My Expansion tank developed a leak (oh no). One new expansion tank and new ceiling later I bled the air out of the system and switched on. No radiators are getting warm. I pulled up the floorboards to see. Only the pipe coming out of the boiler is getting hot. Once it gets to a radiator it all gets cold. I replaced the pump, but to no avail.I flushed the pipes out – no blockage.The only thing I can think of it being it a one way valve which is fitted where the hot water goes up. Can anyone suggest any ways of testing? Or if they think it is down to anything else?Thanks in advance.Lee