➀ Automatic Reset High Limit
The unit heater comes standard with an automatic reset high
limit switch that will shut-off the gas should the discharge air
temperature become excessive. See Figure 15.1, indicator
7
for the location of either the standard automatic 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.
Flashback
Floating Flames (See Figure 30.3)
Flame Rollout (See Figure 30.4)
Not Enough Heat
Too Much Heat
1. Too much primary air
2. Main pressure set too high.
3. Orifice too large.
1. Insufficient primary air.
2. Main pressure set too high.
3. Orifice too large.
4. Blocked vent.
1. Main pressure set too high.
2. Orifice too large.
3. Blocked vent.
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. 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. Reduce primary air.
2. Adjust to maximum of 14” W.C.
3. Check orifice size with those listed on the
serial plate.
1. Increase primary air.
2. Adjust to a maximum of 14" W.C.
3. Check orifice size with those listed on the
serial plate.
4. Clean/correct venting system.
1. Adjust to a maximum of 14" W.C.
2. Check orifice size with those listed on
the serial plate.
3. Clean/correct venting system.
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.
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.
Trouble Possible Cause Possible Remedy
Figure 30.2
Lifting Flame Condition
Figure 30.3
Floating Flame Condition
Figure 30.4
Flame Rollout Appearance
SERVICE & TROUBLESHOOTING
Figure 30.1
Wavering Flame or Misalignment
GOOD
BAD
30
