low voltage electric UFH e.g. Warmfloor

[tonybythesea]

Does anybody have useful experience of 'warmfloor' underfloor heating: carbon-enriched semi-conductive polymer mats with copper elements down each side? It claims to be adequate for complete room heating (not just taking the chill off e.g. small bathroom floors). And to do this using far less electricity than other electric UFH systems for the same achieved room temperatures.

 

I gather the technology was first developed in Norway for things like warming car seats. Two brothers who developed it seem then to have fallen out so that they now run two competing businesses offering this, one based in Norway, one in the US. Both have distributors in UK. Another version of a similar technology (as far as I can see) is sold as Ecofilm.

 

I'm considering using this in a newbuild, so could equally go for a wet system. I know that is generally more economical to run. Gas is not an option, so in this case for a wet system we'd be using either an electric boiler or ASHP. But we have plenty of PV to effectively reduce running costs for any electric system.

 

A dry system would have its attractions. Warmfloor being low voltage can be run directly from the PV when the sun shines, or uses a transformer when run on the mains. And if warmfloor lives up to its publicity we'd get satisfactory heating and there'd be no boilers to replace, no annual servicing, no danger of leaks etc. Unlike other electric UFH it is apparently suitable under any floor covering, not a danger to timber etc.  NB our design already includes good insulation values and MVHR so a high proportion of heat 'lost' in transformers should still be put to use.

 

Problem is, although I can get assurances (and some figures) from the people dealing in warmfloor, I've failed to find anything online that gives solid independent evidence that I won't end up with rooms that don't get warm enough in winter. I've had some positive feedback, I should say, from past clients whose contacts one of the warmfloor distributors gave me.

[Jeremy Harris]

 

The technology used for resistance type underfloor heating has absolutely no impact whatsoever on the performance or running cost, I'm afraid.  Any resistance heating element will be 100% efficient at the element itself, in terms of converting electrical energy into heat energy. 

 

Similarly, any underfloor heating system will always be less efficient than free standing heating, as there will always be some heat loss down into the ground, or the ventilated underfloor void, even with very good insulation (we have 300mm of insulation under our floor and still lose ~8% of the heat from the UFH to the ground beneath).

 

The voltage the system operates at has no effect, other than that a low voltage system of a given power may lose a bit more in the wiring, because of the higher current needed.  The only advantage of using a lower voltage for any resistance heating system is that it may reduce the electrical insulation requirements around the heating elements, or make the installation safer if there is a risk that someone may put a nail or screw through it.

 

With any underfloor heating system, of any type, the most important factors are to ensure there is good insulation beneath it, to reduce heat losses, and to ensure that the heating requirement for the room/house is low enough that it can be met by the modest heat output that UFH can provide.  A reasonable rule of thumb is to assume that UFH can provide around 50 W to 60 W per square metre of heated floor area.  If this is enough for the room in question then it should work OK, if not, then UFH may struggle.  You can push UFH up to about 80 W/m², but the losses increase.  If you can aim to run the UFH at around 20 W to 40 W/m² then the losses shouldn't be too great, assuming adequate underfloor insulation.

[SteamyTea]

Not sure how things have developed in 35 years, but we used to incorporate this sort of heating element into moulds for insitu post curing.

They were very 'patchy ' to say the least. Some areas got very hot, other areas had no heating at all.

Also I don't think you can reliably run directly from PV. You will need some voltage regulation. A normal domestic solar module runs at about 72V. So outside the scope of ELF I think.

And that is before power point tracking is considered.

If you can fit normal wet UFH, the water can be heated with just about any energy source i.e. Heat pump, wood burner, bottled gas, oil fired, electricity, small boy scouts rubbing their knees together etc.

[Conor]

Sounds like marketing BS, as Mr Harris says, all resistance electrical heating is the same in terms of efficiency and running costs. No different than using a 30 year old electric oil radiator. Only differences would be the rates at which heat is transferred from the element to the environment.

[Jeremy Harris]

9 minutes ago, SteamyTea said:

Also I don't think you can reliably run directly from PV. You will need some voltage regulation. A normal domestic solar module runs at about 72V. So outside the scope of ELF I think.

