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Can't get Auto mode to work for Air Handler Blower
I have a Unico air handler with a hydronic heating coil, 24V control circuit with Therm-O-Disc time-delay relay. When the thermostat Fan switch is turned from Auto to On, I see 24 volts move from the blue to the green terminal and the blower relay is activated - fan comes on, no problem. In order to use Auto mode, I strapped a JC/Penn Aquastat A19DAC-1 to the supply pipe, and connected its Red (common) and Yellow (close at temp) to the same bus bar terminals as the thermostat's Blue (load) and Green (fan) -- in parallel with the thermostat's "Fan On" circuit -- and the transformer's primary coil blew. Same result on both air handlers, so it's not a matter of one faulty component or wire. There is no continuity between the Aquastat's terminals and ground, when cold or hot; no indication of a short anywhere. If I put a jumper across the same (blue and green) terminals of the bus bar, the blower comes on. Why would the Aquastat cause this problem when it's in parallel with the thermostat or a jumper? Any ideas would be greatly appreciated.
Wiring Diagram attached: