I have been told by British Gas that the reason my overflow pipe is running and sometimes pouring (despite having a new float fitted in my water tank) may be due to a faulty cartridge in my electric shower allowing water to fill up the cold water tank in the loft from the bottom. Which part of my shower will I need to replace exactly?
If your shower is of the mains type, i.e. instantaneous and only fed od the mains I don't think this advice is correct. You would need to give more information to identify the problem. Sorry to be a bit vague but can only answer as to the symptoms given.
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