And that is before power point tracking is considered.

 

 

The other point about PV is that there is virtually no excess PV generation for the months of the year when heating may be needed.  PV generation pretty much drops off a cliff around October and doesn't pick up again until about March, so it's really of no use as far as heating is concerned.

 

9 minutes ago, SteamyTea said:

If you can fit normal wet UFH, the water can be heated with just about any energy source i.e. Heat pump, wood burner, bottled gas, oil fired, electricity, small boy scouts rubbing their knees together etc.

 

Agreed.  Wet UFH run from an ASHP is relatively cheap to run, especially if the UFH pipes are arranged like ours, inside the concrete floor slab.  This allows us to pretty much only heat the slab overnight, during the E7 off peak rate period, so the running cost for the ASHP is much less than the cost of a boiler run from mains gas.  The ASHP has a COP of over 3, and off-peak electricity is about 8.148p/kWh at the moment, so we're paying about 2.716p/kWh for heating.  Mains gas is currently about  4.3p/kWh, but when boiler efficiency is taken into account this increases to about 5p/kWh.

 

 

[tonybythesea]

I appreciate you guys responding so promptly. 

When I asked for more detail from the UK distributor of Step Warmfloor (American) this is part of what I received: 

"For 1 Kw of electricity the area heated  and cost would be as follows

            For 230/240V products

 100W/m²                                 1000                 =         10.m²/unit price

                                                     100

 

150W/m² mat                           1000                =          6.7m²/unit price

                                                       150

 

200W/m²mat                            1000                =          5.0m²/unit price

                                                      200

Consider now Step Warmfloor. In order to provide the same room temperature and 28°C at floor level Step Warmfloor requires only 54.3W/m²

 

Therefore for 1 Kw of electricity the area heated and cost would be as follows

 

            24W Element                           1000                =          18.4m²/unit price

                                                                54.3

 

Therefore to summarise for 1Kw of electricity a 230/240V system would heat an area of only 10.2m² using 100W/m²,  6.7m² using 150W/m²  and only 5.0m² using 200W/m²

Whereas Step Warmfloor heats an area of 18.4m².  Therefore Step Warmfloor heats an area of between 2-3 times larger with the same 1Kw of Electricity.  ...

As regards to cost it is easy to see that Step Warmfloor costs 2.75 less to run than the 150W/m² product and 3.68 times less to run than the 200W/m² product."

 

I am suspecting that the assertion that I've put in bold italics is the crucial bit I'm being asked to take on trust. I realise this is only a comparison with more usual forms of electric UFH, not with wet systems.

[Ed Davies]

To keep an area at 28 °C, assuming the same room temperature, surface finish and insulation above and below the heating element, will take the same amount of power independent of whether it's 230 V, 240 V, extra-low voltage, water or molten sodium.

 

If the amount of insulation above the element (wire, foil or pipe) is increased then the element will need to be hotter which will increase the losses downwards unless the insulation below is also increased to compensate but, as @JSHarris points out, with sensible amounts of insulation below this loss will be relatively small anyway. It's just possible that a low-voltage element which can be very close to the surface could make significant savings compared with elements buried a bit deeper but only if the insulation below is completely inadequate. I suspect their figures are done on the assumption of no insulation below the elements.  If so, that's a cheat IMNSHO.

Edited October 14, 2019 by Ed Davies

[Jeremy Harris]

It seems that this information is disingenuous at best.

 

If a room takes a floor heat output of, say, 100 W/m² (which is very high indeed - such a floor would be verging on being uncomfortably warm to walk on) then it needs a heat output of 100 W/m².  If the floor heat output was reduced to 50W/m², the heat to the room would be halved.  Of course, halving the heat to the room would be cheaper to run, but then the room would be a lot colder in cool weather.

