Conventional central heating/hot water system installed. Expansion/feeder tank in loft for c/h is too hot to touch as though pump is pumping hot water back up into expansion tank. Also knocking when central heating starts up.
Any ideas what problem might be??
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This unfortunately is a problem that has cropped up here a number of times before. The important thing to understand is how dangerous this can be. Header tanks are plastic, and will not withstand boiling water for long periods, and there is a real risk of the tank unexpectedly collapsing cascading hot water into the rooms below! You may recall, a young child was scalded to death last year after such an occurrence.
If you have an immersion heater switched on, turn it off and see if the header tank stops being heated.
The noise you hear may be the C/H boiler boiling causing water to spurt out of the expansion pipe. Be in the loft when the noise occurs and see what is happening.
I would also strongly recommend you have the tank inspected by an experienced engineer to ensure it is not damaged. But whatever happens, do not let this fault to continue.
I thought it may be helpful to others if I explain what I found when I visited her home concerning the expansion/feeder tank getting very hot.
The vent pipe had been left touching the water in the expansion tank and a separate circulation was taking place. The tank water had become boiling hot, had filled up and was overflowing.
By draining surplus water from the tank and removing a section from the vent pipe so it could not touch the water, it appears to have solved the problem. Obvioulsy I will be keeping an eye on it for a while.