Problem:- My hot water expansion tank keeps filling up to overflow I have replaced the ball float still fills up and have now turned off the in inlet supply. Still have the same problem. I have checked the stop valve and it does not leak. So I now think that the coil in the hot tank has a leak ( we have acid water ) . How can I test for this leak so that I do not have to go out and buy a new tank to find out it is not leaking.
It is not very likely that the coil will be leaking.
Your problem is probably caused by the water level in the expansion tank being set too high. When water heats it expands and the extra volume, on an open system, goes to the "expansion tank" rising the water level.
If the water level in the tank when cold is high enough, when it heats it will rise to overflow level. When it cools again the ballcock will top the system back up again. This cycle will keep repeating every time the system is run.
Fix this by adusting the lever arm of the ballcock or bend the arm down if it is brass. You should only have half a tank of water when the system is cold with enough room for the expanding water. This is assuming you have a standard sized system. If the system is larger capacity you may need a larger tank to allow for the greater volume of expansion.
You can check the coil by marking the water level in the tank when the system is cold. Leave system off for a few hours then recheck the level.
You could also drain the system and disconnect the flow and return on the cylinder. You will see any leak.
You may want to check if you have a shower mixer that has tank fed hot water and mains fed cold water. Quite often if no non-return valve is fitted to the hot water side mains water can seep through to the hot water cylinder and fill the system up to overflow level. To check this leave the mains cold water turned off over-night and check cold feed cistern level in the morning.
I wonder if anyone can help. I have just been in to my airing cupboard and found that I have a leak on the top nut of my hot water cylinder. It seems to be a small leak. is there a gasket/washer I can replace rather than having to relace the whole tank. Fingers crossed. Any advice wellcome.
Its probably a compression coupling and could be cured by tightening Debbie . Best idea is to undo coupling and refit using ptfe tape. Should get a plumber in for this as the water needs to be turned off.