I have read a number of posts about water based underfloor heating but very little mentioning electric underfloor heating.
My understanding is that there are two varieties of electric heating.
1) The single wire running at around 70c.
2) Carbon mats or amorphous metal ribbon cables running at around 25c.
The second option is supposed to be pretty efficient and appears to be highly favoured in other countries e.g Scandinavia. It also seems to be a relatively easy way to obtain underfloor heating.
With rising prices and reducing gas supplies, I would have thought the electric route might now be more sensible.
My floors are 18mm chipboard laid on top of 35mm expanded polystyrene. I was planning to use these mats under an oak cliplock floor (3.5mm oak layer).
Any comments would be appreciated.
Tony
posted on 23-05-06
Electric heating is expensive to run you do have an advantage already having some insulation under the floor. Mats are better and 3.5mm of oak sounds awfully thin? Electricity is 100% efficient from your meter but is inefficient to produce and distribute. In the very long term future you will win but you may go broke getting there!
Paul in Montreal
posted on 23-05-06
The efficiency of electric underfloor heating is the same irregardless of the temperature: 100% of the electricity put into the heating system will be converted to heat. Of course, the conversion of raw energy to electricity at the generating station (and then distribution to your house) is not 100% efficient.
The question you need to address is whether you want the heated floors as a primary source of heat or an auxiliary source. Bathrooms (under ceramic tile) are a good place to put the systems that operate at a higher temperature since there's nothing nicer than stepping out of a bath or shower onto a warm floor. For living space, you don't want the surface temperature much above 23C otherwise it starts to become uncomfortable (the optimum is between 19C and 23C) if the radiant floor is your primary heating source.
Another consideration for primary heating applications is flexibility. As energy availability changes in the future, it may make sense to have as flexible a delivery mechanism as possible. Hydronic systems (i.e. water based) are the most flexible since you have a great choice in the energy source used to heat the water: passive solar, high-efficiency gas boiler, water-to-water heat pump, large heap of manure with embedded pipes (really, I'm not making this up) etc. If you put in an electrical system it can only operate with electricity. For a primary heating system, the energy requirements are currently too large to be serviced by renewables such as solar or wind. If you wanted to go this route, a ground-source heat pump would be a better bet as the energy output is 3-4x the energy input - even then, the cost of a renewable system to power it is still infeasible IMHO (believe me, having two GSHP systems I've researched this).
Hope this helps,
Paul.
Chris
posted on 28-05-06
Thanks for the replies.
Tony,
The 3mm Oak layer is on-top of a 12mm composite wooden structure making 15mm depth in all.
As for the cost, the manufacturers claim that these newer lower temperature electric systems are significantly cheaper to run than the older wire based systems, as they have greater surface area.
Paul,
Unfortunately in the tiny houses with small gardens that we have to put up with in Britain, flexibility is tricky. I could fit solar panels on my roof but a wind generator would be a definite no-goer.
My only other option is to put in radiator heating with a condensing boiler, but that is disruptive to the infrastructure of the building, tearing up floors etc and is obtrusive with visible pipework and radiators.
Ah well.
Cheers.
Biff
posted on 01-06-06
Using grid-generated electricity is amongst the least green methods of heating your house as rather little of the energy in the gas/coal/nuclear/oil ends up in your house.
The next several years will see a developing crisis in electricity generation capacity as older powerstations are retired and we become dependant on Russian gas for new generation. Electricity prices will only move one way.
There might be a case for using electric underfloor heating as an intermittent heat dump for your surplus energy from your domestic wind turbine on windy days, or one day, when we have dynamic metering systems to use it as a supplement at times when electricity generation from intermittent renewables is in surplus.
fostertom
posted on 01-06-06
Biff, agreed, if you mean taking just the right amount of electricity from your turbine, when currently available, in preference to grid electricity. If you mean using massiveness to store peaks of generated energy till it's needed - I'm interested in that, but not so simply in walls/floors, I'd have thought?
The exception is daytime sun coming in through e.g. big south windows, absorbed by falling on heavyweight walls/floor. (I'm sure you know this but anyway ...) In that case, the gain stops neatly by suppertime and the stored heat can be re-radiated through the evening, as outside temperatures drop. By morning the walls/floors are coldish and this coolth is re-radiated (not exactly but it seems that way) through the next day, keeping the interior cool despite the renewed insolation. Big south windows, indispensably coupled with heavy walls/floors, are the first rudimentary step towards energy-harvesting (as opposed to merely well-insulated) buildings. Big south windows are a disaster with lightweight walls (e.g. plasterboard faced) and floors (e.g. timber faced - even rugs on a tiled solid floor aren't so good) - but may be OK with just one or the other.
But beyond this modest input and without the automatic suppertime cutoff, storing energy in massive floors/walls has been tried before. High temperature underfloor heating - mainly electric but also wet - was put into tower blocks and caused swings between extreme under- and over-heating, especially when the electricity was used nightstore-style. Underfloor heating went hand-in-glove with the bad reputation of the tower blocks themselves, and the 'new' low temperature underfloor heating industry took decades to live it down. Despite the massive slow-response principle of modern underfloor heating, accurate heat input is still essential. So using it as a 'dump' for unspecified amounts of generated energy isn't, I think, a good idea - you can't stop it all coming out again within a few hours. Especially wind-generated electricity, which surpluses overnight (unlike solar), so the walls/floor start re-radiating just as the sun comes up!
So - 'naked' massiveness works well when coupled with big south windows. Other 'naked' situations have to be carefully calculated. E.g. a massive uninsulated south wall storing heat collected by a trombe/conservatory, or a 2ft thick cob first floor storing energy ducted, piped or wired there from an outside source. The thickness, density and conductiveness of a mass gives it a characteristic time-lag, which can be tuned to requirement. That's the time it takes (in the range of 2 to 12 hours) for the peak of a temperature wave to pass through and appear as a temperature peak on the back face, so the re-radiation of stored heat peaks when you want it there. If the back face is insulated, then the wave is more or less reflected and so returns to the original front face and peaks there at about twice its time-lag period. The time and quantity graph-curve of the energy input has to be matched carefully to the desired release curve, allowing for calculable attenuation and 'blurring' of the curve. Anyway, it'll all be over within 24hrs.
For longer-term storage, the massive store has to be remote and highly insulated. The hotter the input, the denser the heat can be stored in given mass, subject to correspondingly better insulation. I.e. electric heating elements (from future PVs too, as they bacome economic) far outperform water or air as input medium. For really longterm storage, i.e. 'easy' summer energy stored till winter, I believe we'll be putting it deep into the ground. From there, much would disperse and be lost sideways - but that's OK - and *not* if your neighbours were doing the same! None would be lost downward, as the natural subsoil temperature rises indefinitely as you go down. By tuning it right, the wave of summer heat input could go down and apparently reflect back to peak near the surface in say January. There it could be re-harvested and used for space heating, much like present ground-source heat pumps but without the heat pump, or at least providing a much warmer source for the heat pump, thus improving its ratio of useful heat to electrical input.
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