just double checked. closed them both down and listened for the noise to change on the pump then opened them both back up and now nice and quiet but still overflowing!
not that i can see.
if i close the gate valve that comes from the tank in the loft into the pipework the water stops overflowing the other way back into the top of the tank in the loft
my 10 year old grundfos 15 50 central heating pump has got noisy recently so i changed if for a new one today and all went rather well! I put the heating on and the house warmed up really quickly.
i could hear running water so went into the loft to investigate and noticed hot water running out...