Follow-up: The HT and LT water pump vent plug thread size is R 1/4″. We added vent valves to these locations during the 500hr maintenance on our 4 W50SG units, but still experience multiple trips on lo LT and HT pump pressure after refilling and restarting the units. The best solution we’ve found thus far is to extend the trip delay timer and hold the unit at min. load for half an hour or so, until most of the air has been vented from the system.
City of Tallahassee Hopkins Station