T022_F or A022_F: Main Limit Switch Open
Status
This is a critical fault. The furnace will not operate in gas heat modes but all other modes (e.g. cooling) should function. The Alert (T022_F) will occur 3 times in a single call before becoming an alarm (A022_F) and will lock the furnace out for one hour. It may be reset sooner manually by cycling power to the furnace.
Description
The main limit has opened or is sensed to be opened. This normally means that the temperature inside the heat exchanger area has gone above a certain predetermined critical value and heating operation is not permitted until the limit cools to within normal parameters.
Expected Operation
If the furnace was in heating operation when this fault occurred, blower operation will immediately stop and the furnace will shut down normally with post-purge at the correct speed. After the post purge (or immediately if no heat call was present), no other operation (including thermostat calls) will occur until this fault is cleared. This fault may be displayed in heating or cooling modes and may also be displayed in heating mode after the main limit control has been opened four times consecutively for more than 150 seconds (2m: 30sec) each time or after the main limit control has opened at 40 heat rate and the main limit does not close within 150 seconds. If this happens, the IFC determines that the motor and/or blower may not be functional and enters a hard lockout condition requiring repair of the blower/motor or determining cause for main limit to open and manual reset of power to the furnace.
Cause
1. No airflow
2. Insufficient airflow
3. Faulty Limit Control
4. Loose or Faulty Wiring
5. Dead (non-functional) blower
6. Input too high
Solution
1. Check for proper blower operation. Is the blower turning during heat (or any other) mode? If not, a blower motor fault should also be present. Check the wiring to the motor then check the motor. It may need replacing.
2. Check duct work and filters. Determine the static pressure and make sure it is not above the published values for the furnace. Check the rate and outlet air temperature for high and low heat using a jumper wire between R and W/W1 respectively. Compare them to the nameplate maximum values and adjust if necessary. If a SA sensor is used cycle power to the furnace.
3. Replace the limit control.
4. Check wiring and connections. Replace and/or repair as necessary.
5. Insure properly sized burner orifices are installed. Check the manifold pressure at high fire and compare to the nameplate values. Adjust as needed.
6. Input too high
