Product Overview

Zone Valve Operation and Wiring
There have always been questions
surrounding how the 3-wire terminal
block design of the Taco Heat Motor
Zone Valve works. The explana-
tion 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
(terminals1and2).24VACnow
flows through the wires wrapped
around the heater section of the
element inside the zone valve heat.
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 completes
thecircuitthroughterminals2and3ofthepowerheadto
theboilercontrol(TT)andseparatesystemtransformer.
Therelayintheboiler(orzonecontrol
box)startsthecirculator.Thepiston
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.
Whenthethermostatissatisfied,itscontactsopenandcut
offthe24VACpowertothepowerhead.Thewaxinside
the element cools and contracts. The force of the valves
spring moves the valve disc up against the valve seat. Hence
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.
Wiring Note
Whiletwocircuits,eachwithitsowntransformer,touchat
terminal#2,theydonotcommunicateorinterferewitheach
otherinanyway(seefigureC).Eachcircuitflowsonlyin
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.
FigureC:ZoneValveCircuits.