Furnace in a 72-Cubic-Foot Closet: Combustion Air, Venting, and a Marginal Gas Line
You are replacing a 100,000 Btu/h natural gas furnace in a 1993 house. The furnace sits in a closet 3 ft × 3 ft × 8 ft - 72 ft³ - with a solid door and no outside-air duct. The existing B-vent rises 6 ft through the closet, turns, and runs 12 ft through an unconditioned attic, ending 14 in. above the roof surface and 2 ft from a bedroom window that opens. The gas line is 1/2-inch black iron from the meter, equivalent length 35 ft, serving only this furnace, with no sediment trap and no shutoff valve in the closet. You plan to install a 100,000 Btu/h, 80% AFUE Category I furnace. The jurisdiction requires a mechanical permit and inspection.
1. Is this closet a confined space, and what combustion air does the new furnace need?
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Run the volume test first. The familiar rule is 50 ft³ of volume per 1,000 Btu/h of total input for the infiltration method. At 100,000 Btu/h that calls for 5,000 ft³. The closet is 72 ft³. It is unquestionably a confined space, and the solid door and lack of an outside-air duct mean the old installation has been starving the appliance - which is a likely contributor to any spillage or flame rollout history you should look for before you start.
For a confined space you need two permanent openings, one near the top and one near the bottom, connecting to a space of adequate volume. Using indoor air, each opening is sized at 1 in² of free area per 1,000 Btu/h of total input, so 100 in² each. Using outdoor air through two openings, the allowance is more generous - roughly 1 in² per 4,000 Btu/h, or about 50 in² each - and a single outdoor-air opening through a duct is about 1 in² per 3,000 Btu/h. Whatever you choose, count free area, not rough opening: a wood louvered door gives up about 75% of its face area to the blades, so a louvered door is almost never the answer for a 100,000 Btu/h appliance in a 72 ft³ closet.
The clean solution is two properly sized openings to the adjacent conditioned space with a documented free area, or a direct outdoor-air duct. Verify against the adopted Oregon Mechanical Specialty Code edition, and pull the mechanical permit so the inspector sees the calculation.
2. Identify the vent termination violations and state the corrections.
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Two clear deficiencies. The termination is 14 in. above the roof surface, but a listed B-vent serving a Category I appliance must terminate at least 2 ft above the highest point of the roof within a 10 ft horizontal radius. It also must clear the highest connected appliance draft hood by a substantial margin - typically 5 ft - so you need to check that dimension against the new furnace outlet, not the old one.
The window is the second problem. A vent termination must be at least 4 ft horizontally from, or sufficiently above or below, any door, openable window, or gravity air inlet, because flue gas will migrate into that bedroom under the right wind and pressure conditions. Two feet is a violation, and no inspector will pass it.
The corrections: extend the vent so it meets the 2 ft above roof and 5 ft above draft hood dimensions, and reroute or raise the termination so it is at least 4 ft from the bedroom window or clearly above it. Also check the B-vent's 1-inch clearance to combustibles the whole way through the attic, confirm the vent diameter is listed for 100,000 Btu/h at that height and connector rise, and insulate or reroute the long run through the unconditioned attic to limit flue-gas condensation. Confirm the appliance is a Category I fan-assisted unit and that the vent is a listed match.
3. Will the existing gas line serve the new furnace, and what corrections are needed on the piping?
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Do the capacity check, not a visual. In the standard natural gas table for 0.60 specific gravity and a 0.5 in. w.c. pressure drop, 1/2-inch Schedule 40 black iron at 35 ft of equivalent length carries roughly 85,000–90,000 Btu/h. The furnace needs 100,000. The line is undersized, and adding the fittings, the new sediment trap, and a shutoff valve only increases equivalent length.
Upsize the run to 3/4-inch black iron from the meter, which carries well over 150,000 Btu/h at that length, and recompute the equivalent length including all fittings and the sediment trap. Confirm the meter and service can deliver the total connected load of the house, not just this appliance.
Then correct the installation details: a sediment trap (drip leg) at the appliance, a full-port manual shutoff valve in the same room and close to the furnace, a union for service, proper support and protection against physical damage, and identification of the shutoff. Pressure-test the new piping and document it. Because this is a gas piping and venting change inside a mechanical permit, sequence the work so the rough inspection happens before anything is concealed, and leave the combustion air calculation and the vent listing on site for the inspector.