User Guide

F-24579-2 © Copyright 2006 TAC All Rights Reserved. 7
Switch Over Point Adjustment
If necessary, adjust the switch point as follows:
Note:
• The switch point adjustment requires changes to the
main air pressure, which could affect other parts of the
system. This adjustment should instead be made on a
test bench at which a variable main air supply is
available.
• Install a test gauge into the branch line, so that this
pressure can be measured while making the switch
point adjustment.
1. Set the main air pressure to the desired switch over point.
2. Move the setpoint adjustment to its farthest clockwise
position.
3. Use a 20-881 thermostat wrench (1/16” hex wrench) to
turn the switch over calibration screw, 1/8 turn at a time,
until the branch pressure just drops to 0 psig. This sets
the switch over point.
4. Recalibrate the thermostat according to the instructions
in Standard Calibration or Special Calibration, as
applicable.
5. Reinstall the thermostat cover and set the thermostat to
the desired setpoint.
Calibration of Dual Bimetal Models
The 2218-301 thermostat is factory calibrated and should not
require calibration upon installation. However, if the summer
or winter setpoint requires calibration, or if the switch point
needs to be raised or lowered, recalibrate the thermostat as
follows:
1. Remove the thermostat cover and install a 22-138 branch
tap gauge adaptor and a suitable 0 to 30 psig gauge into
the branch pressure tap hole (Figure-10).
2. Measure the ambient temperature with an accurate
thermometer. This temperature
must be within the range
of the thermostat
. Take care not to breathe on, or place
a hand near the bimetals, as this will result in an
inaccurate reading.
Summer Setpoint Calibration
1. Position the summer setpoint cam to the measured
ambient temperature (Figure-10).
2. Set the main air pressure to 16 psig.
3. Adjust the summer calibration screw, using a 20-881
thermostat wrench (1/16” hex wrench), until the branch
tap gauge reads 9
±1 psig. Clockwise rotation increases
the branch line pressure. Counterclockwise rotation
decreases the branch line pressure.
Winter Setpoint Calibration
1. Increase the main air pressure to 25 psig.
2. Using a 20-881 thermostat wrench (1/16” hex wrench),
rotate the winter calibration screw until the branch tap
gauge reads 9
±1 psig. Clockwise rotation increases the
branch line pressure. Counterclockwise rotation
decreases the branch line pressure.
3. The winter setpoint is in calibration when the winter
setpoint dial indicates the ambient temperature within
±2 °F. If not, adjust the winter setpoint as follows:
a. Rotate the winter setpoint screw until the dial gently
contacts its stop. Clockwise rotation increases the
ambient temperature reading, and counterclockwise
rotation decreases the ambient temperature reading.
b. Continue rotating the setpoint screw approximately
1/8 turn, slippint the screw inside the dial.
c. Turn the setpoint screw back and check for 9 ±1 psig
branch air pressure, with the dial indicating the
ambient temperature within ±2 °F.
d. Repeat steps a, b, and c, as necessary, until winter
setpoint calibration is obtained.
4. The winter setpoint screw may now be used to position
the dial to the desired winter control point.
Switch Over Point Adjustment
Adjust the switch over point as follows:
Note:
• The switch point adjustment requires changes to the
main air pressure, which could affect other parts of the
system. This adjustment should instead be made on a
test bench at which a variable main air supply is
available.
• Install a test gauge into the branch line, so that this
pressure can be measured while making the switch
point adjustment.
1. Set the main air pressure to 15 psig.
2. Position the summer setpoint dial to 85 °F and the winter
setoint dial to 44 °F.
3. The branch tap gauge should read 15 psig, minimum. If
not, recheck the summer setpoint calibration.
Summer
Setpoint
Branch
Pressure
Tap
Setpoint Adjustment
Summer
Calibration
Screw
Cover Scale
Setpont Indicator
Celsius
Scale
Fahrenheit
Scale
Limit Stops (2)
(Accessory)
Winter Calibration and
Setpoint Screw
Winter Setpoint Dial
Summer
Bimetal
Winter
Bimetal
Switching
Screw
2
Figure-10 Dual Bimetal Thermostat Calibration
Features and Limit Stop Accessory.