Hi folks, further to my recent post (suspected faulty gas valve and scaled/sludged heat exchangers) I've run a sludge remover thru the system but noticed the pressure does not return back to the previous pressure at cold. I've reset it twice now to 1 bar (filling loop is off!) but when the heating is on it rises up to 3+bar even at low. I'm replacing the secondary HE tomorrow and will look at the primary HE and check the pressure vessel . I can understand why the pressure might rise while heating but can't understand why it doesn't return when cold...any thoughts?