DIY Doctor

underfloor heating again!

Postby chris_on_tour2002 » Wed Apr 23, 2008 2:36 pm

ok, so i am almost ready to install my underfloor heating, i have a couple more - hopefully straightforward - questions:

1) the resistive cable that actually does the heating is directly connected to the wire that carries the current to it. this cable connects to the supply via the thermostat. there are two wires only, and it doesn't matter which is connected to the feed and which to the return. the earth is in the form of a sheath around the cable. dumb question but does this have to be connected to earth? if not, there will be no earth connection at all.

2) there is no provision for an earth terminal in the thermostat so instead i was going to connect the earth sheath to the earth terminal in the steel pattress box. then run an earth wire through the conduit to the fused spur and into the earth terminal in the pattress box of the fused spur, then finally into the spur itself to tie it into the earth circuit. so - what size of earth wire should i use? as the fused spur is running from the upstairs ring does the earth also need to be 2.5sqmm? also does this method of connecting to earth make sense and can it be done this waY? i cant see any reason why not...


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Postby ericmark » Wed Apr 23, 2008 5:06 pm

Table 52.3 gives min cables sizes as 1 mm² the exact size dependent on fuse size and if RCD protected and you don't give any sizes. If not protected 4 mm² and underground 16 mm² min but I should not expect this to be case and when it says underground it means outside not underfloor. I would expect 1.5 mm² to be used.
Often it does not matter which cable is which on the cold tails. see manufactures recommendations. Remember both Line and neutral are considered as Live.
Earth requirements depend where the heating is and manufactures recommendations. In bathrooms etc earth is a must.
Some systems require an earth mat fitting above the heating cables in others it is built into the main cable again manufactures recommendations must be followed.
Most manufactures recommendations can be found on their web sites there are many different under floor heating systems.
Sorry such vague answers but you have not given any types, makes, or sizes but I am sure it's all in the instructions that came with the cable.

Postby chris_on_tour2002 » Wed Apr 23, 2008 9:40 pm

hi ericmark,

sorry for my vagueness...

this product was made in india and the instructions are a little vague. the uk-based distributor adds their own additional instruction leaflet but this too does not supply answers to my questions.

it doesn't tell me whether or not the earth sheath needs to be connected (i guess i will play safe and connect it to earth anyway) and nowhere does it even tell me the size of the fuse!

also the circuit (well, the whole house) is protected by an ELCB rated at 63amp load, 30mA trip.

so i think i may need to contact the manufacturer for further clarity as clearly the info provided is insufficient.

thanks anyway.
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Postby ericmark » Thu Apr 24, 2008 10:34 am

The fuse size can be given as Watts instead of amps and you need to divide the watts by 230 (volts) to get amps. And I should think any earth provided should be connected the question is should you have also fitted an earth mat over the heating wires? Section 753 of the wiring regulations covers underfloor heating. Here is one small bit of the regulation.
753.514 Identification and notices
The designer of the installation/heating system or installer shall provide a plan for each heating system, containing
the following details:
(i) Manufacturer and type of heating units
(ii) Number of heating units installed
(iii) Length area of heating units
(iv) Rated power
(v) Surface power density
(vi) Layout of the heating units in the form of a sketch. a drawing, or a picture
(vii) Position depth of heating Units
(viii) Position of junction boxes
(ix) Conductors, shields and the like
(x) Heated area
(xi) Rated voltage
(xii) Rated resistance (cold) of heating units
(xiii) Rated current of overcurrent protective device
(xiv) Rated residual operating current of RCD
(xv) The insulation resistance of the heating installation and the test voltage used
(xvi) The leakage capacitance.
This plan shall be fixed to, or adjacent to. the distribution board of the heating system.
Furthermore. the requirements of Figure 753 apply to floor and ceiling heating systems to inform the owner and the user of the installation.

As you can see quite involved and there is loads more like max temp and max wattage etc. I would be very careful that it does meet BS 7671:2008 before fitting.

Postby chris_on_tour2002 » Mon Apr 28, 2008 2:24 pm

thanks - the system is 115 watts, so by my calculations thats only half an amp! seems a bit low for a heating system... admittedly its only a small area (0.59sqm) and its the voltage that generates the heat i suppose, not necessarily the current alone. i will contact the supplier and at the same time ask if it conforms to bs7671:2008.

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Postby ericmark » Mon Apr 28, 2008 4:49 pm

That will take chill off floor its only same heat as one light bulb seem to remember my dads wet room about 4 amp and that is not enough to heat room without towel rail too. Its watts that count for heat.
BS 7671:2008 does not come in until July 1st 2008 and is how it is installed rather than the mat it's self but without good instructions it will be hard to know what is required. Main point is earth mat. Some do not require an earth mat as all built into the heating wires and others do without detailed instructions how do you know? And yes your right 1/2 an amp.

Postby sparx » Tue Apr 29, 2008 5:29 pm

Hi Chris,
been watching this thread and will just say regs state underfloor heating requires earth screen between elements and people, either by earthed metal mat or by direct screen around cable which is most usual and seems to be what you have.
so as you have rcd at mains really don't NEED more but a local RCD fused connection unit (fused RCD spur) would be my favourite and is what I have in bathroom here. Loading seem a bit low but if set to come on early enough it should warm floor noticably on cold mornings,
regards SPARX
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Postby chris_on_tour2002 » Wed Apr 30, 2008 9:06 am

thanks for that sparx - i have confirmed with the uk supplier that an earth mat is not required as the cable contains an earth screen. and i opted for the rcd fused spur in the end, not pretty but worth for peace of mind.
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