Immersion heater rushing water

A question about : Immersion heater rushing water

Hi guys,

This morning, as usual, I put my immersion heater on for half an hour. After this half hour had passed, I suddenly heard the sound of rushing water. I went to investigate and discovered it was coming from the cupboard where my immersion heater is. I've never seen (or heard) this before, and my water meter was ticking over like crazy so I'm quite concerned about what it was. The only way I could get it to stop (it was going for a good ten minutes without fail) was to completely turn off my water supply.

So I thought I'd best get showered and everything then I'd turn the water off to stop it. But after the shower, the madly rushing water was reduced to just a steady drip.

Does anyone have any idea what this was? Is it a problem or a normal function of the water tank due to overflow or something?

Here's a photo of the water tank - the pipe in the background with the black capsule thing on it was the one that had water flowing through it.

And here's a close up of the problem pipe (no idea why it has this little viewing window...)

Best answers:

• You've got a pressurised water system and the viewing window is called a "tundish" and is there to let you know that you've got a problem.
  The red valve on the front of the tank is a pressure relief valve and when the tank pressure exceeds the safe working level it discharges water through the tundish.
  You should turn off the immersion heaters immediately and get it checked as you might have a faulty thermostat which is allowing the tank to get too hot.
  Other causes are the pressure relief valve is faulty, the pressure limiting valve is faulty or the pressure vessel has an internal leak which stops it working properly - in any case you need to turn off the water and get it checked immediately.