My Heil forced air furnace (about 15 yrs old)shows some "white stuff" dripping down. The stuff is dried white and crusty, although at some point, it must have been liquid or at least a slurry for it to drip.

The furnace has a combustion blower that draws heat over a serpentine heat exchanger and up the flue. The crusty stuff is coming from a double wall 4" flue connected directly above the blower exit at the top of furnace. A drip path is seen down to the blower housing, then to a metal shelf below. The white stuff doesn't appear to hurting anything and hasn't dripped on anything electrical. Other than some faint dust, it vacuumed up.

A friend told me this was a by-product of combustion. I have never seen this before and have always keep the furnace clean, vacuuming up dust, etc.

What is this "stuff" and is it any cause for alarm? The furnace has a 4" flue feeding at an angle into a 6" flue which it shares with a 3" flue coming from the hot water heater. I do not see this white stuff anywhere else than at the junction of the angled flue to the pipe exiting above the exhaust blower.

I don't think this is pertinent but I'll add it just for the heck of if. Temperatures in MI have averaged about 34 degrees. We also have a wood stove in another part of the house (family room) with it's own class A chimney. Proximity of the wood stove chimney is at least 30 ft from the roof exit of the furnace flue.

We have been burning wood for the last couple months. When we burn, warm air from the family room migrates to the living room where the thermostat is located. It greatly reduces the gas furnace from kicking on which is the intent and we have done this for the past 15 yrs.

Another bit of information which i don't think is pertinent either but will add. Three years ago the combustion blower was making noise (bearing) and I replaced it myself. I had trouble keeping the gasket mated with the plenum and used high temp silicone to keep it in place and make a seal.

The furnace operates fine and has a 20 yr warranty on the heat exchanger so hopefully, i have a few more years of operation.