UFH 40m2

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Nobby862

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A customer has asked for a quote for a garage conversion, one of the requests is to have underfloor heating, the problem is that they have asked for the electric mat system and the area is 40m2. is that even possible with the load on the wiring. it seems like a lot of cable under the floor. also the running cost would be frightening!

The garage is detached so no central heating available, the sub floor will be the standard floating floor with engineered wood floor covering of some sort, i would like advice on an alternative as i think the area will take to long to warm up unless the UFH is left on all day and night. convector heaters or skirting board heater (know nothing about these) cam to mind but this customer expects a high spec finish and has the funds to do so, saying that convector heaters can look a little cheap. by the way im a general builder with limited electrical experience..

Thanks Nick
 
Nobby862 said:
A customer has asked for a quote for a garage conversion, one of the requests is to have underfloor heating, the problem is that they have asked for the electric mat system and the area is 40m2. is that even possible with the load on the wiring. it seems like a lot of cable under the floor. also the running cost would be frightening! The garage is detached so no central heating available .... i would like advice on an alternative ... customer expects a high spec finish and has the funds to do so ...
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I'm sure you are right in wanting to find some alternative to UFH to suggest to the customer!

Is there no possibility of getting a gas supply to the garage conversion. Even if electric UFH could effectively heat the place (which I doubt), the running costs would mean that the cost of installing a combi boiler and 'local' CH system in the garage conversion would soon be re-paid by much lower running costs. Mind you, if the customer feels that electric convector heaters 'look cheap', you might have to look to something other than conventional CH radiators (maybe 'skirting' or 'enclosed' radiators?).

Kind Regards, John
 
The mats I've used are rated at about 150W per sq. m so you could have up to 6kW which wil need an electrician to install. I'd think at least two circuits with thermostats, maybe one at each end. It shouldn't be fitted under furniture or appliances. It would cost about 90p an hour to run on max. Good insulation of the walls and ceilings will save energy and cost and 6kW might be enough despite the size of the room. You can calculate heat losses with the dimensions and construction information.

I suppose if the client has solar, it wil save some money on sunny days, but I find that winters days are often cloudy and short.

If they have big windows facing the sun, they might do better with split air conditioning units that can be used as heaters in winter.
 
By 31st October he'll probably be out of a job again.

He's been sacked before for lying.
 
cheers guys. you confirmed what i was thinking. ill put it to them concerning the running costs but also the extra cost in upgrading the fuse box and supply cable as i think its only a 4mm. i think i might suggest a heat source pump, heating in the winter and cooling in the summer. cheers
 
A floating floor has no thermal store so isnt really very efficient as as soon as the mat is turned off the heat will dissapate.

Also covering it with engineered timber flooring will slow down the heat getting through making for a poor response time.

It would probably heat the room ok as the insulation will be to current regs. Its worth overspeccing the floor insulation.....although Im sure its too late for that now.

If thats what the customer wants, Id say do it but warn him of the disadvantages. If hes got the budget for underfloor heating and a split air con.
 
Wet UFH and a heat pump would be cheaper to run - typically around 1/2 to 1/3 of the cost vs just electric. But you need to look at the specs carefully as many systems lose efficiency quickly when it's very cold outside, even to the extent of having an electric heater to take over when the heat pump would have less than unity coefficient of performance :whistle:
Or isn't it possible to run a pair of pipes from the heating in the house - you can buy (for a price :eek:) pre-assembled insulated pipes in a direct bury duct ? Then they could use the (assumed) gas boiler in the house to drive the UFH. It'll give much faster warm up as the gas boiler will (or should) have the oomf to put heat into the slab quickly. Wet UFH also doesn't have the problem of having to know before you install it where any furniture might be put - see JohnD's comment about electric UFH not being put under furniture or insulating rugs etc*.

* Reason for this : Wet UFH is a constant temperature system, heat transfer is down to difference in temperature between the water and the slab, and the slab cannot get hotter than the water. Electric UFH is a constant power system, if you insulate the floor, then it will get hotter until heat loss matches power input, and that could be very hot indeed under furniture or thick rugs.
 
Well, yes you can - not widely advertised. Gets round the hotspot problem, but doesn't do anything about running costs or limitations in heating rate due to limited input power.
 
Reychem do a self regulating cable, however all under floor heating needs insulation below the mat to stop the heat escaping down, when we did wet room we had to break out original floor and install around 4 inches of insulation first, by time tiles etc, looking at 5 inches of stuff, so first question have you got the height or would your need to dig out floor.

I have said before the ivector radiators which are fan assisted with multi-speed fan are likely the best way to get heat into the room, as items placed in front do not stop it working and the fan means quick to warm up, non modulating boilers had a problem as also as fast cooling down, but real down side is price.

There are electric versions, but then same problem as under floor need a good supply. So underfloor with good insulation looking at 35 to 60W per meter² so only 3 kw which kind of gives you an idea of how good, with a maximum of 28°C for the floor, it is not very fast, and looking at Reychem T2 Red your looking at £3,500 for the cable, so likely will work out more expensive to standard gas central heating, there is some cheaper systems but they are not self regulating so place even a dog basket on the heated section and it can over heat. So you have to split into sections with pockets to slide in sensors to stop it overheating.
 
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