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Energy Saver: Furnace, Boiler & Woodstove Combustion
(continued ...) sealed flue side. A third possibility is the most common in older residential systems. This third type is a completely open path into the burner and a flue with draft-hood for the flue gas exit. Residential equipment is usually one of the two later possibilities. They are referred to as gravity since there is reliance on the fact that hot-air-rises.
Today, most jurisdictions' Building Codes that I'm familiar with require openings or ducting to the outside. However, there are past Building Code versions that under certain conditions allowed for no specific outside opening. There was a past reliance on the inherent leakage a building had. But you are energy conscious and will be doing many other things that will plug up that leakage. In installations that comply with recent Building Codes, I would expect to find an outside air opening or duct, maybe two depending, running from the outside into the room with your combustion equipment. If you have this arrangement we are close to done. Dual openings are sometimes required at high and low locations into the room with combustion equipment.
Most locations do not require that you upgrade an installation to current Building Codes unless you are doing some related renovation. However, If your combustion air is not ducted in from outside, it should be to save energy. The energy savings is that you are not using heated indoor air for combustion. This is your first task *1. You can consult your local Building Department to find out what the current Building Code in your area requires. The opening size and arrangement is a function of the input Btu/hr listed on the equipment nameplate ...