gregh

This white residue is nothing uncommon or out of the ordinary whatsoever. It does NOT indicate IMPROPER VENTING at all PERIOD. As other members have stated, it is what is left behind after the condensate has evaporated. Used to see it form in new furnaces back in the 1980s. Any appliance that uses a draft inducer fan like the one in the photo above, will have an afue of 80% and does not have a draft hood to introduce dilution into the venting system. This means the flue gasses will contain a higher concentration of water vapor which tend to condense inside the vent until the vent gets up to temperature. In short, any appliance which uses an inducer fan will eventually develop this white powder inside the vent connector and will show up around the fittings just like in the photo above. Older furnaces with standing pilot used a draft hood to assist venting, this introduced an equal volume of air to flue gas thus less humidity going through the vent connector, we would still see a bit of this white residue over time. So..... this residue is NORMAL for the type of heater in question. I would recommend replacing the single wall vent connector with b vent though, as the original vent is likely to corrode through eventually as the condensate and residue is highly corrosive.