Another underfloor heating question

[wilbobob]

I'm planning a bathroom refurb. I'm looking into electric underfloor heating. The free floor area is only small and a 1 sq meter mat at 150 watts will be all I can get in. There's an existing high level 2kw fan heater already fitted, fed by a fused spur inside the bathroom up at ceiling level and outside Zone 2. I'd like to use the same supply for the UF mat. As the existing main supply board is a split type with RCD's to both sides, do I really need to replace the fused spur with another RCD as some sources seem to say, or can I rely on the RCD in the main board for protection?
I intend placing the air/floor thermostat under the spur/RCD, within the bathroom at high level outside Zone 2. The thermostat has IP2x protection. Would this be considered satisfactory?
I'll intend getting the installation tested and certified by a suitably qualified person but want to present him/her with the correct fittings.

[redsturgeon]

I'm planning a bathroom refurb. I'm looking into electric underfloor heating. The free floor area is only small and a 1 sq meter mat at 150 watts will be all I can get in. There's an existing high level 2kw fan heater already fitted, fed by a fused spur inside the bathroom up at ceiling level and outside Zone 2. I'd like to use the same supply for the UF mat. As the existing main supply board is a split type with RCD's to both sides, do I really need to replace the fused spur with another RCD as some sources seem to say, or can I rely on the RCD in the main board for protection?
I intend placing the air/floor thermostat under the spur/RCD, within the bathroom at high level outside Zone 2. The thermostat has IP2x protection. Would this be considered satisfactory?
I'll intend getting the installation tested and certified by a suitably qualified person but want to present him/her with the correct fittings.

I have had electric underfloor heating installed in 2 bathrooms and neither to my knowledge has a separate RCD. The thermostat for the underfloor heating is installed within the floor with the mat.

John

[PrincessB]

There's an existing high level 2kw fan heater already fitted,

As an option would you have space for one of these?

https://www.cnmonline.co.uk/dimplex-rad ... 30832.html

After a lot of thought, I decided to use one of these for a smallish 3x3 metre bathroom and have found it brilliant.

We usually leave it set at 2 which is sufficient to remove the chill while not using much power, over the weekend we set it to 5 which makes the room toasty warm for longer showers and bathing.

If there is prosecco involved the 7 setting makes things really cosy in short order.

As the bathroom was built into an existing room, I specified lots of insulation so there is celotex and rockwool everywhere, four inches of the stuff in the floor along with an insulated ceiling, the stud partition walls are packed with it and the external walls are lined with insulation backed plasterboard.

It's not the prettiest heater but that 1kw output is greater than most towel rails can manage. Unlike a fan heater it is completely silent (no clicking of a relay due to the design of the electronics) I wanted two of them and then found I had no space for the second one - It turns out that one is ample.

Regards,

B.

[richlist]

All the people I know with underfloor heating have it powered by gas not electric.

Electric is significantly more expensive than gas.

[redsturgeon]

All the people I know with underfloor heating have it powered by gas not electric.

Electric is significantly more expensive than gas.

Two very different systems for different purposes.

John

[wilbobob]

Thankyou to all the responents. I think I'll make provision for a local RCD and leave it to the tester to decide what's needed
Bob

[csearle]

I'm planning a bathroom refurb. I'm looking into electric underfloor heating. The free floor area is only small and a 1 sq meter mat at 150 watts will be all I can get in. There's an existing high level 2kw fan heater already fitted, fed by a fused spur inside the bathroom up at ceiling level and outside Zone 2. I'd like to use the same supply for the UF mat. As the existing main supply board is a split type with RCD's to both sides, do I really need to replace the fused spur with another RCD as some sources seem to say, or can I rely on the RCD in the main board for protection?
I intend placing the air/floor thermostat under the spur/RCD, within the bathroom at high level outside Zone 2. The thermostat has IP2x protection. Would this be considered satisfactory?
I'll intend getting the installation tested and certified by a suitably qualified person but want to present him/her with the correct fittings.

Hi, no, the RCD in the consumer unit is perfectly adequate* so long as it is a 30mA one. In fact if you place another RCD of the same sensitivity in series with the one in the CU you introduce the problem of discrimination (i.e. you won't know which one will trip first).

So long as your controller is out of zones (i.e. beyond zone 2) then it need not have any particular IP rating. I would question putting it at high level though because the length of lead on the floor sensor may be insufficient, and even if it is sufficient, it is best to use a 10mm flexible conduit running from the controller down the wall and into the floor so that you can replace the sensor should it develop a fault. If the controller is high up you may not be able to push any replacement around the 90 degree bend at the bottom of the wall. Having the controller (or the fused connection unit) low down allows this to be done fairly easily. The sensor and the power to the mat should be run in separate conduits.

The insulation resistance (and series resistance) of the UFH element should be tested upon receipt, after the mat/line is laid but before it is tiled/covered, and at the end. The sooner a fault is detected the cheaper it is to fix.

Regards,
Chris
(Electrician)
*Make sure the existing heater is not on an unprotected circuit.

[wilbobob]

Thank you Chris, that's the information I was hoping for. I understand what you are saying about the sensor and the flexible conduit. The high level option is first choice as it's above the door in a stud wall. Perhaps I'll bite the bullet and chop out sockets in the brick wall next to it. It's an old house and the bricks are really hard, so not an attractive option.

[csearle]

The cable between the controller and the UFH mat/line is not allowed to run just anywhere, it has to be run within zone, i.e. orthogonally from the controller (assuming the cable's depth is within 50mm of the wall surface) so if the existing fused connection-unit is above the door then you might have difficulty adhering to this wiring regulation. There are other zones that you might be able to use though. See here.

The precaution of allowing the sensor to be replaceable is one that not everybody bothers with.

Regards,
Chris

[csearle]

PS If you are anywhere in the vague vicinity of Tunbridge Wells and in exchange for a pint of beer I'd be happy to come and chase out a channel through your bricks.

C.

[wilbobob]

Leicestershire is only very vaguely near Tunbridge Wells, and I don't think I can get the wall on the back of a lorry, but thank you for the kind offer and the further advice. Apologies for the delay in replying, been away for a few days.
Bob

[csearle]

Ha ha, never mind.
C.