 

The starting point is to work out the heating requirement for the house.  This should be on the design EPC for your build.  Work out the actual heated floor area that's practical (i.e. floor area not covered by fitted furniture etc) and then divide the power needed in the coldest weather by the heated floor area and you will end up with the floor heat output needed per m².  For example, our house has a floor area of 130m², but a heated floor area of 75m².  In the coldest weather we're likely to get here (-10°C outside) the house needs about 1.6 kW of heating.  1600 W / 75m² = 21.33 W/m²  This heat output, with a room temperature of 21°C, needs a floor surface temperature of 23.2°C

 

This is the worst case heating requirement, for most of the winter it will be a lot lower than this.

[Jeremy Harris]

2 hours ago, tonybythesea said:

Therefore to summarise for 1Kw of electricity a 230/240V system would heat an area of only 10.2m² using 100W/m²,  6.7m² using 150W/m²  and only 5.0m² using 200W/m²

Whereas Step Warmfloor heats an area of 18.4m².  Therefore Step Warmfloor heats an area of between 2-3 times larger with the same 1Kw of Electricity.  ...

As regards to cost it is easy to see that Step Warmfloor costs 2.75 less to run than the 150W/m² product and 3.68 times less to run than the 200W/m² product."

 

 

OK, lets take a look at some real numbers here and see whether any of this stacks up.

 

The formula normally used for approximating  the heat output from UFH is pretty well proven from experience, and is:

 

Heat output per m²= 8.92 * Δ_T^^1.1 , where Δ_T is the difference in temperature between the room and the floor surface, in °C (it's really K, but °C is the same for this purpose)

 

Let's assume that the room temperature is 21°C, just because that's a fairly typical figure. 

 

if the electrical input is 1 kW, there are no losses down through the floor and the floor area is 5m², then that 200 W/m², working from the approximation formula above, and assuming a 21°C room temperature, will give a floor surface temperature for the 5m² of heated floor area of ~34.85°C (which would probably be uncomfortable to walk on).

 

If the heated floor area is increased to 18.4m², then for the same 1 kW of heat input the floor surface temperature drops to ~25.24°C, a far more reasonable temperature.

 

Note that the heat output to the room is exactly the same for both these cases.  All Step Warmfloor are doing is trying to pull the wool over customer's eyes by playing around with the heated floor area.  They aren't lying, they are just being a bit creative with the way they are presenting thing, in the hope that they can fool people.

 

 

 

[SteamyTea]

1 hour ago, JSHarris said:

Heat output per m²= 8.92 * Δ_T^^1.1

Where does that come from as it is pretty useful for estimating.

[ProDave]

Floor temperature of 28 degrees.  If ours was that warm, we would cook.

 

Those figures a few posts above are just a load of meaningless twaddle.  you cannot change the laws of physics.  The amount of KWh of heat a room needs to keep it at a particular temperature depends on the levels of insulation, and what the outside temperature is.  

 

If you choose to use resistance electric heating, then any system will cost the same. There is (ins spite of what some manufacturers like to imply) any particular make of resistance heater that is in any way more efficient than another make.

 

 

[Jeremy Harris]

32 minutes ago, SteamyTea said:

Where does that come from as it is pretty useful for estimating.

 

 

It's the approximation used by UFH installers/system specifiers.  That's where I got it from, anyway. 

 

Looking at it, it seems to be based on a combination of the radiated heat output (from the Stefan Boltzman equation), with a frig factor added to allow for the conducted/convected contribution.

 

The Stefan Boltzman equation is:

 

P = eσA(T⁴-T_C⁴)

 

Where:

P = power (W)
e = emissivity
σ = Stefan's Constant = 5.6703 E-8 W/m².K⁴

A = area (m²)

T = temperature of the radiating surface (K)
T_C = temperature of the surroundings (K)

 

 

[SteamyTea]

Ah, that makes sense, my least favourite of the equations.

 

[Ed Davies]

The Stefan Boltzman equation is one of my most favourite of the equations. ?

 

The emissivity, e, typically equals about 0.9 to 0.95 for most building materials other than shiny metal.

 

A simpler approximation to @JSHarris's first equation is to just use 10 W/m²·K. I.e., 10 watts for each square metre of warm floor for each °C of temperature difference between the floor surface and the average room temperature.

