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Energy Saver: Furnace, Boiler & Woodstove Combustion
(continued ...) and the equipment itself. If you ask, sometimes a Building Department representative will visit your house for free to assess your situation. When you know what needs to be done, use rigid metal HVAC duct you can buy at your local home center. If you are starting with no connection to outside and going to some connection to outside, I would think that you cannot get the size wrong. However, for most modest size residential houses, I would expect the answer from the Building Department official to be two 8 to 12 inch (20 to 30 cm) diameter openings or ducts.
This does not apply well to a simple wood stove or equipment without electronic controls. For most other heat making equipment with electronic controls, this does apply *1. Install two louvers in the nearest outside wall. Louvers usually have rain excluding devices and bird/rodent screening. This means the louver area usually needs to be two times larger than the duct area. For our 8 to 12 inch duct example, that would be a louver area of 100 to 226 square inches (628 to 1400 square cm). So, the square louver size is then 10 x 10 inches (25 x 25 cm) to 15 x 15 inches (38 x 38 cm). Transition from the square louver to the round duct size indicated. Route the two ducts into the room with your boiler or furnace.
Some current Building Codes require specific things about where these ducts are to be terminated (namely distance from floor and ceiling). But since this should not be a Code required upgrade, you do not necessarily need to follow those requirements but it is up to you. I recommend that you terminate the first duct as close to the burner air intake as possible without obstructing service access. Terminate the second duct near the draft hood intake on the flue. Keep in mind ...