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Carrierman hit on the second most overlooked aspect.
You need dilution air for drafting as well as combustion air for any carbon-based fueled appliance.
In answer to your 'older home' question, the older homes were less air-tight in construction, so air was able to move from outside to inside or from floor to floor with ease, so the problem was not as serious as it can be in a more air-tight home. I would size my dilution and combustion air as though this house were air-tight in all cases. You can't have too much air available for efficient combustion and proper draft.
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"My clients’ wishes are the center of my attention." -- David Guido, a contractor in Woodstock, N.Y.
New York Times, July 20, 2006
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