Adding underfloor heating to existing circuit? Share switch?

[SJRSJR]

Hello

When my ensuite gets renovated I'd like to add 100W electric underfloor heating. The room is 4 sq m so am I right in thinking the load is 400W?

The lighting circuit that serves the bathroom includes the landing and two bedroom lights as well as the extractor fan and electric towel radiator. I plan to put a new towel radiator on the CH circuit but make it dual fuel for which I will need a 400w heating element. At the moment the radiator and extractor fans are on switched fused spurs outside the room. (How do they differ from isolation switches?)

I've read that it should be on its own circuit but it won't be worth the aggro to run new cable to the consumer unit. Would my electrician be able to add the u/f heating to the existing circuit? And could an isolation switch/fused spur serve two appliances e.g. the extractor fan and u/f heating? Thanks

PS The house has full RCD protection at the consumer unit.

 

[Taylortwocities]

At the moment the radiator and extractor fans are on switched fused spurs outside the room. (How do they differ from isolation switches?)

A fused spur IS an isolation switch in that it disconnects both live conductors- line and neutral.

And could an isolation switch/fused spur serve two appliances e.g. the extractor fan and u/f heating?

Possibly, like many things “it depends”…

Would my electrician be able to add the u/f heating to the existing circuit?

You’d have to ask him/her.

 

[ericmark]

It seems you're asking if UFH can be supplied from the lighting circuit? In the main, a lighting circuit is limited to 6 amps, as many ceiling roses are only rated at 6 amp or 1380 watt, I know new LED bulbs are well below 100 watt, but that was the traditional amount we were told to allow for, and you're talking about using 800 watt for non lighting. So 5 bulbs left to go.

"Every installation shall be divided into circuits, as necessary, to take account of danger that may arise from the failure of a single circuit, such as a lighting circuit" It may well comply, but it is not cut and dried, I would not criticise an electrician either way.

 

[Murdochcat]

If the room is 4 square meters you won’t be fitting a 4 square meter pad. It’ll be much less, maybe 1.5 m

 

[Taylortwocities]

If the room is 4 square meters you won’t be fitting a 4 square meter pad. It’ll be much less, maybe 1.5 m

Eh? How?
Stop smoking that stuff.

 

[Murdochcat]

Eh? How?
Stop smoking that stuff.

Gap from the wall all around the room, similar gap around bath, toilet, basin etc.

Not good to have electric underfloor heating under anything

 

[SJRSJR]

Thanks for the replies.

you're talking about using 800 watt for non lighting. So 5 bulbs left to go

The maximum from the downlights would be 4 x 4.7W. I use LED lighting but even if a subsequent owner put 3x150W old style bulbs in the other rooms (why would you?) it would surely still be within capacity?

Gap from the wall all around the room, similar gap around bath, toilet, basin etc.

It's an ensuite, (there's a separate circuit for the main bathroom) and it'll be done as a wetroom, so no bath or shower tray, but yes, I'll take account of the w/c and basin, I was just rounding up. Speaking of which, it seems better to get the loose wire heating so you can work round the dead areas, is there a downside to doing it like that? Thanks.

 

[ericmark]

When I did my late mother's wet room, I could not find chemical underfloor heating elements with the built-in earth braid needed for a wet room, I could only find the pure resistive type. Nothing attached to the floor, toilet, and wash basin were wall mounted. We used the maximum permitted, and it was useless.

OK, got the floor warm, but with the required extractor running it did not heat the room. Lucky the towel rail did. We had 9" of insulation under the floor, and even with floor off, the floor was not cold. The main idea was the UFH would dry the floor, but unless mopped, it would take 1.5 hours to dry, and once the shower had been on, the floor was cold, we had expected to shower if anything to warm the floor but no, it cooled it, and it would take an hour to get warm again.

 

[SJRSJR]

Thanks ericmark.

I wasn't expecting to heat the room with it (why I was going for the 100w product) but rather 1) to take the edge off when in bare feet and 2) to help speed up the drying of the wet area after using a squeegee blade.

