FAQ for programmable stats - and how to wire them?

[Set Square]

Do we need one - or maybe some extensions to existing FAQs?

There are already FAQs about thermostats and wiring but - as far as I can see there's one aspect which is not currently adaquately covered. That is, when replacing a mechanical stat which uses an accelerator heater with an electronic one which doesn't, what to do with the redundant neutral wire.

Questions relating to this seem to crop up fairly often. Only yesterday we had one bright spark(!) who couldn't bear the thought of having an unused wire, and so chose to connect it to the spare contact of the stat's change-over switch - with the inevitable pyro-technics and, in his case, wrecking the CH side of his programmer.

In addition, the habit of some 'professionals' of using the earth conductor in T&E cable as a switched live can present a bit of a challenge when trying to identify which wire does what.

What do you reckon?

[Dave Plowman (News)]

Insulate and tape up out of the way - it's only common sense.

Then he really shouldn't be messing with things he doesn't understand - the presence of batteries says the device doesn't need mains power even before reading the installation instructions.

Well, I'm sure some use plastic water pipe for gas, but it's difficult to include every bodge in an FAQ. And often best *not* to cover such things.

[Mungo]

As a transient lurker/poster to uk.d-i-y I feel that Dave's suggestion (if not extensively discussed before) needs further debate.

There is a risk by broadcasting bad practise that you encourage (or educate) people to use unwise or dangerous methods in their DIY. The alternative argument is that by explaining the bodge and then going on to comprehensively denounce it you will prevent someone reinventing a bad practise and in their ignorance marvelling at the new method that they have invented.

Of course, there are many ways to mess up a job and I guess a lesser quantity of ways to do the job properly.

What say the panel?

Mungo

[Phil Addison]

Yes. Is that an offer?

Not necessarily redundant. The CM67RF relay box is mains powered so needs 4 wires, 2 for the relay and L and N for power. The 'redundant' neutral could in principle be used for the N. A L may also be needed depending on whether the existing stat was connected to a 'volt free' boiler feed, of was switching a live feed.

That said, the CM67RF is built to accept 2 separate 2 cores and it is decidedly awkward to do it the first way above.

May be just the relay that needs replacing.

If its the green/yellow, isn't that ok provided it is sleeved red? (or is it brown these days).

I reckon you would do a good job of a FAQ on it ;-)

Phil The uk.d-i-y FAQ is at

The Google uk.d-i-y archive is at Remove NOSPAM from address to email me

[Set Square]

Not really! I would contribute my five penn'th towards it, but I have absolutely no experience of RF stats - so I would be guessing as far as they are concerned.

I'm not so sure! Bearing in mind that in most T&E cables, the earth conductor is bare except where it's sleeved at the ends, I have a feeling that you ain't supposed to use it for anything live, even if you do sleeve the ends red or brown.

As above.

[Phil Addison]

Probably! But the green/yellow I mentioned is found in multi-core not flat cable. AIUI the inner sleeve is the insulation, the outer is mechanical/secondary protection, so perhaps you can sleeve the bare earth so long as it is inside a protective pattress of some sort.

Phil The uk.d-i-y FAQ is at

Google uk.d-i-y archive is at NOSPAM from address to email me

[Dave Plowman (News)]

With any permanent fitting like a thermostat (mains 'powered') I'd always provide an earth even although not needed. A bit like lighting switches, etc. The current one might well be safe without - being plastic - but future types may have metal plates.

Many central heating installers seemed to leave the wiring to the tea boy

- everything done in 0.5mm three core flex. Not really to be condoned on here.

[Set Square]

Agreed - but the question being debated in this sub-thread is along the lines of "Is it permissible to use the earth wire in flat T&E cable for a live signal (e.g. switched live) if you sleeve the ends in some suitable colour?". The thing is that it has no primary insulation, and relies entirely on the secondary outer sleeve.

Do you have a view on this?

[ARWadsworth]

Reg 412-02-01

Live parts shall be completely covered with insulation which can only be removed by destruction etc.

So the CPC should never be used as a switched live.

Adam

[Dave Plowman (News)]

Yes - it's a bodge and unsafe. Instead of TW&E for a stat, use triple and earth. It's hardly expensive. If needing flex get some multi-core where the marked earth conductor remains the earth. That's hardly expensive either for the amounts used - and pro installers should have a drum of it.

[Dave Plowman (News)]

Perish the thought, but if I was forced into using TW&E for a thermostat with shunt coil, I'd use the earth for the neutral.

[ARWadsworth]

You will never be forced into doing such a bodged job, although you could pay a professional to do it for you :)

Adam Adam

[Tim S]

Aye, you're not supposed to do that either. The IEE regs have been moving towards the notion that neutral is to be regarded in the same way as live, and technically T&E sheath doesn't count as insulation.

If your neutral fell off at the CU (or in the street - happens, especially in rural areas with overhead wires) all your "neutrals" would rapidly float to 240V relative to earth. Guess that's their line of thought, amongst other things.

Regards

Tim

[Andrew Gabriel]

As others have said, never. However, you should certainly not assume some plonker has not done this.

[Set Square]

I found that my stat was wired like this (not by me, I hasten to add!) - with the earth wire used as switched live - when I came to replace my mechanical stat with a programmable stat. Since I only needed 2 wires, I used the red and black - making the necessary adjustment at the other end of the cable.

[Dave Plowman (News)]

Yes. The big problem with using wiring that's not intended for the job is that what is 'logical' to one may not be to another. If the wiring is all insulated then this may just be an inconvenience resulting in a possible circuit trip. But using an uninsulated earth conductor as a live is asking for trouble at a later date.

[ARWadsworth]

Not according to the NICEIC. They quote reg 514-04-02 "the bi-colour green and yellow shall be used exclusively for a protective conductor"

Adam

[Phil Addison]

But is it still green & yellow if its sleeved with say red?

Phil The uk.d-i-y FAQ is at

The Google uk.d-i-y archive is at Remove NOSPAM from address to email me

[ARWadsworth]

Yes.

Adam