How good is underfloor heating in a large room

[BeeDee]

Morning all,

I'm about to embark on some home renovation and the kitchen is the first target. We are looking at making it bigger, all in the room would be 32m2. I have read that electric underfloor heating systems are better and more efficient for smaller rooms, but that they can take a while to heat the room and are more expensive to run than wet underfloor heating systems. I'd prefer electric however, ideally throughout the house as I'd like to get away from gas entirely and move towards renewables.

So my question is, how practical is it to have electric underfloor heating in a room of this size? Could it be the sole heating source in the room (bare in mind just last week we had temperatures of -10C)? And would it be prohibitively expensive to run (I realise this is subjective but given it's one room in the house I'd hope it would be no more than a £5-£10 a month in running costs)?

Thanks,

 

[James]

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.

 

[pc1966]

I would imagine a ground or air source heat pump would be your best long-term option as you don't need (or want!) too hot a floor and the running costs would be much lower (by 2-3 times) than normal electric heating.

However, the installation cost of that would need consideration, and any practical issues of where it can be located.

 

[DPG]

This is the problem when people want to go 'eco' and use renewables. But then end up using 6 kilowatts of power just to heat the kitchen!

No offence intended to the OP.

 

[littlespark]

is your area calculated by the room size, or the floor area AFTER the kitchen units have been put in? No point heating under the units.

 

[MJPD29]

5 quid or slightly more per day is more realistic in a room that size

 

[Wilko]

Hi - it’s good to think this through before you build. So well done .
If you do choose to go electric underfloor heating can it can work very well. It has very few moving parts and it is easy to install. But, if the mat gets damaged it just stops working. This can happen during tiling and it can be quite difficult to fix, ie tiles up. Meanwhile you’ve no heating. So perhaps make sure the responsibility to install and commission is clearly linked to ‘significant’ payments.

It takes power to increase the room temperature and the more insulation you have the less power is required. So perhaps spend more on this aspect .

Unless you generate your own power, gas is going to be the cheapest per kW but that too may change in future.

 

[Mike Johnson]

Wet underfloor heating can be accommodated with an electric boiler if you are dead against gas, essential to insulate under the floor to ensure heat always goes where you want it and as above don't heat under the units or fixed furniture, but I am sure you have identified that in your research.

 

[Mike Johnson]

I would imagine a ground or air source heat pump would be your best long-term option as you don't need (or want!) too hot a floor and the running costs would be much lower (by 2-3 times) than normal electric heating.

However, the installation cost of that would need consideration, and any practical issues of where it can be located.

And the noise from the external compressor can be considerable if you live in a Rural environment, mine are very noticeable if you are near them.

 

[HappyHippyDad]

I put 160w P/sqM electric underfloor heating in my 12m2 conservatory. It is absolutely wonderful! I walk in, in the morning in my bare feet and it just feels lovely ?. 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!

It is lovely, but it feels extravagant and is very expensive to run. My electric bill is usually around £40 p/month and it went up to around £130. The install was done properly with good insulation below. Also, it's not just the winter months you use it. The tiles are cold throughout spring and autumn so it is used (albeit less) in those months too.

I don't know much about wet underfloor heating, but if I had the choice I would go with that, as I assume it is far less to run. I'm sure it most be quite a bit more to install, but I bet you make that back over a few years with the reduced running costs.

 

[HappyHippyDad]

With regards being a primary heating source, the answer is 'yes', it can be a primary heating source. However, you would need the correct heating mat/coil for that. You would also need all the correct underfloor insulation. You would also need to have the house very well insulated. Without all of these things it will not be a very warm house in the middle of winter.

In this cold spell we have just had, my conservatory (bear in mind lots of windows and pvc roof) could reach about 17 degrees(C) with the underfloor heating on constantly throughout the day.

 

[pc1966]

And the noise from the external compressor can be considerable if you live in a Rural environment, mine are very noticeable if you are near them.

The one place I have actually stayed at that featured an air-source heat pump and under floor heating was one of the Landmark Trust properties up north (The Shore Cottages No 2) and it made it a very comfortable stay, nothing like the freezing cottage it would have once been had you not got a big fire on.

In that case they have an out building and the external compressors/heat exchangers were behind it and really not noticeable at all inside the property, though of course there was also the sea noise to mask it.

 

[Mike Johnson]

As I said it all accords on the ambient noise level, if you live in town with constant traffic passing by you probably will not hear it at all, I live in an area where I want to know what that car was doing driving past my house. ???

 

[Zerax]

I live in an area where I want to know what that car was doing driving past my house.

Probably 'casing the joint' !

 

[Mike Johnson]

Funny you should say that my nearest neighbour had their camper van stolen over night just recently, they drove it through my paddock and out of the gates at the other end so not passing either hose at close quarters.

 

[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]

https://www.NoLinkingToThis/p/dickies-cushion-crew-socks-black-size-7-11-5-pack/49250

 

[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.

 

[Simon47]

... for design purposes, you can consider the oversite concrete to be a constant 5 degrees.

