Steam coming out of boiler

Airlock? Not sure about the remedy, try making sure the air separator valve as its cap unscrewed slightly.

Andy.

Basically, the water isn't circulating, so it is boiling the water within the boiler.

1. You haven't refilled it properly and there are excessive airlocks that can't be pumped through.
2. You have an S-Plan system with no bypass and the valves have failed.
3. The pump has failed.

My bet is on "1".

How did you go about refilling the system? Did you notice the bleed valve on the top of the boiler? Do you have any drop loops to radiators (there's often additional bleed valves on the top of the loop).

Christian.

Number 3 actually. The pump had failed.

Thanks for all the replies.

Steve

...

Though if you really had been getting steam out of the "type pressure valve thingy" (by which I assume you mean same as a car tyre valve) that would suggest the pressure vessel diaphragm had also gone. However the automatic air vent is more prominent and outwardly similar-looking so I guess that's what you meant - ?

Yes, it was the automatic airvent doing it's job.

In this case the auto-steam-vent.

The noise, cavitation and pressure pulses when this happens can be so severe that the boiler can begin leaking afterwards. However on a newer unit it will likely have survived.

The noise has been described to me as 'it sounds like rocks are are being crushed inside'.