White powdery substance

1998 Rheem furnace installed in garage. What is the white stuff and/or how is it formed? A form of efflorescence perhaps? Additionally, there was some on the B-vent joints in the attic. Any help/insight is appreciated.


On the vent pipe it appears at the seams of the elbows and appears to be concentrated in one area. Is it possible that there was an external factor? eg Could the stuff have fallen, landed, or dripped in while other work was being done. The pattern in the furnace looks the same as what you would find on the floor if you cut out a piece of wallboard to install another outlet…dust picked up by the exhaust vent and allowed to exit at the seam of the elbow. The elbow shows signs of corrosion. The seams show signs of corrosion, pinholes through which the dust/material coould exit???
I am not a HVAC guy, this is just a SWAG.

On the flue, I’m guessing this is what corrosion looks like on galvanized metal. I assume you referred it to a specialist because it’s too close to the wood.

Corrosion… most likely caused by wind driven rain leakage past the rain cap. In the case of extremely low flue gas temps, moisture could be from condensation in the flue.

Flue gases need to be at a high enough temperature to prevent the condensation that is taking place in this flue vent.What you see are the mineral deposits left after the water evaporates. The moisture will cause premature failure of the flue vent & possible damage the vent motor & cause the reference tube on the inducer housing to plug up with minerals. If the unit is not operating in the top 2/3 of the temperature rise chart found in unit you can try lower blower speeds & check gas supply pressure.

Condensation from condensing type furnace. HVAC video training class for home inpsectors on NACHI.TV it is a outstanding class.

Excellent post - Thanks:)

Additional information about a possible different cause of this conditionhere.