Venting an oil furnace

As a general rule, chimney codes restrict venting appliances that use different fuel types into the same chimney liner. There are some exceptions, but they are fairly specific, and unfortunately there are also installations where the rules were simply not followed.

What you have could be one of those exceptions, or it could be a situation where the system was not installed correctly when the original furnace was put in, and the issue is only becoming apparent now that the new furnace has been installed.

Since you mentioned that you are smelling fuel-oil odors from the basement on the second floor, a couple of possibilities come to mind.

One possibility is that there is damage or a defect in the chimney tile liner around the second-floor level, allowing flue gases to leak into the house at that point.

Another, and in many cases more likely, possibility is that the second-floor furnace is pulling basement air into its return ductwork, and that air may contain oil burner fumes from the basement. If that’s happening, the furnace would then distribute those odors to the second floor through the supply ducts.

If you can, please post some photos of the new furnace, the other furnaces, and how they are vented into the chimney. It would also help to see how the ductwork and return air connections are arranged.

That will make it much easier to narrow down what’s going on.

And while you are at it in getting us more information, determine whether the appliances are venting into the same flue, or into the same chimney but with different flues…