Immersion Heater not working - is it just the Thermostat?

5 Oct 2008
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United Kingdom
My immersion heater in my tank is not working (i do not have a boiler) :( just an electric shower and an immersion heater. For heating I have electric heaters.

basically immersion heaters can be a bugger to remove as I have gathered and most of the time the thermostat is the problem and can be replaced a lot easier and quicker.

But how exactly do I know the thermostat has failed and it is not the heater element? I have a multi meter, digital type, and have used it before and know how to safely.

Any instructions on how to check if the thermostat has definitely gone and can be replaced soley will be great.
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is their power to the stat ?
is their power coming out the other side of stat ? if not has the stat tripped check reset button.

if stat has power both sides turn mains power off and check for continuity across the heating element.

if thats ok do a ohms test a good element will be about 18 ohms.

Yes there is power to the stat

when the immersion is on there is no power coming out of the other side of the stat though.

there is no cut out button, as this heater is pretty old and before the regulation came in to place.

so basically from this the thermostat has failed?

also, sorry for wasting your time, but A) why would turning the power off do anything, there would obviously be no continuity? and B) the resistance what difference does that make? sorry i probably sound like a right thick-head! But i am curious now from what you said. i have never tested for resistance before though.
doing a continuity test uses the battery from the multi meter to test for a circuit.

i wouldn't want to slip and touch 230v with my fingers.

example i've had a element before that takes ages to heat and never to full temp.
continuity showed the element had a circuit.
ohms only showed very low.
which proved element was on its last legs and breaking up inside.
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yeah sorry i'm :oops: now! ha

anyway there was no voltage coming through the stat when on (needing heat). so i take it the thermostat has def. failed.

i just am wanting to be sure as if i do not need to replace the whole heater it will be a lot easier for myself!

the heater circuit goes to live, then through to the T terminal the other side of the stat and if there is voltage (it is working) through to neutral, that is correct isn't it?
you can by-pass the stat just to check the element is ok if you like but not as long term
yeah i guess not as long term as if there's no voltage going from live to T terminals then the thermostat has def. failed. so i'l need to get a new thermostat of the correct size i suppose.

if the element is fine, do i just test from live to neutral terminals for resistance then? thinking about it now, i suppose it is possible that both the thermostat and elemtn could BOTH have failed! just thought of that situation.
so i'l need to get a new thermostat of the correct size i suppose.
Yes. For a top-mounted element on most cylinders the size is 18".

if the element is fine, do i just test from live to neutral terminals for resistance then?
Yes. The resistance should be in the region of 18-19 Ohms.
ok that's gd then. 18-19 ohms is a strange amount isn't it! i wonder why it's that!

anyway will do and hopefully there will be resistance between live and neutral, or i'll need not just a new thermostat but also a new element, so a whole heater replacement :(
hi - sory yes i know what resistance briefly, i am not an electrician! obviously as you will know, but basically i never thought about working it out with the triangle!

i know that i have used the triangle (s) before for working out what size fuse to use, but never had any call to use resistance.

seco and softus you have taught me something today. thanks.
just to throw a spanner in the works - why can't a voltage test be used to test the element (from live to neutral) won't it do the same thing, ie show the element is working correctly?! please excuse me if i am way off it's been a looooong week!

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