dhw diaphragm which is underneath boiler. black box attatched to brass round housing. as water is drawnthrough boiler pin comes out and presses on microswitch in black box to fire boiler, if it doesnt move or only moves slightly then diaphragm has failed but if it makes contact with microswitch then red sensor for dhw has failed. dpmartin17@aol.com
paul, October 2007
