Immersion heater wiring

[David Hearn]

Our old hot water cylinder (inherited with the house) came with an immersion heater wired via a 13amp plug. This hasn't been used by us for years (since the thermostat blew, boiling the water and shooting it into the loft!). It's been unplugged ever since.

I'm planning on replacing the hot water cylinder, and therefore need to decide whether to fit an immersion heater or not. I'm thinking to fit one, but not actually wire it in yet (or ever!). I have no intention of ever wiring it into a plug! I know that's totally wrong...

Anyway, if I remember correctly (I haven't looked at it for a couple of years), the socket which the immersion heater used to plug into was on the one of the lighting circuits, which is wired into a 16A MCB (plug in type for old fuse box). The other light circuit has, I think, a 6A MCB.

Is it permitted (I doubt it's recommended) to replace the existing socket with a suitable double isolator immersion switch and wire the immersion in that way? It's the only source of power in that cupboard, and would be the easiest way of getting power to the immersion heater.

I just want to think about things for the future, rather than actually going ahead and doing anything about wiring it in now.

Thanks

David

[spamlet]

Presumably you mean the water is normally heated by gas. In which case you will be very grateful for keeping an immersion option open should there be any problems with the gas supply (eg roadworks) at a later date. Don't see why you would need any special arrangement, other than a big enough 'fuse': you no doubt have similar devices in your kettle and washing machine, and having them on plugs makes them easier to replace when they blow. Thermostats do get stuck occasionally, and the fact that your system was able to relieve the pressure into the loft, is a good sign, though you would normally arrange things so that the overflow would go back into the cold water tank, or outside, rather than doing any harm.

S

[David Hearn]

Yes, normally heated by gas.

The vent pipe was vaguely aimed into the (open topped) cold water tank, however, due to the spluttering nature of it boiling, and it's vague aim, and a bit of insulation lying between the tank/vent probably most ended up going through the ceiling and running down a light fitting onto a bed... all whilst we were out the house.

D

[ARWadsworth]

I would start by checking that out. 16A is not a typical rating for a lighting circuit.

Adam

[David Hearn]

Might it be for a light + immersion heater circuit though. ;)

D

[spamlet]

You wouldn't normally want to run the immersion element while you were out: I've always considered these to be just 'emergency' boosters, and rather expensive to just leave on. All the same, the next one you get, make sure it has a thermostat that turns off at a reasonable temp. This one

says it has an "overtemperature cut out mechanism" so that it can be reset rather than replacing, every time the thermostat plays up (Which, given my experience with kettles, I still wouldn't rely on 100% but may be better than the one you have.) Loosen the old one while the hw tank is full, and you are less likely to distort it or the pipes that run in and out of it: then empty the tank to unscrew it completely and put the new one in.

Make a cover for the water tank and make sure the vent pipe goes through it - but not under the water or below any level it is likely to rise to. If it has been some time without a lid, you may want to empty it and clean out all the dead birds and rats etc. (tie up the ball valve with a bungee cord up onto a roof beam, I often find is a handy way of stopping it filling without having to hunt for the stopcock: while you open the cold tap on the bath to empty it.).

If you really don't want to use a plug, and you still want the option of using the heater while you are out, and don't want to trust it, you could use one of these:

[Dave Plowman (News)]

Modern ones have two thermostats to prevent this. The standard single ones did have a habit of jamming and boiling. However, a properly installed header tank should be able to cope with this fault without overflowing.

[John]

I seem to think it is now mandatory to have an overtemp trip thermostat on a new installation.

[NT]

16A lighting circuits arent compliant, and I cant help but wonder if a 5A circuit has had its fuse/mcb changed.

If your supply isnt up to running 3kW, I hear you can get dual element heaters that go in one standard hole fitting. If thats correct, just wire the 2 elements in series and you've effectively got a half power heater there, which can be run on a 10A lighting circuit or a 13A plug.

NT

[ARWadsworth]

It still sounds like your lighting circuit is running from a 16A MCB unless I am misunderstanding something.

That would be amost unusual setup.

And YES, intall the immersion even if you do not wire it up.

Adam

[John]

16A lighting circuits arent compliant, and I cant help but wonder if a 5A circuit has had its fuse/mcb changed.

If your supply isnt up to running 3kW, I hear you can get dual element heaters that go in one standard hole fitting. If thats correct, just wire the 2 elements in series and you've effectively got a half power heater there, which can be run on a 10A lighting circuit or a 13A plug.

NT

What??

Wiring 2 x 3kW heaters in series will still take a 3kW load. I assume you mean they are 1.5kw per element. That would need checking carefully.

[Harry Bloomfield]

John pretended :

No, two 3kw elements wired in series would double the resistance and half the wattage/current demand.

[John Rumm]

No, because at 16A there is a fair chance that light fittings and pendent flexes will be inadequately fault protected. Note also then any SES or SBC fittings are only rated for use on circuits up to 6A - so even a 10A MCB would be to much.

[Roger Mills]

What size of cable does the circuit use? Lighting circuits are usually wired in 1.0 or 1.5 mm^2 cable - which wouldn't be adequately protected by a 16A fuse - the cable would probably overheat before the fuse blew!

Immersion heaters should really have their own dedicated circuit (not sure whether it's mandatory?) but, for occasional use, you could in practice get away with powering it via a fused spur from a ring main - but *not* from a lighting circuit!

[Dave Plowman (News)]

1.5mm cable is rated at a max of 16 amps. So if the cable run was such it didn't have to be de-rated, it would be safe. And likely adequate for a 3 Kw load. Although most would use 2.5mm.

[Andy Wade]

The old 6 A limitation for SES (E14) and SBC (B15) lampholders has been removed in the 17th edition and these holders are now allowed on circuits up to 16 A [reg. 559.6.1.6].

However (i) lighting circuits exceeding 10 A are rarely required in domestic installations and (ii) an immersion heater should have a separate circuit; combining it with a lighting circuit is totally non-standard - quite bizarre in fact - and would fail to meet the requirement for safe and convenient division of an installation [section

314] IMHO.

[NT]

Yeah. A bodge, but safe enough and ok for backup use.

NT

[John Rumm]

Well spotted - I had not noticed that!

Retract that bit then!

Was that just a change in the rule, or has there been a shift in manufacturing quality / spec of the fittings to match?

Yup agree with that...

[Graham.]

they are 1.5kw per element. That would need

0/10 See me.

[NT]

[e-mail]...

[e-mail]...

ou mean they are 1.5kw per element. That would need

Perhaps he has 340v mains? :) Then he'd be right.

NT