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I've recently had my immersion heater changed and it worked fine for a couple of months but now it isn't heating the water properly. When it is switched on it appears to be heating up fine but when I switch the hot tap on it runs hot for a minute or two then turns freezing cold. It does this even if it has been on for several hours. I have tried changing the timer and the thermostat but don't understand what can be causing this problem. please can you help.

hi merlin if your thermostat is the new version it will have a reset button on top try reseting, if it doesnt then you have the old illegal version which should not have been sold to you .you can test the thermostat live with a mains tester if you are confident as to what you are doing it should read live both sides all the best john