Hot water cylinder going cold over night

1 Dec 2015
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United Kingdom
We have a hot water cylinder with two immersion heater, one standard, one booster. We are with SSE on the Total Heating Total Control system which switchers the main immersion heater on at various times during the day. We had a power cut last Friday and since then the water has been cold and we've been using the booster. A chap from SSE came out yesterday and replaced the switch that switches the water on and off and everything was fine. Plenty of hot water.
This morning it is cold. Not warm. Stone cold.
The immersion heater appears to be working.
The cylinder supplies hot water to taps only. No heating, and our showers are electric. I can understand that perhaps the water was not heated during the night, but surely it wouldn't go cold in such a short space of time? Can someone be drawing the water away? A problem with the intake, or the overflow? Any help greatly appreciated.
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You say the immersion “appears” to be working, have you tested this?
Well when we stopped getting hot water I put a meter across it and everything appeared to be fine. And it was working again yesterday after the SSE guy replaced the switching thing. However, it should be on at the moment and there doesn't seem to be any movement on the meter. Perhaps just an intermittent fault with the immerser?
Could be intermittent fault. Did you check resistance of the heating element?
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Sounds like the switch hasn't been set up properly, if you've tested the element Are you on electric heating only? Economy 10/7?

I'd be getting them out again to check.
surely it wouldn't go cold in such a short space of time?

The lower immersion heater is designed to heat the water in the entire hot water cylinder. Usually this takes place only when cheaper electricity is available, generally during the night, and sometimes with a mid afternoon top up. (Hence the timer requirement and as per @Madrab Economy 10/7 question above).

The boost element higher up in the cylinder will work whenever it is switched on, and be charged at the current £ rate at the time. However, it will only heat up a small quantity of water at top of the cylinder, (usually enough for a sink full or two) as a result this water will gradually loose heat to the cold water immediately below it, if it hasn't been heated up by the bottom element earlier.

Therefore, I suspect in your case, that the off peak immersion heater isn't working, and you are just using the boost immersion heater at the top, and hence why the water is cooling quite quickly.
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