You should check your exhaust vents from your hot water tank to your chimney and inside your chimney wall for the chimney liner to see if it is not deteriorated or closed off for any reason... [1]if for any reason it's broken or deteriorated...you could end up with Carbon Monoxide poisioning....
Links:
[1] http://www.wisegeek.com/what-is-a-flue-pipe.htm