Page 29
31-5000724 Rev. 0
5.
If low pressure switch opens turn off inducer. No further
action.
6.
If low pressure switch is still closed, decrease inducer
speed 1/2 of the low pressure switch open RPM
margin. Allow 5 seconds to stabilize.
7.
If low pressure switch is open turn off inducer. No
further action.
8.
If low pressure switch is still closed, set flag for
calibration on next call for heat and turn off inducer.
Measuring Pressure Differential (Figure 18)
Checks of pressure differential can aid in troubleshooting.
GE Appliances provides a kit (10L34) if necessary. When
measuring the pressure differential, readings should be
taken at the pressure switch. Lack of differential usually
indicates problems in the intake or exhaust piping, but may
indicate problems in the heat exchanger, condensing coil,
header boxes, combustion inducer or other components.
The differential pressure is the difference in pressure
measured across the cold end header box orifice.
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 inducer 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 pressure
switch hoses.
The pressure switch measures the pressure differential
across the CAI orifice (difference in the channel and
cold end header box).
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.
Unit
Set Point
High Fire
Set Point
Low Fire
All
1.00 ± 0.05
0.25 ± 0.05
*Units over 7500 ft will require a conversion kit. See Table 28.
Table 17. Pressure Switch 0’ to 7500’
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 measuring device with hose from the negative (-)
side of
tive (-) line and with hose from the positive (+) side of the
measuring device to the tee in the positive (+) line.
NOTE -
ever the (+) port reads less negative pressure than the (-) port.
4 - Operate unit and observe measuring device 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 20.
5 - Remove thermostat demand and allow to cycle off.
6 - Remove measuring device and tee's. Reinstall combustion
air sensing hoses to the pressure switch.
RED and
BLACK
TUBING
or RED TUBING NEGATIVE
BLACK TUBING
POSITIVE
To Cold End Header Box
Field Installed
Measuring Device
Red
Figure 18.
Measuring Pressure Differential
