Upon arrival tested the auto feed and found the auto feed was good and only the backflow preventer needs to be replaced. Valved boiler off and drained the pressure. Took the old backflow preventer out and removed old gaskets. Replaced with new backflow preventer and new gaskets. Added pressure back to system and bled the air through the hi-vents on and around the boiler. Had to put a cap on the boiler drain since it was leaking. Opened valves at boiler back up and turned system back on. Unit is working properly.