Rust flakes and scaling are found inside a furnace burner compartment. What should the inspector consider?

Steel in a burner compartment rusts only if it has been wet, so rust flakes and scaling are a record of moisture that has been arriving repeatedly. Several sources are worth checking and each has a different significance. Condensation from flue gases backing into the compartment points to a venting or draft problem. Water from an air conditioning coil or a blocked condensate line above the furnace points to a drainage failure. A leak from plumbing overhead is simple but common. And a cracked heat exchanger can allow moisture-laden combustion products where they do not belong. The inspector's finding is that moisture has been present and that the source should be identified, because two of those causes are combustion-safety issues, and the rust itself may also be shedding into the burners and disturbing the flame pattern.

Rust and scaling inside a burner compartment mean water has been reaching steel that should stay dry. Identifying the source matters because several of the possibilities — flue gas condensation, a failing heat exchanger, condensate overflow — are safety concerns rather than cosmetic ones.

Corrosion inside a furnace cabinet indicates moisture intrusion, whose sources include flue gas condensation from poor draft, condensate from a cooling coil above, plumbing leaks and heat exchanger failure. Several of these are combustion-safety concerns requiring HVAC evaluation.

Rust Is a Water Record. Steel does not corrode from heat or from running — something wet has been arriving, repeatedly.

Look at the flame pattern too. Rust flakes falling into burners disturb combustion, and a disturbed flame is itself a reportable observation.

• •Attributing corrosion to operating mode or burner alignment
• •Reporting the rust without pursuing its source
• •Overlooking that rust debris can disturb the flame pattern