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White Powdery Substance


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Hey all,

This is my first inspection...ok I'm not really up and running yet but I will be soon.

I'm looking to purchase this duplex so I did the inspection myself.

When I inspected the gas heater, I found a white powder like substance in the unit itself. It seemed to be coming from where the blower housing is connected.

It also seemed to be corroded inside the flue itself. There was white corrosion leaking from the joints in the exhaust flue.

The heaters in both units had the same situation. The heaters are about 10 years old.

Any suggestions?

I've got photos, however they would not load when I attmepted to link them to my message.



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Originally posted by hausdok

Flue gases are acidic, so if they cool too rapidly they will condense inside the flue and drain back to the furnace and into the inducer housing where they normally show up at the bottom as mineral salts as the acid leaks out the bottom of the housing.

It usually indicates poor exhaust buoyancy caused by gases that are cooling too rapidly. Sometimes it can be so bad that it rusts out a flue, like the photo posted earlier today by someone else on another thread.

Poor exhaust bouyancy? Ooooooo...Mike's been reading again. [:D]

I once heard a CO guru claim that anytime you found heavy rust in a heat exchanger it was a sure sign of poor venting. Any thoughts?

Brian G.

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In addition to what Mike said, it can also be caused by a furnace that short-cycles.

When the furnace first starts up, the vent is cold and condensation forms in it. If the furnace runs long enough, the vent warms up, the condensation re-evaporates and everything's fine.

However, if the cycle's cut short then the condensation forms over and over again without having enough time to evaporate.

Short cycling can be caused by a whole bunch of things.

- Jim Katen, Oregon

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Great information. I will be starting my home inspection business in March, and finding this site has been an incredible help. It can feel a little daunting to knock on that door for your first inspection, but with the help of all of you on this forum, I do feel slightly more competent. (or at least more informed)

I am currently reading ALL old posts, going through them and making notes as I go.

As far as the acidic build up on the heaters goes, I will ask the seller to have that looked at prior to closing.

Thanks Guys...

Rob Renfro

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