Powermax overflow

[WillQ]

I visited my daughter in Chester this week. On arrival the tank overflow was discharging hot water and steam over the front door - not a pleasant welcome! The boiler is a powermax 155xP sealed system i think. I climbed in the loft and found that the vent was discharging into the tank. I trained as a pipe fitter/welder many moons ago but haven't worked as one for years. Any ideas what could be causing this. British Gas refused to service it as they said it didn't have enough room around it. The house is only 3 years old.
Will

 

[Softus]

1. Are you sure it's sealed?

2. Which cistern is the hot water discharging into?

3. Is the immersion heater on?

 

[WillQ]

There is a small expansion vessel on the front of the boiler. I thought that this would indicate that it was a sealed system. I think that the tank is a cold feed tank. I asked my daughter if there was an immersion heater fitted and she said there wasn't. Please excuse my vagueness but heating systems have moved on considerably since my days on the tools.

 

[Softus]

A faulty cylinder coil would be consistent with the symptoms you describe.