Venting, Combustion Air, and Gas Piping
Venting, Combustion Air, and Gas Piping
This domain tests whether you can select and install venting, provide combustion air, and size gas piping without creating spillage or CO hazards. Vent categories are defined by pressure and temperature, not by material alone.
Venting categories
- Category I: negative-pressure, non-condensing. Typically natural draft or induced draft, with a vertical vent and a slope of at least 1/4 inch per foot upward from the appliance toward the vent.
- Category II: negative-pressure, condensing.
- Category III: positive-pressure, non-condensing.
- Category IV: positive-pressure, condensing; commonly PVC or CPVC with a condensate trap.
Combustion air
For a confined space, the standard rule is 50 cubic feet of volume per 1,000 Btu/h input. If using two permanent openings to an unconfined space, each opening is 1 square inch per 1,000 Btu/h. With outdoor air through vertical ducts, each opening is 1 square inch per 4,000 Btu/h; horizontal ducts use 1 per 2,000 Btu/h.
Gas piping
Use approved pipe and fittings, a sediment trap at the appliance, a listed shutoff valve, and proper support. Size by the longest run and total connected load, keeping pressure drop within the table allowance. Test the piping before putting it in service and purge air safely.
Worked example
A 100,000 Btu/h furnace is in a confined closet using two openings to outdoor air. Each opening = 100,000 ÷ 1,000 = 100 square inches. If the same installation used vertical ducts to outdoor air, each opening = 100,000 ÷ 4,000 = 25 square inches. The smaller number is not automatically correct; the duct method must match the actual installation.
Traps
Do not vent a Category I furnace into PVC. Do not count a louvered door as free area without its rating. Do not install a sediment trap after the valve or omit the drip leg. Do not pressure-test with the appliance connected.