Weil-McLain Heater CG-5-PIDN residential gas fired/hot water
Every once in a while when the thermostat calls for heat: 1) automatic vent damper butterfly valve opens, 2) circulator comes on, 3) gas/flames comes on however the zone valve fails to open and the water temperature continues to rise and will go above 230 degrees. This occurs even though the gas/flames shut off at preset temperature (185) as heated water is continuing to rise. Consequently the water in the pipes at the heater begins to boil and causes very loud pipe banging.
Very random: not every time thermostat calls for heat. Maybe once or twice a day. Always seems to be in the evening or overnight.
Also, sometimes the automatic vent damper butterfly valve stays open after the gas and circulator go off.
Works fine other times: 1) automatic vent damper butterfly valve opens, 2) circulator comes on, 3) gas/flames comes on, 4) gas shuts off at 185 degrees (preset temp) and 5) comes on again at approx 160 degrees.
Anyone have any idea what could be causing this problem?
Yep, the CG series don't play well with Taco zone valves. You need to swap them out for Honeywell's or some other fast opening valve.