Vernon Winnelson

Taco Zone Valve Chart

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Taco Zone Valve Chart






There have always been questions surrounding how the 3-wire terminal block design of the Taco Heat Motor Zone Valve works. The explanation below and associated diagrams explain step-by-step the operation of the zone valve.

As the air temperature at the thermostat drops to the thermostat setting, a normally open contact in the thermostat closes. This completes a circuit between the thermostat, the zone valve transformer and the heat motor of the zone valve power head.

The expandable wax substance inside the element is heated. It expands and pushes the piston inside the element down against the valve stem. Since the valve is an upside down globe valve, this downward push on its stem moves the valve disc away from its seat, opening the valve. As the piston continues to move down, further opening the valve, the contacts inside the enclosed end switch close as the actuator tab attached to the piston moves away from the end switch. This dry contact end switch closure complete The relay in the boiler (or zone control box) starts the circulator.



(terminals 1 and 2). 24 VAC now flows through the wires wrapped around the heater section of theelement inside the zone valve heat.the circuit through terminals 2 and 3 of the power head to the boiler control (T T) and separate system transformer.continues to move down until the valve is fully open. An interruption of current to the power unit heater occurs in the fully open mode because the outer blade of the heater switch is deflected causing the heater element to open, interrupting the 24 VAC power from flowing through the heater wires As the piston retracts slightly, the contacts on the heater switch meet again, resuming power to the heater wires wrapped around the wax filled element. This slight back and forth motion is repeated as long as the thermostat contact is closed. the element cools and contracts. The force of the valves spring moves the valve disc up against the valve seat. Henc the valve stem also moves up, pushing the piston back into the element. Now that the heater switch is closed, the snap-acting end switch between terminals 2 and 3 is open. that circuit. This occurs because the transformer in either circuit cannot cause a voltage or current driving force to be realized in the other circuit when joined only at one point to that circuit

When the thermostat is satisfied, its contacts open and cut off the 24 VAC power to the power head. While two circuits, each with its own transformer, touch at terminal #2, they do not communicate or interfere with each other in any way.

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