Immersion heater - timer installation

3 Jan 2008
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United Kingdom
Hello all,
I currently have an immersion heater and I am on Economy 7.
The immersion heater does not have a timer, but is switched on when the night rate switches on.

I was thinking of fitting a timer as I don't think the immersion needs to be on for the full 7 hours.

Could anyone tell me what timer I need and could it be installed in the airing cupboard next to the on and off switch? Presumably I would not be able to replace the on off switch with the timer?

Many Thanks
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I hope your immersion heater has a thermostat. If it is working, it will stop drawing power when the water is hot.
Sorry should have said, yes it does have a thermostat. it does turn off after a while, but obviously when it cools down a bit it starts up again. Normally if you have used a bit of water early in the morning and the cold incoming water must be enough to drop the temperature so it turns back on again.

Could it be the thermostat has gone then?
if it is properly insulated, then once hot, it will stay hot for the best part of 24 hours. Tell us how the cylinder and surrounding pipes are insulated.
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The Cylinder has a green polystyrene jacket sprayed on. the flat is 17 years old and as far as I know the original. The pipes are not insulated as far as I can see. the water does stay hot for about 2-3 days if the immersion is turned off.
But it seems to turn its self on and off during the night (E7 times) so maybe the thermostat is turning the element on prematurely?
The thermostat is set to approx 55 degrees.
is the water out of the taps scorchingly hot if you put your hand in it? If yes, the thermostat has failed and it is too hot. If not, the thermostat is OK (you can use a thermometer for a more accurate guide)

If it is being kept hot with off-peak electricity, then it does not really matter if it tops itself up during the night or in the early hours - off-peak electricity is quite cheap and heat is not being wasted as it is well-insulated.

the insulation sounds OK, you could do the hot pipes round the cylinder for a small extra benefit - modern pipe lagging is stiff polythene foam tubes and can be cut and shaped with a breadknife. The hot pipes above the cylinder keep hot by water convection so are best insulated.

Verify that you have not got a dripping hot tap or a leak, as this will waste hot water and cause the immersion to come on again. So will a mixer tap or shower which allows cold water from the mains to leak up the hot water pipe into the cylinder.

How did you discover the heater was going on and off? i would only expect that to happen if someone had been running a hot tap.
I have heard about these econmy 7 immersion heaters. There must be a timer installed somwhere - it would not be able to work otherwise peoperley on economy 7.

where the wire joins the connection unit, between that and the immersion heater or between that and the consumer unit there should be a small timer somewhere.

i wouldn't think your thermostat had failed, otherwise you would get a( no hot water at all - the overheat cut out button would need resetting or b) you would have water spouting from your vent and a lot of noise!
The hot water comes out very hot.
I found out because my E7 time ends at about 8am, after I have gone to work, when I run a little hot water into the sink a few seconds later the immersion will start up again as though it has dropped enough in temperature for it to turn back on.

I have not located a timer anywhere, it is only flat and has 2 consumer units one for E7 and the other for day rate. Therefore I presume it uses the E7 switch as a timer.

Many thanks for everyones help. Sounds like the thermostat has gone then. Probably best to replace it as it is old and doesn't have the trip switch by the looks of it.
If you have an E7 meter, it probably incorporates a timer and cuts power to the E7 circuits during the day.

Your Immersion heater should have a thermostat adjuster under the cap, and an easily replacable thermostat. It is very very very much more work to change an immersion heater itself.

If you can take a photo under the immersion cap :!: turn off the power first

and of your meters, consumer units and the various cables between them, we can probably tell you more.

Many Thanks. I will take some pictures tonight, but I think you are right.

The thermostat / immersion is an 11" one. It seems like the thermostat has gone to me. I measured the temperature of the water last night and it was 67 degrees C.

I then turned on the Day unit heater, which is set to the same temperature and and this did not start heating up, presumably because the water was not cold enough to trip the thermostat.

I am going to get a new thermostat today and fit tonight anyway. As far as I know its 17 years old, and the new one will have the safety over temperature switch also, which would be safer.
I took the old thermostat out over the weekend and fitted a new one. all seems better now.

I tested the old one buy putting it in a cup of hot water, about 80 degrees C. Then put a meter across the contacts of the thermostat and turned the dial. The thermostat was on at anything above 20 degrees!

Many thanks for everyones help.

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