Electric underfloor heating disaster

[ishaqmir]

Hi everyone

12 months ago we got a builder to do a detached double garage conversion into a studio annex. He quoted 2 months and took 12 months, and that was after I had to intervene and tell him how to do a lot of the building work by having a chat with the building regs man who was overseeing things.

Anyways, finally kicked the builder out of our property, and most things are ok. However, I had to do the electric underfloor heating myself. Laid it directly on the new 50mm screed WITHOUT laying any insulation on top of the screed (I know, I don’t know how this slipped my mind). I thought it would be ok because of the insulation under the screed (builder laid 50mm kingspan type on the bare ground before the 50-60mm screed)

Tiler came and he also thought it would be ok. He put a layer of latex levelling over the heated mats, then laid the tiles with flexible adhesive. Now its come to trying out the heating and it takes HOURS and HOURS to even get just the chill off the tiles, never mind heating up the room. What a disaster! The mats are definitely all working fine as I did the multi meter test before, during and after installation, its all good.

Now I had a chat with the tiler and he has suggested that I start from scratch ON TOP of the existing tiling: lay the insulation, then mats, then latex (optional), then the tiles again with flexible adhesive. Problem is, now the fitted wardrobe and full kitchen has been installed, and the toilet and basin unit too.

I’ve had a look at the main door sill (see photos) and the shower tray height from the tiles and determined that I have about 40mm to play with. The tiles with adhesive will be around 10-12mm. What would be the best course of action?

As you can see from the photos, I’m mega annoyed as the garage conversion otherwise looks pretty good. But this flooring is a major disaster…

Any advice much appreciated…

 

[johnny2007]

Get an electric heater in there, unless you have money to throw away.

 

[ishaqmir]

Thanks for the reply, yes, I was thinking of that. Have put one in there for the time being. Problem is the bathroom, what would we do about the heating there?

 

[johnny2007]

Thanks for the reply, yes, I was thinking of that. Have put one in there for the time being. Problem is the bathroom, what would we do about the heating there?

Same.
I didn't mean portable electric heaters, but fixed ones, the ones the use in modern flats that stay fixed to the wall.

 

[robinbanks]

"it takes HOURS and HOURS"

Of course. It takes a 1.5kw kettle about a minute to increase the temperature of 1 litre of water by 20 degrees. A 15sqm room would draw about the same power (1.5kw) but that electric mat is bonded to about 600 litres of screed. If that were water it would take 10 hours to heat up by 20 degrees. The screed has a heat holding capacity that is about half what water is, so you're still looking at around five hours to lift the floor temp from eg 10 degrees to 30

It does however, take a long time to cool, and emits most of its heat into the room so basically, you should just leave it on on a thermostat all the time. Screed based UFH systems are NOT supposed to be blasted on and off like radiators. They're also not supposed to run at high temperatures - if you walk on the floor and think "gosh that feels nice and warm" then it's too hot- a screed pad at about 26-28 degrees will warm a well insulated room to 22 degrees but will not feel warm under your feet because your feet are between maybe 26 and 37 degrees depending on what you've been walking on. Even cold feet (26 degrees) walking on a 26 degree floor will not register it as warm

"builder laid 50mm kingspan type"

It's a bit low; kinda surprised your building inspector let him away with that one, 70mm probably more typical but under a UFH floor I'd have put 150mm personally. Bit late to change that now alas; get a builder more capable of thinking things through next time and advising you not to go for the absolute bare minimum bulding regs requires. Take solace however, that you saved a large amount of money on insulation that you can now spend on electric!

