If you are having that much carbon buildup with a gas furnace then something is wrong with the mixture or possibly your flue needs cleaning/adjustment as it may be causing a puff-back due to improper draft or blockage.
If that is the case it may be more serious as to Carbon Monoxide buildup. I would have a pro look at for proper burn and ventilation IMO.
BTW Don't let my Fl location fool you, I am originally from NY and had a gas heating system for over 15 years
Last edited by R2D2; 02-07-2013 at 07:36..