Flue Condensation
I had an 80% 70k Payne gas induced draft single stage furnace installed two years ago, it replaced an old 80% 75k Lennox natural draft furnace . There is a gas water heater with a y pipe and both go into a shared 6" b-vent after a few feet.
It's fine when I am here and have the heat set around 70 degrees, but when I leave in Jan I turn the heat down to 55 and shut off the water heater, at times condensation will form in the b-vent and flow down the flue to the cap end of the flue and it's causing corrosion. The b-vent is about 25 ft and through an unheated attic and into a roof cap.
I measured the furnace air temp rise and it measures 60 which is at the upper end of the recommended range. I also measured the stack temperature at the furnace at 265. I have never observed any short cycling.
I called the company that put in the furnace, explained the situation and they gave me a chimney guy to call and mentioned insulating the flue. I know you can't wrap b-vent, so I am holding off calling him.
Any suggestions?
