dhw halstead ace high?

i have followed the procedure in the fault finding in the manual and have, replaced the dhw thermistor and now the pcb, set up the board according to the M.I. and know hot water, does not even fire up unless by doing it manually myself. when leaving it about half an hour and scratching my head miraculously had hot water for approx four hours. And now nothing at all again. I see from previous questions it is now pointing to the diaphram and flow switch. But there is NO mention of this in the M.I. Please Help!!!