When immersion heater is put on water comes out of overflow

What is the likely cause of this, elderly uncle has questioned this. Don't know what make of boiler he has as he doesn't seem to know.

Please let me know ASAP, thanks

Immersion heater is electric element.
So the boiler make is irelevant, If it is electric immersion, the t/stat may have failed & so the water is boiling.

Thanks for your response, will the immersion heater need replacing?

They usully have an inbuilt thermostat, not certain if they can be replaced. Hang on for a while & maybe you will get a further reply on DIYNOT.

Yes, thermostats can be replaced fairly easily, best to take the old one out first so you can get the same size. They fit in a watertight sleeve, so that no water squirts out when you remove them. You do of course have to turn off the electricity supply to the immersion heater, and carefully observe the connections and draw a diagram so that the new one is connected the same way. You might do it yourself or call a plumber. it is a common maintenance job.

If the immersion heater itself is very old, it can be worth fitting a new one (which includes a new thermostat) to save having another call later. Changing the heater is more of a job.

Thanks JohnD much appreciated