can be if the main heating wire fails, or got damaged when I was tiling

 

[brianmoooore]

The consventory on the other hand is not so good, I think the reason for this is that underneath the heating wire is concrete floor. I think I am losing heat into the concrete and in hindsight I should have done something to insulate below the element so all of the heat will be going upwards into the slate tiles and into the room rather then heating up some concrete, so might be worth thinking about or looking into insulation etc

The consventory is about 3m x 2.5m and although the floor gets warm so is nicer to walk on, it does little to heat the room

You are attempting to heat planet Earth, so it's never going to work properly. You need to have at least 75mm of Celotex underneath the screed, preferably more.
Like an electric shower, electric underfloor heating is always going to be a poor substitute for those powered by hot water.

 

[Mike Johnson]

It's always difficult to retro fit underfloor heating due to the depth required for efficient insulation, most of us do not have the luxury (if you can call it that) of designing our houses from the ground up, so floor levels can be taken into account, but even 12mm of insulation is better than none, with the heating mat any leveling screed adhesive and tiles or engineered wooden flooring we are talking about increasing the floor height by at least 25mm, this can cause all sorts of problems at the threshold to the next room, unfortunately wet UFH would increase this height even more, so unless you have nice high ceilings and intend to raise the floor over the whole ground and first floor (not many of us have more floors) UFH is not a viable solution, but it is nice and wet UFH is cheaper to run than most any other form of heating not matter what the fuel.

 

[BeeDee]

For an open room like that you are looking at 100 to 200w per sq m
Therefore 3 to 6kw

At 15p per kwh
This is roughly
45 to 90 pence per hour.

I think £5 to £10 is a realistic cost to run PER DAY during the winter.

Per day... ouch!

Thanks for this, the bit I was missing in the maths was the W/m2.

 

[andyb]

Per day... ouch!

Thanks for this, the bit I was missing in the maths was the W/m2.

It's not as simple as working out the full load then multiplying this by the hours that you have it on. As already mentioned previously in this thread by others, a massive factor is insulation. A 200w/m mat will take more power than a 100w/m mat, obviously, but it will heat faster and bring the room to temperature sooner. After that it's down to insulation, if it's poor it will be on most of the time but a well insulated room and floor will require a lot less power to maintain the temperature.

 

[telectrix]

£5/day? that could be a pack of 3 socks every day. no brainer.

 

[nicebutdim]

Struggling to get my head around these costs for heating a single room.

Also struggling to find packs of 3 socks anywhere online.

 

[telectrix]

Dickies Cushion Crew Socks Black Size 7-11 5 Pack

Order online at Screwfix.com. Size 7-11. Combed cotton finish. Good colour retention. FREE next day delivery available, free collection in 5 minutes.

www.screwfix.com

 

[nicebutdim]

That pack of 6 seems better value ?

 

[pc1966]

Think of poor Jake the Peg, he needed 3 socks.

 

[telectrix]

Think of poor Jake the Peg, he needed 3 socks.

and now he's got 10 years.

 

[Simon47]

It stays on all day in the winter keeping my feet lovely and warm. It also costs a bloody fortune!!!!! I wish I'd bought a pair of slippers!

Our kitchen has a solid concrete floor - I believe it's insulated. But no amount of insulation will make an unheated slab of concrete anything other than a cold slab of concrete. Even with two pairs of sock and slippers it's 'kin cold underfoot. Yes most new-builds I've looked over the wall at have all had unheated slabs of cold concrete to make sure the occupants always have cold feet ? And of course, with a solid concrete floor, it's the hardest to retrofit UFH to.
My plan is that eventually everything downstairs, and possibly much of upstairs, will have wet UFH. "Eventually". It's going to take some work to dig 2" off the slab but it's going to get done ... eventually.
One time a while ago when "going on about it" (according to SWMBO) she turned round and asked if I wanted under-coffin heating when I went ?

 

[Mike Johnson]

This all gets down to how your house is constructed, if its a Raft foundation you are stuck with possibly a screed on top of a concrete floor that can be taken up, but it will only be 50mm thick, taking that up and putting in 25mm of insulation will be better than nothing.

If you ground floor is built in the traditional way with footings and an independent floor slab, then taking that up and re-laying with insulation will be hard, but a worthwhile job.

If your ground floor is a beam and block construction you have all sorts of problems with end bearings of the beam and its possible intermediate supports if any, but what ever screed is on top of the beam and block can be hacked up and insulation and thin UFU put in.

There is 10mm wet UFH available, but the key is always the amount of insulation you can lay under it.

 

[Simon47]

My assumption is that the kitchen (modern extension) is a footings and separate slab - I'll cross the road of whether there's a separate screed on top when we get to that. As it happens, the guy that built it (DIY) now lives a few doors down the road, I need to catch him when he's free and pick his brains over a few details (like where the 2off 2.5 T&E leaving the CU changes into the SWA going out to the greenhouse, and whether the tee in the water supply pipe is buried under the kitchen floor slab ?) sometime - would sure beat having to find everything myself.
Part of the original ground floor is also solid concrete, and I suspect that's integral to the walls as a stability measure. After 80 years, I think that's going to be very well cured and solid.

But actually, apart from around the outside, insulation doesn't make much difference to energy requirements. In the middle of the room, you've got "lots of thickness" of earth underneath. So while it will take a lot longer to heat up, once heated up it won't take more energy. The only complication really is that around the outside walls, you have a cold bridge to the outside ground surface - especially if (as seems to be a requirement) they've filled the cavity up to DPC level with concrete to reduce the insulating properties of the building.
But if the cavity were left empty, and say it's 1/2m down to the footing slab, then you've 1m of earth between the floor slab and the outside ground surface.

My eventual plan is to have a radiator to provide rapid response, and UFH to provide comfort underfoot - the flow to the rad going via the UFH manifold first so the (smart) rad TRV will control both.

 

[brianmoooore]

But actually, apart from around the outside, insulation doesn't make much difference to energy requirements. In the middle of the room, you've got "lots of thickness" of earth underneath. So while it will take a lot longer to heat up, once heated up it won't take more energy.

Nice theory, but unfortunately it doesn't work like that. The mass of earth is very good at conducting heat away, and for design purposes, you can consider the oversite concrete to be a constant 5 degrees.
It's not something I tend to publicise on here too often, but my business had a plumbing and heating arm to it as well as electrics, and I've designed and installed several complete house wet UFH systems since they became popular.