Cold water coming out of hot tap on mixer ?

Dear momotha. Great question. There are perhaps 2 possible reasons / solutions. 1. If hot water is tank fed, cold water may be higher pressure (mains fed) and thus pushing back into the hot pipe when the mixer is opened... have a check valve fitted to the hot pipe. 2. If tank fed your loft tank expansion pipe may be dipping into the water and siphoning off some cold... have this sorted by a plumber. If a combi, the boiler needs attention perhaps. hope this has given you some ideas. Bob.

By what has been said I am assuming it is a combi boiler , if so so the plate heat exchanger requires attention.

Hi, the first things to find out before answering your question is what type of system you have, Combi boiler, Regular boiler, has it been serviced of late. Your mixer valve, when was it installed? 28/22mm pipe feed? Is it fed by a combi boiler or through your how water cylinder, if regular boiler? What ever the issue is, if no experienced in plumbing works would suggest that you call in a plumber who can best analyze your issues!

Put a non returnable check valve on hot and cold