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Domestic HW zone not calling for heat until another zone calls for heat
I have a 4 zone system. Zone 1, 2 and 3 are baseboard hot water. Zone 4 is domestic hot water with a circulator to a coil in Vaughn Top Performer insulated storage tank. System is 12 years old. Worked fine until recently. Seems the super store tank water is dropping below the point where it would normally call for hot water to its coil from the boiler. Odd thing is when zones 1-3 thermostats call for heat in the morning it is then that the zone 4 circulator turns on. It heats the tank water to what I assume is the high set point. Once it is at temp THEN zones 1-3 circulators turn on...despite the fact that they'd been calling for heat all along while the tank was domestic hw tank was heating up. The taco "zone 4 priority switch" is enabled and always has been. Question - why isn't zone 4 turning on when it needs its water heated? Why does another zone's call for heat seem to be needed to turn on circulator 4?