Combustion Appliance Venting and Draft
Vent categories
- Category I: negative pressure, non-condensing; B-vent or lined masonry.
- Category II: negative pressure, condensing.
- Category III: positive pressure, non-condensing.
- Category IV: positive pressure, condensing; PVC or CPVC.
Draft rules
- Slope vent connectors upward toward the chimney at least 1/4 in. per foot.
- Single-wall connector: 6 in. clearance to combustibles. B-vent: 1 in.
- Termination: at least 2 ft above any roof surface within 10 ft horizontally, and 3 ft above the roof penetration.
- Natural-draft overfire draft typically runs -0.02 to -0.04 in. w.c.
- A draft hood admits dilution air and caps draft; a spillage test at the hood confirms the vent can carry the products.
Worked example
A 40,000 Btu/h water heater shares a 6-in. masonry flue with an 80,000 Btu/h natural-draft furnace. The furnace is replaced with a 90% Category IV unit vented in PVC, orphaning the water heater in a flue sized for 120,000 Btu/h. Flue gases cool, draft falls to -0.008 in. w.c., and the hood spills. The fix is a correctly sized liner or a power-vented replacement, not a larger draft hood.
Traps
- Reading draft at the wrong point or before the appliance stabilizes.
- Ignoring depressurization from bath fans, dryers, and range hoods.
- Assuming any negative number is adequate draft.
- Venting a condensing appliance into an unlined exterior masonry chimney.
- Forgetting that a common vent must be sized for the appliances still connected to it.