Trim it to 4 short sentences. I don't think normal folks would follow the detailed explanation or read past your 3rd sentence. "I'm a home home inspector and notice stuff that's wrong with houses. When I walk my dog by your house, it's obvious your furnace exhaust is really messed up and it could be dangerous to you and your family. At the very least, it could ruin your expensive furnace. I strongly suggest you have an experienced heating contractor look at it right away and get it fixed. Call me if you or your heating contractor have any questions. Regards, Your concerned neighbor, Richard the inspector".