I have a myson orion boiler and the vent id from the first floor by the divertor valve. When the system reaches temperature and shuts down, I get a banging noise and red hot water venting into the overflow?
Any advice please? i have already changed the 3 port valve and the pump.

John, March 2010

check the pump over run on the boiler is working, plus check that the by-pass is open some times they can collet a bitt of muck in, turn it closed then open it fully
then back 1/2 way and see what happens

erj, March 2010