Page 17
DIFFERENTIAL SWITCH CLOSED
−1.0
−0.2
0
−1.0
−0.2
0
−0.4
−0.6
−0.8
−1.2
−0.4
−0.6
−0.8
−1.2
BURNER BOX STATIC PRESSURE
COMBUSTION
AIR BLOWER ST
A
TIC PRESSURE
DIFFERENTIAL SWITCH OPEN
(Furnace will not operate)
CHART REPESENTS NORMAL OPERATING
CHARACTERISTICS OF THE PRESSURE SWITCH ONLY
AND SHOULD NOT BE USED FOR TROUBLSHOOTING
FIGURE 19
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 the required inches differential (see table 12)
, 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 the required 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 19) the unit may have a restricted flue outlet or oth
er problem. If both readings are farther from zero (figure 19)
the unit may have a restricted flue inlet or other problem.
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 20.
FIGURE 20
TEE AND 1/4"i.d. RUBBER HOSE FIELD PROVIDED
USED FOR MEASURING PRESSURE
ACROSS BURNER BOX AND COM
BUSTION AIR BLOWER
TO DRAFT GAUGE
TO PRESSURE
SWITCH
TO PRESSURE
SENSING HOSE
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 20.
8 − Install draft gauge to open end of Tee.
9 − 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). Both readings should
fall above the line shown in figure 19.
10− Compare readings to figure 19. Be sure to compare
only like readings (compare startup reading to startup
reading, then compare steady state reading to steady
state reading). Subtract the absolute steady state
readings from one another. This will be the pressure
differential.
In order for the furnace to operate, the
larger negative must always be on the combustion