If you mean the domestic diaphragm, it's right at the back of the boiler underneath the burner chamber, you would need to remove the diveter valve to change the diaphragm
Carefree
January 2011
Automatic Answer
While you wait for answers, we may have found some information which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.