This might elicit criticism but when it's really cold I tend to shower without the extractor fan, then put it on when I'm leaving the room or open the window and leave the door open. I might be tempting fate but I haven't had any ventilation issues so far.

We had 9" of insulation under the floor, and even with floor off, the floor was not cold.

So, are you saying insulation will help take the edge off the cold tile and not to bother with u/f heating?

 

[ericmark]

So, are you saying insulation will help take the edge off the cold tile and not to bother with u/f heating?

Yes, it was the insulation that helped.

The floor was dug out to access the drains, so there was actually less work having 9" to 3" but I did not do the groundwork, a builder was to have done the wet room but ran off in the middle of the job, so I had to take over, mother had lost her leg and the wet room was for her, the builder destroyed the first UFH the tile glue was out of date and not setting, so easy enough to lift tiles and re-lay a new heating mat, our error was the pocket for the sensor had too tight of a bend, and when years latter the sensor failed it would not come out of the pocket.

Switch on and ½ hour before you could feel any heat, 2 hours to reach full temperature which should be limited I seem to remember at 27ºC, so I would come down at 6:30 am to turn it on for mother having a shower with carers at 8:30 am but at 9:30 am the floor was still wet, and had to squeegee blade the floor for it to dry quicker.

We selected sculptures tiles for grip, but this resulted in more water being held on the tiles, all in all it was a failure.

 

[Diver Fred]

My thoughts based on the experience of fitting my own bathroom first followed by a couple more.

If the room is 4 square meters you won’t be fitting a 4 square meter pad. It’ll be much less, maybe 1.5 m

Not entirely sure I agree completely. it will be less but not that much less - maybe 2.5 to 3 sqMtr

Gap from the wall all around the room, similar gap around bath, toilet, basin etc.

Not good to have electric underfloor heating under anything

Do plan your room out on the insulation (you are insulating the floor, yes?) particularly where the toilet (and Bidet) will sit, Hand basin pedestal if you are using one. If the pipes go through the floor mark those positions also. Cupboards can be hung from the wall. Mark all the positions in a bold colour. If you are using 'heating mat' you will need to separate some of the element off of the mat to negotiate around intrusions into the floor.
Things to think about - where you stand when 'pointing Percy at the Porcelain' it's not nice to have one foot warm and one cold and the wife may agree (especially if you also install a bidet. That will be a loop either around the toilet base or a loop either side.
With care you should be able to 'loose' the remnants of the mat under the bath if necessary - very useful if your bathroom is cold and possibly frost prone. One square crossing shouldn't cause a hot spot.
Make sure you have plenty of duct tape to hold the element down but don't leave any air gaps between the tape and element.

If the temperature sensor is cast in the floor - install 2. If the temp sensor goes in flexible conduit make sure it slides easily.

Do the electrical tests at every step and record the results in the installation document.

 

[Diver Fred]

It's an ensuite, (there's a separate circuit for the main bathroom) and it'll be done as a wetroom, so no bath or shower tray, but yes, I'll take account of the w/c and basin, I was just rounding up. Speaking of which, it seems better to get the loose wire heating so you can work round the dead areas, is there a downside to doing it like that? Thanks.

Loose element you use a 'bubble' mat laid first over the sub floor to clip the element in to give the correct spacing and stop bending the element excessively.
Ask your supplier if the element can be installed under the shower tray if that is bedded on the same flooring compound as the floor tile that the element is cast in.

 

[ericmark]

If the area was not originally a bathroom, it will need planing permission, I got in trouble as builder had not applied, seems down to the owner, but builders often help.

 

[SJRSJR]

Thanks for the replies, very helpful.

I intend to be very careful about the positioning, that’s why I’m thinking about loose cable on a grid - maybe run the cables one notch closer together in traffic areas. I’ve had it in a kitchen before - the fitter told me he’d not laid it under the units, but turns out he’d positioned it so the first third of your feet were cold when standing at the sink.

Insulation - rolls or PIR board? I’ve got some 50mm PIR left over, would that be thick enough?
Below is a heated room.

It’ll be a wall hung sink/vanity unit, WC and tiled shower area, so not a shower tray.