Steam escaping from indoor boiler

[darwin]

Hi, I'd be most grateful as to any suggestions for the following problem:
I have an indoor Grant Euroflame domestic boiler. Beside the boiler is
a vertical pipe on top of which sits a valve of some description (pressure relief valve ?)
Today I noticed steam (and black water) erupting from the valve so
I immediately switched off the heating. This stopped when the radiators had cooled. Could anyone shed any light as to why this has suddenly started happening ? (I had the boiler serviced last October). Will it be an expensive job to fix ?

Many thanks for your help.

 

[ChrisR]

It may well be a pressure relief valve. It may be doing its job , letting pressure out before the system explodes. You'd might want to check for problems before assuming the more likely alternative - that the thing has decided to leak for no particular reason. If it's the latter, I'd freeze the pipes either side to put a new one in - you don't lose inhibitor and you don't get air locks that way.
If you just changed the valve, and it still leaked, you'd KNOW you had problems elsewhere.
Not a long job, but I'd charge a bit for frreezing on top of the time. Part is a few quid.

 

[breezer]

could it be an auto air vent, (small brass thing) ours does that sometimes