"Aidan" wrote in message
ups.com...
In normal operation it releases any air that gets into the system.
Typically dissoled oxygen in the boiler return wil be released from
solution when the water is haeted in the boiler and is discharged from
the open vent.

In the event of a control failure, it is a safety device; it releases
steam, prevents the system becoming over-pressurized and bursting. In
your system, such a failure would probably now cause the contents of
the heating system to be discharged into the loft and the boiler would
dry-fire to destruction.

If the boiler has a blow-off valve the boiler is most probably suitable for
sealed systems ..... and a combined feed and expansion pipe. If so, then he
has a one pipe F&E system, except the pipe is probably 15mm not 22mm. Then
it will not dry fire itself to destruction as a high limit is on the boiler.