Copied from this site, not my words, but maybe this is the issue. Also pasted the link to the site, seems to have some good information.
“A source of fresh air to replace the exhausted air.” Replacement fresh air is often overlooked by the do-it-yourselfer. The rule of thumb for replacement air is to provide fresh air at the same rate that combustion air is being exhausted. This means that if you are exhausting 400 CFM (cubic feet per minute) of combustion contaminated air, you also have to provide incoming fresh air at the rate of 400 CFM.
http://mikeaurelius.wordpress.com/20...basics-part-1/