G6RA, G6RK Service Manual 29
Main Air Limit Control (See Figure 35)
The main limit control is an automatic reset type. It reacts to
abnormally high air temperatures in the heat exchanger area.
If the main limit opens, the gas valve is de-energized and the
induced draft and main blower motors continue to run. The
main limit will automatically reset after the temperature is
reduced.
Vent Limit Control (See Figure 35A)
The function of the vent limit switch is to sense the heat from
the flue gas in the event the vent system becomes blocked
or restricted.
Check-out Procedure: Same as for main limit switch.
Possible causes of vent switch tripping:
1. Blocked vent system.
2. Improper installation of special vent systems.
3. Inadequate combustion air. Furnace may be operating in
a space classified as "confined." Consult Section 2 of
this service manual.
Check-out Procedure:
1. Shut off power to furnace.
2. Remove wires from limit (Be sure furnace has removed
any heat surrounding switch).
3. Check for continuity across switch.
a. If continuity is present, switch is closed and assumed
good.
b. If continuity is infinite, the limit is open and should be
replaced.*
*Limits should be replaced with their exact replacement.
Check-out can also be performed using a voltmeter:
a. Put meter on at least 24 vac scale.
b. A voltage reading across the switch
indicates an open switch if there is voltage
present at the switch.
Possible causes for Main Limit Tripping:
1. Dirty filter
2. Dirty cooling coil
3. Oversized furnace
4. Restrictive duct system
5. Main blower failure
6. Improper speed selection
7. Over-firing of furnace (gas pressure too high)
8. Main or induced draft motor cycling on internal overload
Figure 35.
Figure 35A.
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