We live in an early 80's built house with oil fired hot water and central heating. The hot water tank is located in the upstairs airing cupboard and has an insultation jacket.
This morning we turned the mains water off to the house (to install a new downstair's loo), the water was off for about 3 hours. When it was switched off, we left the taps running until the water tank was drained.
Should the hot water tank have cooled during this time, as the hot water hadn't been switched on since at least 4 hours? (time between normal heating/water switch off through to when work was completed). It felt very hot to the touch, almost as if the hot water had been on the whole time. The immersion switch is switched off. Could there be a fault with the immersion switch, or is it normal for the tank to stay hot for so long?
I'm guessing it's a cylinder - then it would have still been full of hot water. You would have drained the tank that feeds the cylinder and the hot pipework. But the cylinder would still be full and would maintain the heat.
If you read the post
'Hot water - gate valves' - by stevehayes -
kenj posts - this will explain why your cylinder doesn't drain.
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