There will be a cone-shaped hood there that admits extra air to the chimney to stabilize draft, and soot will show up around it if the appliance has been backdrafting significantly. Even if you find no signs that your furnace or water heater has been backdrafting, it's a good idea to perform the simple test described in the sidebar. If one or more of your combustion appliances is backdrafting for longer than about 30 seconds at start-up, you should fix the problem. As long as the eater is correctly tuned, the situation is not life threatening. If you ignore the problem, the performance of the burner will deteriorate, and the situation could well become deadly.
Experts take two approaches to solving backdrafting problems.
One method goes after the mechanism of backdrafting, the other eliminates sources. The potential for backdrafting can be controlled by balancing indoor and outdoor air pressure with a fan that pushes air into the house. Jim White points out that air will get into the building anyway, so it may be best to heat it and push it in intentionally. Source control, as advocated by Lstiburek and Nelson, is done by a procedure called aerodynamic uncoupling. Fortunately, this isn't as complex as it first sounds.