Restricted air inlets often produce a noticeable disruption of the flame and a change from blue to yellow. Another cause of insufficient primary air and incomplete combustion is excess gas flow to the burner. Excess gas flow seldom produces a noticeable disruption of the flame or change of color. Excess gas flow can be caused by excess gas pressure or by oversized gas orifices. Gas regulators can fail, go out of adjustment, not be correctly field-derated for altitude, or be adjusted upward by field technicians to increase heat output.
Excess heat is produced which can damage heat exchangers.
Excess combustion products are produced which can exceed capacity of heating appliance flue vents and vent systems. The flame can continue to burn blue, giving no obvious visual indication of the problem. To find and correct problems caused by excess gas flow it is important to check gas flow, gas pressure, orifice sizing, and carbon monoxide concentrations in combustion products. Air forced through holes by the circulating fan increases the production of CO by disrupting the flame. The air flow increases the amount of combustion products spilling into the living quarters by disrupting the air flow through the appliance vent flue.