For three winters now the chimney that vents my hot water heater has been developing a back draft of cold air when the temperature drops to the mid 20's or colder. This blows out my pilot light. We've been in the house for over 10 years and the only thing that has changed, that I'm aware of, is the addition of glass block windows in the basement (basement is sealed up tighter now?). What's going on or more importantly, what's the fix for this situation? It seems unlikely that any kind of baffle in the flu will be allowed by code or would even be safe. Is there any type of chimney cap that would prevent this back draft from setting up? Do I need to change to direct venting? Any help or coments are appreciated.