Page 36
FIGURE 15
1 −
Remove thermostat demand and allow unit to cycle off.
2 −
Install a tee in the negative (−) line (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 negative.
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 19.
5 −
Remove thermostat demand and allow to cycle off.
6 −
Remove manometer and tee’s. Reinstall combustion air
sensing hoses to the pressure switch.
BLACK TUBING
POSITIVE
RED TUBING
NEGATIVE
Measuring Pressure Differential
The CAI is installed on the cold end header box. The cold
end header box is a single piece made of hard plastic. The
box has an internal channel where the combustion air in-
ducer creates negative pressure at unit start up. The
channel contains an orifice used to regulate flow created
by the CAI. The box has pressure taps for the CAI pres-
sure switch hoses.
The pressure switch measures the pressure differential
across the CAI orifice (difference in the channel and cold
end header box). See table 18 for orifice size per unit.
If
replacement is necessary the gaskets used to seal
the box to the vestibule panel and the CAI to the box,
must also be replaced.
TABLE 18
SLP98UHV Unit
C.A.I. Orifice Size
−070
0.780
−090
0.920
−110
1.030
−135
1.170
TABLE 19
Pressure Switch
0’ to 7500’
SLP98UHV Unit
Set Point
High Fire
Set Point
Low Fire
−070
1.00 + 0.05
0.25 + 0.05
−090
−110
−135
*Units over 7500 ft will require a conversion kit. See table 29.
