Venting and Draft
Vent categories, why draft matters, and the failures that put combustion products into the building.
Safety
What it is
Venting is the system that carries the products of combustion from the appliance to the outdoors. Draft is the pressure difference that makes it happen, whether created by buoyancy in a natural draft appliance or by a fan in an induced or forced draft appliance.
Vent categories
| Category | Vent pressure | Flue gas temperature | Typical material |
|---|---|---|---|
| I | Negative | Non condensing | Type B vent or lined masonry |
| II | Negative | Condensing | Corrosion resistant |
| III | Positive | Non condensing | Listed special gas vent |
| IV | Positive | Condensing | Listed plastic or stainless as the manufacturer specifies |
A condensing appliance produces acidic condensate in the vent, so the material, slope and drain provisions all matter. A positive pressure vent must be sealed to the manufacturer instructions because any leak pushes flue gas into the building.
What can go wrong
- Blocked vent terminal from a nest, snow, ice or debris.
- Disconnected or corroded vent sections, often at a joint above a furnace.
- An oversized or cold chimney that never establishes draft.
- Depressurization of the mechanical room by exhaust fans, a clothes dryer or a range hood, pulling flue gas back into the building.
- A common vent left serving a single appliance after another was removed or replaced with a condensing unit.
- Improper slope on a condensing vent, allowing condensate to pool.
Spillage is an immediate hazard
How to check it
Draft measured at the appliance after a few minutes of run time
Confirms the vent is actually moving products of combustion
Spillage test at the draft hood with the building in a worst case depressurization condition
Reveals problems that only appear when other exhaust equipment runs
Ambient carbon monoxide in the mechanical room and living space
Detects spillage that has already occurred
Vent connector slope, support, joints and clearance to combustibles
Mechanical defects are a common and easily missed cause
Terminal clearances and obstruction
Blocked or poorly located terminals cause recirculation and spillage
Local requirements
Vent sizing, materials, terminal clearances, common venting rules and permitted repairs vary by jurisdiction, adopted code edition and the manufacturer instructions for the specific appliance.
Local Code Notice
Symptoms
Practice & Mastery
- Questions1
- Venting and Draft Troubleshooting Scenarios2
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