I have a standard efficiency gas furnace that is nearly 20 years old.
The furnace has a regular exhaust vent that goes to the roof. There is an open 12-inch duct near the furnace which I’m guessing is combustion air supply from somewhere up above, probably the attic.
This is a 2-story house and the furnace is in the basement. There are loose gritty grains on the furnace under the exhaust vent connection. These are apparently forming on the inside of the exhaust, because tapping on the exhaust vent causes them to break loose. Is this a reason for concern?
It won’t be too long until we replace this furnace. If I want to upgrade to a high-efficiency model, what modifications will be required to the exhaust vent? There does not appear to be a good place to exhaust to an exterior wall.