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Immersion boiler - no hot water overnight, but boost works fine


My immersion boiler has recently stopped heating water overnight like it used to for the morning, but boost is working as expected to heat the water up.

I've checked all my fuses and switches & the timer is ticking away happily.

I've rotated the clock on the timer (horstmann eco 7 quartz) and checked that it does turn on the timer light at the correct time, but then once put back to normal time, it fails to heat over night.

Is it the timer unit? I just thought that maybe I should turn the timer on tonight (so make it think it's eco 7) time and check if water is actually being heated?

Any suggestions or advice welcome - I'm considering swapping the timer unit to a horstmann eco 7 digital anyway as I hate the ticking but happy to do that sooner if it'll fix the water too!


4 Answers from MyBuilder Electricians

Best Answer

If you are handy with a multimeter you can isolate the power, unwired the BOTTOM immersion and test for resistance across the two element terminals. A reading between 12-14ohms indicates that the time clock is at fault. The operation of this can also be checked with a multimeter. If there is no reading or there is continuity then the element will need to be replaced. I am happy to attend and carry out these checks for free to provide a quote to fix this for you. Please get in touch. Many thanks, Alec.


Answered 26th Jan 2017

If its a dual element set up then its most likely that the overnight (larger of the two elements in the tank) element has gone and needs replacing. If its a single element set up, then it points towards the timeclock being faulty and not switching the supply on to energise the element during the off peak period overnight.


Answered 26th Jan 2017

Hello like the post from electrical safety services said it could be a fault with the timer ( this happens quite about) or it could be the element or the thermostat if you have a 2 element system. I would never advise using any electrical equipment unless you are a faulty electrician or plumber who knows their way around a heating system. So best advice I can give is to call in a professional to sort it for you. It's a quick job to do so shouldn't cost you very much and will pay for itself in wasted electric bills. Hope this makes sense. Max Electrical


Answered 26th Jan 2017

Another thing to check is that the circuit breaker or fuse for the economy 7/night rate circuit is in the on position. If it has tripped, it could indicate a fault, probably with the element, but worth turning back on and see how it goes.


Answered 27th Jan 2017

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