G
gimmer
after spending some considerable time on this piece of junk i am now befuddled. burner on both ch and dhw mode burner goes out for 3 mins relights for 3 mins goes out and on and on the whole system was full of sludge so i have removed all the crap and cleaned out hex replaced worn pump head and high limit stat . ch and dhw sensors good. cleaned out flue and fan assy. turned to gas valve adjusted max rate to 10mb burner goes off burner pressure fluctuating all over down to to a minus. eventuall y got the dhw to stay on through altering gas pressure but water only warm. heating still going off got 20 vdc at the mod any ideas anyone me thinking blockage in main heat ex