DHW problem - pressure release valve?

[b33k34]

We have a conventional DHW system with a hot water tank running most of the house. However the last owners added a pump to the airing cupboard that supplies the en-suite basin and shower (this is above the water tank so pressure is very poor otherwise.)

This is connected to the hot water tank using "Hep" style plastic pipe that runs up into the attic and back down to the shower room. In the attic there is a t-junction in each pipe and a short section sticks up with a brass cylindrical object about 7cm tall and 3cm across. On the top of these is a little plastic 'pen cap' that can be screwed down or opened.

I've always assumed these are some soft of pressure release valve. They've always had a bit of scale on them as if a very small amount of water is released at times.

Last night one of them let out enough water to leave a wet patch on the ceiling and blow all the paintwork on a section of picture rail.

Why do we need these valves? How should they work? How can i stop them from soaking my ceiling?

[Andrew Gabriel]

They sound like automatic air vents to prevent an air lock. You should just be able to screw down the cap to seal them. They would only need opening if you are trying to remove air locks, which is only likely to happen if you've had the water turned off for some reason. A build up of hard water scale can cause them to start leaking.

Here are some pictures, although they vary from one manufacturer to another:

[b33k34]

It's one of the Robocal ones. I wonder if it's failed - when you unscrew the black plastic bit completely you can see water inside. We've done it up tight and watched it while someone showers and it's not leaking now.

Thanks