5-565.1
51
service & trouBlesHooting
trouble
Possible cause
Possible remedy
not enough Heat
1. Unit cycling on high limit.
➀
a. Obstructions/leaks in duct system.
b. Main pressure set too high.
c. Blower motor not energized.
d. Loose belt
e. Blower speed too low.
f. Blocked/damaged venting system.
g. Air distribution baffle removed (high
temperature rise units only).
h. Defective high limit switch.
2. Main pressure set too low.
3. Too much outside air.
4. Thermostat malfunction.
5. Gas controls wired incorrectly.
6. Unit undersized.
1.
a. Clean/correct duct system.
b. Adjust to a maximum of 14" W.C.
c. Check/correct to insure blower motor
operates within 45 seconds of when
- gas controls are energized.
d. Adjust belt tension.
e. Check/correct blower drive settings for
proper rpm.
f. Check/correct venting system.
g. Replace air distribution baffle.
h. Replace high limit switch.
2. Adjust main gas pressure.
Minimum for Natural Gas — 6" W.C.
Minimum for Propane Gas — 11" W.C.
3. Adjust outside air damper to decrease
outside air percentage (if possible).
4. Check/replace thermostat.
5. Check unit wiring against the wiring
diagram.
6. Check design conditions. If unit is
undersized, an additional unit(s) or other
heat source must be added.
too much Heat
1. Thermostat malfunction.
2. Gas controls do not shut-off.
a. Gas controls wired incorrectly.
b. Short circuit.
3. Main gas pressure set too high.
4. Defective gas valve.
1. Check/replace thermostat.
2. a. Check unit wiring against the wiring
diagram.
b. Check for loose or worn wires.
3. Adjust to a maximum of 14" W.C.
4. Replace gas valve.
➀
automatic reset High limit
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 14.1, indicator
➆
for
the location of either the standard automatic or optional manual
reset high limit switch. The switch should operate only when
something is seriously 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
Service & Troubleshooting.
figure 51.1
lifting flame condition
figure 51.2
floating flame condition
figure 51.3
flame rollout appearance
