Poor combustion / flame problems - Goodman/Amana GM9C96

What is going on

Burner flame is unstable, lifting, yellow or the furnace has repeated flame or pressure switch problems with no single clear code. The manual lists the main venting, drainage and burner factors to check.

Likely causes

• Venting too long, sagging or with too many tight elbows; flue gas recirculating into the intake
• Furnace holding condensate (bad drain hose connections, dirty trap, wrong slope, A/C coil drain interfering)
• Wrong manifold pressure or orifices for the gas type
• Leaking inducer-to-collector gasket or burr in the collector box orifice
• Burners dirty or misaligned; heat exchanger orifice plate loose or hanging
• Gas line binding the burner assembly
• Not enough combustion air (single-pipe installs) or cabinet air leaks

Check in this order

1. Vent keep it as short as possible, use 45 degree or long-radius fittings, no sags that trap condensate; if 3 in. pipe is used, transition from 2 in. as close to the furnace as possible; make sure exhaust is not re-entering the intake.
2. Drainage check drain hose connections, clean the condensate trap, confirm the furnace is level end to end with about 3/4 in. tilt toward the front, and that the A/C coil drain is not interfering.
3. Set manifold pressure for the gas in use at both low and high fire; if converted to LP, confirm every orifice size.
4. Remove the inducer and check the gasket between inducer and collector box cover; check the collector box orifice hole is free of burrs on both sides.
5. Check burners are clean, seated and lined up with the heat exchanger tubes, and that the orifice plate between burners and heat exchanger is tight and not missing a screw.
6. Make sure the field gas line is not pushing on and distorting the burner assembly.
7. Single-pipe installs confirm the space provides enough combustion air; seal cabinet air leaks that let supply air affect combustion.
8. If heat exchanger integrity is in doubt, follow the manufacturer's heat exchanger inspection procedure (Service Bulletin SF-041).

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