"BillR" wrote in message
...

There could air in the boiler. There may be some way of bleeding the
boiler
itself.
Some combis have an automatic bleed valve. See installation/user manual.
Another possibility is the pump has locked due to running dry.
There is a standard procedure to try to free it.
Worst case is that something has been damaged by trying to run the system
without sufficient water

Yeah I'm thinking that myself. Any idea what the standard procedure to
unlock the pump would be ? I've only got a single sheet of user
documentation that doesn't go much beyond tic-tac-toe. It's an SD620F if
anyone knows it ?

Would shutting it down completely and restarting help free the pump ?