Thanks very much to the three guys who replied to my earlier posting.
Much appreciated. I turned the hot water on and watched the tank to see
what happened. There was a continuous steady flow of hot water from the
vent pipe into the expansion tank. It showed no sign of stopping. Very
disconcerting. I have now shut the system down pending a plumber
visiting on Wednesday. I had a new pump fitted two months ago. Perhaps
they set it too quick. Although the system was flushed and inhibitor
put in at this time, I think there is sludge in the system. The water
coming out of the vent pipe was a darkish brown.
Does anyone know how much it would cost to convert to a sealed system
and whether it would be worth it?