immersion heater thermostat melt down

[skilso]

Recently a plumber changed our immersion heater and after being calle

back twice to rectify afailed thermostat he embarked on other wor nearer home and I ended up changing the thermostat myself.I have no changed it three times and each time the same problem arose.even thoug the thermostat is set to a low temperature the water overheats and th power wire to the thermostat melts and eventually disconnects.I hav tried a heat resisting wire but this burns too as do the plastic top o the thermostats.The fuse is 15amp and has not blown atall during thes traumas.The cylinder when connected gets very hot at the top and i often quite cold below.I wonder if anyone can suggest something befor I purchase another thermostat .Thanking you ...Skils

-- skilso

[Timothy Murphy]

Are you sure you have the correct connections? From my recollection it is easy to make a mistake.

[Andrew Gabriel]

I wonder if you have a long thermostat and a short element? Not that I would expect this to cause a wire to melt, but it could cause the water to be too hot at the top.

[Chris McBrien]

If you can obtain a multimeter from someone, check the actual resistance of your immersion heater element. Make sure that you disconnect all wires from the heater and thermostat. The resistance of the heater should be about 20 Ohms assuming it is a standard 3kWatt heater. With this same multimeter you can also check that there is no electrical leakage between the heater and the copper tank itself. The copper tank should be connected to Earth.

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[John]

What make of thermostat are you installing ?

Try a different brand, which has suitable British Standards markings on. Also the temperature does not need to be high for a immersion, and often they are set way too high, around half way or less on screw setting should be enough.

J

[Aidan]

A sudden thought about this. Is there an open vent pipe from the top of the cylinder, which terminates over the cold water storage tank?

It might be that sometimes part of the heater and the thermostat are not immersed in the water. This could explain the melting and why the cylinder is cooler lower down. It shouldn't be possible for the heater to become un-immersed, but there are some lunatic installations around.

If there's no open vent, the water level may drop as air accumulates at the top of the cylinder.

You should be able to find a diagram of the correct cylinder installation on the web or in a DIY book, to compare with your set-up.