Which is (I assume) a true statement, but does not actually support your blanket statement. "For design purposes" really means "if you want to avoid calculating actual figures, then this will be near enough for common situations". U (or R) values for soil varies significantly, but it DOES have thermal resistance, and it is not "negligible".
And as I said, part of the reason it will appear to have exceedingly poor insulating qualities is the thermal mass that will suck heat away until equilibrium is reached. Put another way, if you did rely on a couple of meters of soil as insulation - you'd find yourself putting lots of heat in initially and might conclude that the soil has no insulating quality, but that heat isn't "gone", that thermal mass is now holding much of it.
And "I've designed and installed ..." is not automatically a qualification to be taken at face value*. If you've always assumed that soil is not an insulator, and you've always assumed that the oversite is a constant 5˚C, then you have probably never even considered the design of an earth insulated system. And if you've never looked at the numbers, then your experience is not relevant to the specific case in point.
Not that I plan on relying on soil for thermal insulation - it won't be necessary.

* Apologies, this really isn't aimed at yourself, but I've met a few "heating engineers" who I wouldn't trust to plug in a fan heater. And often the reason for "disagreement" is them relying on stuff they take for granted - like "it's illegal for a homeowner to touch anything to do with gas" (it isn't) and "the boiler must be connected with a fused spur, it's in the instructions" (I made them put the socket back when doing mum's boiler, and it wasn't). And my favourite was the guy who was 101% adamant that Grundfoss, DAB, Wilo (and I assume others) don't make these modern modulating pumps.

 

[Peter Earthmother]

In China, we recommend Dry UFH system(water pipe), save one third of the gas cost compared with the traditional wet method, also fast heat up, while the initial installation cost is slightly higher.
We also provide electric system, however the result is low market acceptance in China, for example Shanghai, due to the cost.

 

[Peter Earthmother]

A resident of a residential area with pre installed floor heating by a developer found us and installed floor heating, and then compared it with other residents in the same residential area.
The result is that the cost is saved.

 

[Simon47]

In China, we recommend Dry UFH system(water pipe), save one third of the gas cost compared with the traditional wet method, also fast heat up, while the initial installation cost is slightly higher.

"Dry UFH system(water pipe)" - so that's pipes carrying hot water embedded in (e.g.) a concrete slab ? We call that a wet UFH system because the heat carrying medium is water. If you say dry, then to most of us (or at least to me) that implies no water, so probably electric.

We also provide electric system, however the result is low market acceptance in China, for example Shanghai, due to the cost.

Ah, so same problem as here then - lecky costs a lot more per unit of energy than gas does.

A resident of a residential area with pre installed floor heating by a developer found us and installed floor heating, and then compared it with other residents in the same residential area.

I don't quite follow that. I read it as the property was built with UFH by the developer - but the resident came to you to install UFH ? Or was it electric originally installed, and you replaced it with a wet (water) system ? Or something else ?

 

[Peter Earthmother]

"Dry UFH system(water pipe)" - so that's pipes carrying hot water embedded in (e.g.) a concrete slab ? We call that a wet UFH system because the heat carrying medium is water. If you say dry, then to most of us (or at least to me) that implies no water, so probably electric.

Ah, so same problem as here then - lecky costs a lot more per unit of energy than gas does.

I don't quite follow that. I read it as the property was built with UFH by the developer - but the resident came to you to install UFH ? Or was it electric originally installed, and you replaced it with a wet (water) system ? Or something else ?

Dear Simon,
Hello! Nice to meet you!
First of all, in China, we identify the heating system as WET or DRY by whether there is cement backfill in the end of UFH process.
I will list some common construction systems in China's floor heating market as below ——
XPS foam board + manual clips + water pipe + cement + tile = WET
EPS foam slotted board + aluminum layer + water pipe + wood floor = DRY
EPS foam slotted board + aluminum layer + water pipe + cement + tile = WET
XPS foam board + aluminum layer + electric cable + wood floor = DRY
These are basically the above types.
Of course, the product types are not involved, e.g. XPS itself has many specifications to adapt to different needs.
In addition, as a result of home renovation, the resident chose our DRY system products.
Meanwhile, we need to conduct actual research on the product, we reached a cooperation with him and collected the gas cost.
Hope you can understand, and thank you for asking questions, cuz most people don't know about China's floor heating market.

 

[Mike Johnson]

In the UK, all of Europe and the USA any underfloor heating that contains water pipes is called WET, any electric UFH is called DRY, the type of construction is not taken into account. China's floor heating market is only of academic interest to most on here.

 

[Peter Earthmother]

In the UK, all of Europe and the USA any underfloor heating that contains water pipes is called WET, any electric UFH is called DRY, the type of construction is not taken into account. China's floor heating market is only of academic interest to most on here.

Good afternoon, Mike,
The development of China's floor heating market is relatively slow, and industry standard documents related to floor heating were established in 2004.
Some technical terms are different from those in Europe.
Fortunately, we can understand each other.
Thank you for your correction.

 

[pc1966]

Ah, so same problem as here then - lecky costs a lot more per unit of energy than gas does.

Very much so, and why water UFH makes more sense as it can be fed from a heat pump.

Not up to date on prices, but until recently electricity was about 4 times the price of gas per unit energy, so even with a heat pump with a COP of around 3 (i.e. 3kW heat out for 1kW electricity in) gas is/was still a bit cheaper. However in the long-term that is going to change and so UFH is likely to work out cheaper.

Also a warm floor is really comfortable in winter!