F-24576-4
©
Copyright 2008 TAC All Rights Reserved.
5
OPERATION
The deadband output (branch) pressure is factory set at
8 psig (55.2 kPa), placing both heating and cooling actuators
in the “off” (or closed) position. The output remains at that
pressure while the ambient (room) temperature varies within
the deadband. When the ambient temperature closely
approaches either the heating or cooling setpoint at either end
of the deadband, the output pressure changes at the rate of
approximately 3.3 psi/°F (41 kPa/°C). This modulates the
respective heating or cooling actuator from its closed position
toward its open position, while the other actuator remains in its
closed position. The temperature sensing assembly,
consisting of a deadband lever and two bimetal elements,
allows only one actuator at a time to be modulated toward the
open position.
CALIBRATION
Deadband Pressure Calibration
The deadband pressure is factory set at 8 psig. If necessary,
check calibration or adjust this pressure to meet application
requirements, as follows:
1. Remove the thermostat cover, using a 20-881 thermostat
wrench (1/4” hex wrench), and install a 22-138 branch tap
gauge adaptor and a suitable gauge into the branch
pressure tap hole (Figure-10 or Figure-11).
2. Be sure ambient temperature is between 65 and 75°F.
Turn the heating dial to 57°F and the cooling dial to 83°F.
3. Adjust the deadband pressure by turning calibrating
screw “A” (see Figure-10 or Figure-11), using a 20-881
thermostat wrench (1/16” hex wrench) until the desired
deadband pressure is obtained.
Setpoint Calibration
Direct-Acting Models
The direct-acting models are calibrated to have a 5.5 psig
branch pressure when the heating dial is set at ambient
temperature, and a 10.5 psig branch pressure when the
cooling dial is set at ambient temperature.
1. Refer to Figure-10. Using a 20-881 thermostat wrench
(1/16” hex wrench), gently turn screw “B” to position the
cooling dial to the 83°F setting. This moves the cooling
bimetal away from the deadband lever. Turn screw “C” to
obtain 5.5 psig output pressure. If there is a difference
between the ambient temperature and the dial reading,
rotate the dial gently to its stop and continue rotating screw
“C”, slipping the screw inside the dial. Rotate the dial back
to again obtain 5.5 psig. If the dial does not match the
ambient temperature, a second try may be required.
Figure-9 Direct-Acting Thermostat Operation.
Thermostat Output
Pressure Increase
Heating
Heating and
Cooling Off
Dead-Band
(adjustable)
Temperature Increase
Dead-Band Pressure
(factory set at 8 psig)
Dashed lines indicate reverse action.
Cooling
Direct Action
