My guess is Ian's right. You need to manually operate the PRV so there is no system pressure whilst pumping up the expansion vessel 12psi should do. Download the manual
http://tinyurl.com/ygwvwxo
pdf 30/64:
8.20.2 Close heating flow and return valves (fig. 6) by
turning 1/4 turn until indicating lines are horizontal.
Pull lever on head of safety valve to
drain primary circuit to boiler.
c
October 2009