Can I replace Immersion Heater Switch?

I take it you mean 'in the wall' if its a flush mounting you are after.

Sure as long as you do a pukka job that meets regulations - your biggest issue might be if you need to extend the wires...

Unless it has it's own seperate circuit back to the CU I would use a switched fused connection unit ( similar to your proposed one but with a fuse). Search on Screwfix for switched fused spur.

Might save quite a bit of money to use a switch that incorporates a time switch.

Google immersion heater timer

I would say no. 240V x 20A = 4,800W. I believe some immersion heaters can draw up to 6kW. Even if yours doesn't, someone might upgrade or replace it.

Is this "increasingly" without other changes (eg in the time the heater is on or the temperature in the house)? If so it may be that you have a bad connection in the switch where temporarily at least tightening the screws might help. See under "High resistance connections" under "Types of Fault" in

But do please note the rubric to the table: "Power must be switched off before any inspection takes place." And - with apologies if I'm instructing in the sucking of eggs - in this case that's most emphatically off at the consumer unit, not at the switch :)

If someone upgrades it then it's their responsibility to check the switch and upgrade it if necessary. Ditto the cable and MCB/fuse.

Usually like fish.

True, though I tend to follow the Brunel approach.

I would still suggest OP needs to check the power rating of the heater unit before deciding whether a 20 amp switch is suitable.

My screwdriver is rated at 1,000 Volts. I think the instruction actually states 'Any appliance should be switched off and unplugged before working on it.' It seems to me this does not preclude an inspection provided suitable precautions are followed.

IT will look silly on a metal backbox, they are intended to be sunk into the wall. Just use a single surface-mount pattress box like this -

If it's a standard 3kw immersion element then simply checking the tightness of the connections inside the existing switch may be all that is needed.

I'd say that provided the OP is sufficiently experienced to check the rating of the existing heater and know whether it is on a ring or radial circuit (checking the fuse/mcb rating of the latter) then they are comptent to replace the switch with a suitable modern one. My main worry would be the state of the existing wiring and earthing if the installation back to the consumer unit is also the same age (or older).

The old swith may have "cooked" the wires close to the terminals since it has been overheating. This should be obvious with the insulation discoloured, hard, and brittle but it should only extend a couple of inches from the switch. Does the switch outlet have a relatively short flex that goes straight to the immersion? If so, well worth replacing that flex with modern, heat resistant flex. That should let the OP move the switch slightly to remove any affected bit of the cable.

Good point. Although a metal box designed for industrial installations with conduit would be OK.

Those boxes in ?incompatible with the switch that the OP was asking about though. They are intended for industrial use.

I think you will find the outside dimensions as well as the corner radii do match up, especially since this is an MK switch.

Compatible switch available:

Except that the OP described it as "a thin-ish one, mounted on the wooden door frame" which sounds very much like a light switch which would only be good for 6 amps!

Personally as long as the wiring is sound and no matter what the law may say, I'd do it now before the existing one causes a fire by the sound of it!

However the one thing is, if its really that old then the wiring could be as well and the heating may have hastened its demise, so you may open it and find a major rewiring job to do. Brian

Yes but I'd certainly put in a fused one to stop the unwary. Brian