Hi all, I have a pottrton performa 28i and i have the strangest of problems. When i run the hot water and theen turn it off the boiler keeps burning even though the heating is off. But when i turn the water temp dial right down it works. and on the diverter vavle the bottom plunger does not return when the water is turned off but again it oes when the temp ial is set to very low. PLEASE Help