Auxiliary limit control (location and purpose)

What is going on

On these select 90% and 95% furnaces there are two auxiliary limits on the blower housing, one on each side. They are there for the horizontal and counterflow installs, where a failed blower would let heat build up with nothing to carry it away. When I see one open it usually means the blower quit or the power dropped while the burners were lit. They also open on a power loss mid-cycle. They are good for both horizontal right and horizontal left, so you do not move them around.

Likely causes

• Main blower failure in horizontal or counterflow installations
• Loss of power while the furnace is firing
• Restricted airflow overheating the blower housing
• Loose connection at the limit terminals

Check in this order

1. Locate the two auxiliary limits on the blower housing, one on each side
2. The auxiliary limit works in both horizontal right and horizontal left installations and does not need to be relocated for airflow direction
3. Test the auxiliary limit per the procedure in the manual that follows this section, power off and at room temperature
4. If one is open, check the blower and filter before resetting it
5. Do not bypass either limit, they protect the cabinet and wiring

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit switch open (blower compartment)
• Blower motor does not have enough information, fails to start 40 consecutive times