What temperature was the thermostat set for, and what did you turn it down to?
The build up of limescale on the inside of hot water cylinders forms an insulating barrier on the inside wall of the tank, effectively masking the real temperature of the water inside. Hence the setting on the thermostat dial is only a guide and should not be relied on.
Try turning the dial down to the minimum setting. If that does not work, then either the thermostat is faulty, or has not been wired in correctly.
Is the immersion heater on? It has its own thermostat my daughter tried turning down thermostat on cylinder and as a result the electric cut in and cost her a lot of money as more expensive than gas. Idea is to have electric set to lower temp so if gas fails it auto cuts in. Note since there are often exposed live terminals inside the immersion heater please ensure it is isolated before you open top. Be safe.
ive turned down the thermostat to the 20d this is the minimun but its made no difference. the water is still scalding hot. do i ned to get a plumber to change the thermostat or is this something i can attmept myself ??????
It is an electrical job and as to if you can do it well I don't know your ability so can't answer. Most likely you could but it could be something else at least an electrician will check it first and should be able to say if thats not the fault. Could be a valve stuck. The valves do have tell tail levers to show their position. Most Plumbers and Electricians do a little cross over on their trades and I would expect either would sort it for you.
I had a similar problem. My thermostat on the side of the tank had slipped down towards the bottom of the tank , where it is always cool. and gives a false reading. So I moved it back up to about a third of the way up the tank and all worked well.
To test if the thermostat is faulty - select hot water only, and turn up the thermostat dial to it's max. Wait for the boiler and pump to start delivering hot water to the cylinder. Then turn the thermostat down until you hear it click, or to it's lowest setting.
The boiler should now shut down, although the pump may continue to run for a minute or two to reduce the boiler temperature.
If this works then the thermostat would appear to be functioning properly.
At this point I would suspect a sticking or faulty motorised valve is allowing hot water into the cylinder coil when only the Central Heating is supposed to be on.
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