Well I has a gas engineer round the other day. He changed a pressure switch and pumped the expansion vessel back up with a foot pump. Took about 20 mins.

Probably an hours labour plus parts. Or you can get those guaranteed fix things from britishs gas etc for a certain amount.

How much £££ are we talking?

Pressure relief valve is my bet.

Expansion vessel could be flat and needs pumping up to put more air in. Would be a gas engineer visit that's required to sort.

It takes a fair bit of pressure to bet the pressure relief to go though. About 3 bar!!

Sounds like the pressure relieve valve or the diverter valve Is about to shit it's pants.

:thumbs:

Don't know too much myself other than if the system isn't regularly flushed etc the build up of rusty water is normal, if it starts coming out the over flow it's normally down to a sticking pressure relief valve so the water leaks out the overflow, the rustiness is just down to age of water in the system etc

A rusty looking stain is appearing on the outside wall of one my rental houses. The boiler sits directly at the other side of the wall to where the stain is, and the source appears to be an overflow pipe of some sort.

I know jack shit about boilers tbh, so before i phone the letting agent to get one of their guys to sort it i wanted to ask what the root cause possibilities are. Pipe needs replacing? Maybe feed it into the down-pipe gutter (if it isn't hot water)?