Edited October 14, 2019 by Ed Davies

[SteamyTea]

Funny how people have different favourite equations, mine is Newton's Law if Cooking.

I would type it properly if I switched auto correct off on my phone.

[tonybythesea]

Help! I thought Mr Boltzman would make an appearance son and that by then you'd have lost me. ?  

 

Just so you know, the explanation offered by Warmfloor of why their resistance heating can be more cost-effective than other electric UFH is as follows. "The reason why cable systems require more W/m² to operate is that a very small diameter cable  only heats 2% of the floor so has to reach a very high temperature in order to pass heat across a 50mm space. Step Warmfloor heats more than 60% of the floor so needs less energy to heat a much much larger area and does not need to reach such a high temperature to do so.  It heats the whole floor at an even temperature with no hot or cold spots."

 

Does this relate to Ed Davies' comment: "If the amount of insulation above the element (wire, foil or pipe) is increased then the element will need to be hotter which will increase the losses downwards unless the insulation below is also increased to compensate but, as @JSHarris points out, with sensible amounts of insulation below this loss will be relatively small anyway. It's just possible that a low-voltage element which can be very close to the surface could make significant savings compared with elements buried a bit deeper but only if the insulation below is completely inadequate. I suspect their figures are done on the assumption of no insulation below the elements.  If so, that's a cheat IMNSHO."

 

I'm intending to post the heat loss calcs and other dimensions of our building FYI later today but my wonderful day job needs me too!

[Jeremy Harris]

The thermal conductivity of concrete is reasonably high, around 1 W/m.K, so heat will fairly quickly even out across the floor.  If it didn't, then there would be a risk of the hot spots from resistance  wire underfloor heating being dangerously hot, from their own description.   I fitted resistance wire underfloor heating in the bathroom at our old house, directly under the floor tiles.  The wires were spaced ~100mm apart and you couldn't tell, when walking on the floor in bare feet, where they were, as the tiles were all very evenly heated.  This pretty much proves that their hypothesis that small diameter heating wires (ours were ~3mm in diameter, including the silicone insulation) will get very hot is simply wrong.

 

It's also worth looking at the running cost for various UFH options.  Using electric UFH at the surface of the floor will not provide much in the way of heat storage, so the heating will almost certainly need to be on during the peak rate period.  The electricity cost is likely to be around 12p to 15p/kWh, so if the house needs, say, 1 kW to maintain a comfortable temperature, and needs to be heated for 15 hours per day, then the cost will be around £1.80 to £2.25 per day.

 

The same amount of heat supplied by an electrically driven ASHP will cost around 1/3rd of this, so about £0.60 to £0.75 per day.

 

The same amount of heat provided by an ASHP that is running UFH in an insulated concrete slab, so can be charged mostly overnight at the off-peak rate, will cost even less, perhaps less that £0.45 per day.

 

As a general rule of thumb, an ASHP will run at a COP of better than 3 when running UFH.  Our's has been running at a COP of over 3.5 pretty much since installed.  This will always make the running cost at least 1/3rd of the cost of direct electric heating, most probably even less most of the time.  Being able to heat up a floor slab overnight on cheap rate electricity gives another significant running cost saving.

[SteamyTea]

Heat does not 'rise' in a solid, so I am not sure how much difference where the elements are located actually makes in practice.

But I still think that fitting wet UFH pipes us the way to go, then from an educational viewpoint, you can play about with different heat sources later.

[tonybythesea]

I can now share some heat loss calcs - document attached. It gives two options as to level of insulation; we are currently thinking of the lower level. 

 

The usage pattern for our studio building will be almost entirely daytime use and with periods of many days together with nobody there. The nature of that use means we will be going for a very durable washable floor surface but not ceramic tiles. Vinyl or vinyl tiles most likely as of now. Therefore I don't think we can expect the floor to hold a lot of heat and may just need to accept we put the energy in often at peak tariff times. Careful management and  control systems presumably can make all the difference to eventual bills. 

Revised Heat Load Calculations 11 10 19.xlsx