Immersion Heater Timer

18 Jun 2019
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United Kingdom
Hi all, I'm trying to change my simple immersion heater to a timer one.
I understand the wiring of neutral and load. But I don't get the earthing. My current switch has a tiny metal strip connecting the earth through the switch. It seems like it's just joining it... however the new timer has an earth terminal...
I've read the old forums. Do I just attach the loose ends of the earth cable together (and run it through the new timer)? Or can I attach the loose ends to the earth terminal on the timer.... (I.e. does the timer itself need to be earthed or is it enough that the heater is earthed)....
Sorry if this is badly worded. Attaching pictures. :)


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The Isolation switch should be retained ,and a timer added . The timer in your pic looks flimsy to me ,are you sure its suitable for an immersion heater ? What is its rating ?
Hi Terry,
Thanks for your reply.
I have two switches on the wall. One for the main tank, one for the small tank. I'm aiming to change the one for the main tank.
There is no other isolation switch.
The timer has a switch embedded (as in the picture)
The switch is 13A (3kw) MAx resistive.
I checked the fuse and the previous switch is also 13A. The heater is 3kw.
So should i retain the switch but add the timer separately ?

What do you think ? :)
The isolator is there, so that anyone carrying out work around the immersion heater can isolate it and know it is isolated. Swapping it for a time-clock would be a mistake, working on the immersion heater, there is no certain way to isolate it a clock could switch at anytime.

A more usual place to add the timer, would be adjacent to where your fuse or distribution board is, or perhaps where your remote switch is, in place of that one - maybe in the kitchen.

As Terry said - Are you sure that timer has a suitable current rating?

The earth terminal on your timer appears to be just a 'parking terminal', which would not be connected to anything. It is just a terminal to joint the incoming and outgoing earth wires.
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Hey Harry, Thank you. The ratings are as above.... Would that not be suitable for the heater do you think?
To carry out work on the immersion heater, I would have thought i would switch the fuse off at the main fuse board. Would that be ok ?
Also there is a switch on the timer. Would that not isolate ?

Thanks for all the help everyone :D
btw. i've been using the current switch to turn the immersion heater on and off. it's primitive. :)
there is no other switch as is.... (just checking!)
btw. i've been using the current switch to turn the immersion heater on and off. it's primitive. :)
there is no other switch as is.... (just checking!)

Mount the clock where that switch is presently located, then add a second box directly below it, to mount the switch in, add a few inches of 2.5mm T&E to join them. That way, the clock would run undisturbed when the immersion heater is safely isolated to work upon. You could also use that same switch, as a quick way to switch the immersion heater off for those times when you might be away from home - the clock would continue to run and normal duty would be resumed on your return, once your switch back on.
Thanks again Harry :) much appreciated. Do you think the current rating is suitable for the heater?

The timer is 13A (3kw) Max resistive.
With a resistive load, the amps go UP when the volts go up.

Working from the nominal figures:

We know that the nominal voltage is 230

We think that the nominal amps are 13

x volts ÷ y amps = z ohm

230 volts divided by 13 Amps = 17.7 ohm (rounded)

Calculating on 240v:

x volts ÷ z ohm – y amps

240 volts divided by 17.7 ohm = 13.56 amps

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