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FIGURE 11
1 - Remove thermostat demand and allow unit to
cycle off.
2 - Install a tee in the negative (-) line (red and black tubing
or red tubing) and a tee in the positive (+) line (black
tubing) running from the pressure switch to the cold
end header box.
3 - Install a manometer with hose from the negative (-)
side of the manometer to the tee installed in the
negative (-) line and with hose from the positive (+)
side of the manometer to the tee in the positive (+)
line.
NOTE - Both sides of the cold end header box are nega
tive. However the (+) port reads less negative pressure
than the (-) port.
4 - Operate unit and observe manometer reading.
Readings will change as heat exchanger warms.
a. Take one reading immediately after start‐up.
b. Take a second reading after unit has reached
steady state (approximately 5 minutes). This will be
the pressure differential.
The pressure differential should be greater
than those listed in table 6.
5 - Remove thermostat demand and allow to cycle off.
6 - Remove manometer and tee's. Reinstall combustion
air sensing hoses to the pressure switch.
Measuring Pressure Differential
To Cold End Header Box
Field Provided Tubing
To Pressure Switch
To Cold End Header Box
Black Tubing
(positive +)
Red and Black Tubing
or Red Tubing
(negative −)
“+” High
“−” Low
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