Have just had new hot water cylinder and immersion fitted. If immersion is left on water is hot and works ok but when switched off and left overnight water is stone cold. There are no obvious leaks.Boiler for heating is oil fired.
My guess is that you have an "Indirect Hot Water" system where the water in the cylinder should be heated by hot water from your boiler passing through a coil of pipe inside the cylinder then gravity feeding back to the boiler. It may be that there is an airlock in the heating coil in the cylinder, preventing the flow to and from your boiler. However I'm no plumber so I can't be certain.
I don't think you've said whether your central heating boiler can also heat your domestic hot water. Can it?
If you go to bed knowing that the cylinder contains plenty of hot water, and yet in the morning it has become cold, then that water must be going somewhere. You're not using an old type hot-fill washing machine or dishwasher overnight to take advantage of low tariff electricity, are you?
If you do indeed have an indirect system, then there is another, albeit only slight, possibility. It is this; if the boiler is not set to fire but for some reason the pump is circulating (cold) water through the cylinder heating coil, then that coil water could be taking the heat out of the domestic hot water and pushing it around the rads. (Are the rads slightly warmer than the ambient room temperatures?)
Your not using the water somewhere are you? Dishwasher/Washing machine overnight perhaps?
Perhaps the immersion isny being left on long enough and not all the water is being heated.... When your heating is on you should be ok with the coil heating the water - we are getting to that time of year.
Id imagine at present, either the water is being used, the immersion isnt on for long enough or the immersion isnt capable of heating more than a small amount of water (wrong size for cylinder)
Why not heat your water for when you need it? ie immersions on a timer.
Also, have you checked the thermostat of the immersion(s)?.... it would be normal to have this set to 60 degress celsius... it is possible the installer set it lower.
Why are you not using your boiler for your hot water anyway? An immersion is normally meant more as a back-up. Even with your heating off you should still be able to heat your hot water using your boiler using your programmer as you should have a 3-port valve.
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