CONQUEST 90 - Page 91

Check. The following voltage test
can be used.
1. Turn power on to the furnace.
2.
Use a voltmeter to check for 24
vac across the limit contacts.
3. A 24 vac reading indicates an
open limit switch.
4. A zero (0) vac reading indicates
a closed switch.
The condition of the limit switch can
be verified with an ohmmeter. If the
meter shows continuity the switch is
good. If the meter shows an open
circuit, and the switch temperature is
below reset temperature, replace the
switch.
There are several reasons for a limit
switch to open and they almost
always involve low air flow through
the furnace.
1.
A dirty or restricted air filter.
2. A dirty or restricted cooling coil.
3. A dirty or restricted secondary
heat exchanger.
4. Undersized or restricted return
air system.
5. Undersized or restricted supply
air system.
6.
Closed supply air registers.
7. A problem affecting the main
blower:
a. A wrong speed tap selection,
b.
Failing motor bearings,
C.
Low voltage to the motor,
d.
Dirty blower wheel,
e. Wrong motor rotation,
f.
Blower wheel slipping on the
motor shaft.
8.
Overtiring the furnace with too
much gas pressure.
y
TABLE26
FIGURE 105
LIMIT IN UNIT
•
FIGURE 106
CHECKING LIMIT SWITCH
•
..
I
MAIN LIMIT SETTING TEMPERATURES
FURNACEINPUT
-GRA/-GRJ
RESET
45,000 BTUH 170° 150°
60,000 BTUH 160' 140°
75,000 BTUH (07 .. A)
200°
0
1so
75,000 BTUH (07' 'BJ 190° 170°
90,000 BTUH 190° 170°
105,000BTUH 180'
0
1so
120,000BTUH 190° 170°
·GSA/-GTA/-GTJ RESET
200°
1ao·
1ao·
160°
190°
170°
200° 180°
150° 130°
170'
0
1so
170'
150°
90