55
5-572.10
SERVICE & TROUBLESHOOTING (CONTINUED)
Figure 55.1
Floating Flame Condition
Figure 55.2
Flame Rollout Appearance
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The duct furnace comes standard with an automatic reset high limit switch that will shut-off the gas should the discharge air temperature become
excessive. See Figure 22.1, indicator (49) for the location of either the standard automatic or optional manual reset high limit switch. The switch
should operate only when something is wrong with the unit operation. Anytime the switch operates, correct the difficulty immediately or serious
damage may result. If the switch cuts off the gas supply during normal operation, refer to the “Not Enough Heat” section of Table 54.1.
Trouble
Possible Cause
Possible Remedy
Floating Flames (See Figure 55.1)
1. Insufficient primary air.
1. Increase primary air.
2. Main pressure set too high.
2. Adjust to a maximum of 14” W.C.
3. Orifice too large.
3. Check orifice size with those listed on the
serial plate.
4. Blocked vent cap.
4. Clean louvers in vent cap.
Flame Rollout (See Figure 55.2)
1. Main pressure set too high.
1. Adjust to a maximum of 14” W.C.
2. Orifice too large.
2. Check orifice size with those listed on the
serial plate.
3. Blocked vent cap.
3. Clean louvers in vent cap.
Not Enough Heat
1. Unit cycling on high limit.
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1.
a. Obstructions/leaks in duct system.
a. Clean/correct duct system.
b. Main pressure set too high.
b. Adjust to a maximum of 14” W.C.
c. Blower motor not energized.
c. Check that blower motor operates
within 45 seconds of when gas controls
are energized.
d. Loose belt
d. Adjust belt tension.
e. Blower speed too low.
e. Check/correct blower drive settings for
proper rpm.
f. Blocked/damaged venting system.
f. Check/correct venting system.
g. Defective high limit switch.
g. Replace high limit switch.
2. Main pressure set too low.
2. Adjust main gas pressure. (Minimum for
Natural Gas — 6” W.C. Minimum for
Propane Gas — 11” W.C.)
3. Too much outside air.
3. Adjust outside air damper to decrease
outside air percentage (if possible).
4. Thermostat malfunction.
4. Check/replace thermostat.
5. Gas controls wired incorrectly.
5. Check unit wiring against the wiring
diagram.
6. Unit undersized.
6. Check design conditions. If unit is under-
sized, an additional unit(s) or other heat
source must be added.
Too Much Heat
1. Thermostat malfunction.
1. Check/replace thermostat.
2. Gas controls do not shut-off.
2.
a. Gas controls wired incorrectly.
a. Check unit wiring against the wiring
diagram.
b. Short circuit.
b. Check for loose or worn wires.
3. Main gas pressure set too high.
3. Adjust to a maximum of 14” W.C.
4. Defective gas valve.
4. Replace gas valve.
Power Exhauster Motor Will Not Start
1. Power supply is off.
1. Turn on main power.
2. No 24V power to thermostat.
2. Check control transformer.
3. Thermostat malfunction.
3. Check/replace thermostat.
4. Defective power exhauster relay.
4. Replace power exhauster relay.
5. Defective power exhauster motor.
5. Replace power exhauster motor.
Table 54.1 - Troubleshooting (Continued)
