Help just got a horrible squealing noise from my water pump , , on closer look the pump was running dry , no water in the pipes at all , tried venting the air from the system and fitted new grundfoss 15 60 pump but still cant get any water into the pipes, a lot of air/water noise in the boiler itself but wont vent from the union inside the boiler housing, have checked all the header tanks etc and all full , any ideas folks