Measuring Pressure Differential - Lennox G26 series User Manual

−1.2
−1.0
−0.8
−0.6
−0.4
−0.2
0
0
Temporarily jumpering the pressure switch when trouble-
shooting will determine if the pressure switch and furnace
are operating properly. However, this may not indicate if
the sealed combustion system is operating properly. If the
unit cannot attain 0.2 inches differential, the unit will not
operate. Be sure to remove jumper when finished. See
Warning this page.
WARNING
Safety Hazard. Turn off gas supply before jump-
ering switch or testing switch differential. If
switch is operating properly and sealed com-
bustion system is operating improperly, a po-
tentially lethal situation will be created when
switch is bypassed. DO NOT ALLOW UNIT TO
OPERATE WITH SAFETY SYSTEMS BYPASSED.
Checks of pressure differential can be made as an aid in trou-
bleshooting. It is important to remember that the switch must
see" 0.2 inches differential in order for the furnace to operate.
Lack of differential usually indicates problems in the intake or
exhaust piping but may indicate problems in the heat ex-
changer, condenser coil, header boxes, combustion blower or
other components. Generally, if both readings are closer to
zero (figure 24) the unit may have a restricted flue outlet or
other problem. If both readings are farther from zero (figure
24) the unit may have a restricted flue inlet or other problem.
CHART REPESENTS NORMAL OPERATING
CHARACTERISTICS OF THE PRESSURE SWITCH ONLY
AND SHOULD NOT BE USED FOR TROUBLSHOOTING
DIFFERENTIAL SWITCH CLOSED
−0.2 −0.4
BURNER BOX STATIC PRESSURE
DIFFERENTIAL SWITCH OPEN
(Furnace will not operate)
−0.6 −0.8 −1.0
FIGURE 24

Measuring pressure differential

The differential pressure is the difference in pressure mea-
sured on either side of the pressure switch:
1 − Remove thermostat demand and allow to cycle off.
2 − Disconnect hose from left side of pressure switch and
install Tee as shown in figure 25.
TEE AND 1/4"i.d. RUBBER HOSE FIELD PROVIDED
USED FOR MEASURING PRES-
SURE ACROSS BURNER BOX
AND COMBUSTION AIR BLOW-
ER
TO PRESSURE
SWITCH
3 − Install draft gauge to open end of Tee.
4 − Operate unit and observe draft gauge reading. Read-
ings will change as heat exchanger warms.
a. Take one reading immediately after startup.
b. Take a second reading after unit has reached
steady state (approximately 5 minutes).
5 − Remove thermostat demand and allow to cycle off.
6 − Remove draft gauge and Tee. Reinstall combustion air
sensing hose to left side of pressure switch.
7 − Disconnect hose from right side of pressure switch
and install Tee as shown in figure 25.
8 − Install draft gauge to open end of Tee.
Page 19
−1.2
TO PRESSURE
SENSING HOSE
TO DRAFT GAUGE
FIGURE 25