HVACopediaGoodman · Furnace troubleshooting
Airflow lower than demanded, furnace runs on low stage when high stage is expected
What is going on
Not enough air moving. Control runs the furnace on low stage when it should be on high. Same story as most airflow calls, check the air path.
Likely causes
- Blocked filters
- Restrictive ductwork
- Undersized ductwork
Check in this order
- Check the filters for blockage. Clean or replace, or remove the obstruction.
- Check ductwork for blockage. Verify all registers are fully open.
- Verify ductwork is appropriately sized. Resize or replace if needed.
- Check the return side too, closed dampers and blocked returns count.
- Measure temperature rise and compare to the rating plate.
- Run a call for high stage after the fix and confirm it stays there.
Applies to: 34.5 in chassis gas furnaces
More furnace guides
- Alternate combustion air opening (upflow or horizontal)
- Alternate vent/flue location (upflow or horizontal left)
- Annual inspection checklist
- Auxiliary limit control (location and purpose)
- Auxiliary limit switch open (blower compartment)
- Blower motor does not have enough information, fails to start 40 consecutive times
Take it to the job
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Source: Goodman GMVC96 Service and Troubleshooting, p. 63. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.