That said, if you leave the UFH on constantly then gradually the ground temp under the insulation will warm slightly and less heat will flow across the insulation, into the soil, so more heat will be lost into the room, warming the air

Make sure the rest of your room is well insulated and free from draughts and leave the floor on permanently on a decent thermostat designed for underfloor (one that can detect tiny changes in temperature so it can cycle on and off without letting the room cool more than maybe 0.3 degrees)

-

Or if you want to run your underfloor like your radiators, blasting it in morning and evening so it feels warm underfoot and having it switch off when you're out, proceed as your tiler said, and lay a new floor on top of the existing. I wouldn't

 

[Notch7]

Anyways, finally kicked the builder out of our property, and most things are ok. However, I had to do the electric underfloor heating myself. Laid it directly on the new 50mm screed WITHOUT laying any insulation on top of the screed (I know, I don’t know how this slipped my mind). I thought it would be ok because of the insulation under the screed (builder laid 50mm kingspan type on the bare ground before the 50-60mm screed)

Tiler came and he also thought it would be ok. He put a layer of latex levelling over the heated mats, then laid the tiles with flexible adhesive. Now its come to trying out the heating and it takes HOURS and HOURS to even get just the chill off the tiles, never mind heating up the room. What a disaster! The mats are definitely all working fine as I did the multi meter test before, during and after installation, its all good

but what you have done is correct.

the electric underfloor heating cables are in direct contact with the screed, so they will heat the screed as they warm, the screed will act as a heat sink and will release the heat over many hours.

Thats how underfloor heating is meant to work, if you want the floor as a primary heat source, but you need a suitable underfloor heating system that can output between say 150-200 watts per sq metre.


You are confusing it with a heating mat for just making the tiles warm - if you insulate below the cables, as soon as the heating is switched off, it will cool down. Maybe you laid a heating mat only designed for this purpose.

 

[Wayners]

I was working on a job 15 years ago where the new electric floor heater never worked in the kitchen and the large tiled floor was taken up.
All new and tested. New tiles fitted again.
Lady turned on and the meter spun at some rate. It took a few days to warm up but when the first bill came in it was switched off and stayed off.
Was a 5m x 4m kitchen.
Think fitters took a big hit.

 

[cdbe]

My sister paid a few hundred extra to have it under her tiled hallway some years ago when it was all the rage - I doubt she even remembers where the switch is now.

 

[JohnD]

when the first bill came in it was switched off and stayed off.

This is quite usual for electric UFH

Since energy from electricity costs about four times as much as energy from gas.

 

[robinbanks]

I was working on a job 15 years ago where the new electric floor heater never worked in the kitchen and the large tiled floor was taken up.
All new and tested. New tiles fitted again.
Lady turned on and the meter spun at some rate. It took a few days to warm up but when the first bill came in it was switched off and stayed off.
Was a 5m x 4m kitchen.
Think fitters took a big hit.

Which could all be entirely irrelevant; maybe the floor was poorly insulated, maybe the room had solid walls and was draughty as anything..

Ultimately the floor is an emitter, emitting the heat energy put into it. Design should be performed so that the emission is into the room and then retained. If the floor and room were well insulated and draught free then a fan heater and an underfloor electric mat of equivalent wattage would cost around the same to run and heat the room similarly

 

[JohnD]

If the floor and room were well insulated and draught free then a fan heater and an underfloor electric mat of equivalent wattage would cost around the same to run and heat the room similarly

Except that very few people run a fan heater 24 hours a day, 7 days a week, to maintain the kitchen at a constant warm temperature.

 

[robinbanks]

The human element is indeed the flaw in the system

 

[JohnD]

The time lag and slow release.

 

[winston1]

This is quite usual for electric UFH

Since energy from electricity costs about four times as much as energy from gas.

Indeed and leaving it on 24/7 instead of say 8 hours a day for a radiator means you are paying 12 times as much.

 

[ericmark]

We fitted electric underfloor heating in mothers wet room, good job also had a towel rail on central heating, it was so slow to heat up, turn on to tiles feeling warm 1 hour, as to room getting warm, the extractor fan resulted in never warmer than hall where the replacement air came from.

And loads of insulation, 9 inches.

@winston1 makes a good point, energy to maintain is a lot less than needed to heat up, using a fan heater is clearly going to be faster than convection.

Under floor heating is great for an old peoples home, but not for a home empty when occupants are at work.