CSLB C-20 HVAC ContractorHeating SystemsMedium
A 90% AFUE Category I natural-draft furnace is replaced with new equipment in the same location. A technician notes the old vent connector had a barometric draft damper installed in the vent pipe. What is the primary function of this component?
- ATo prevent condensation from forming inside the vent pipe
- BTo regulate draft by admitting room air to stabilize vent pressure
- CTo increase the furnace's combustion efficiency by adding excess air
- DTo filter particulates from the flue gas before venting
Show answer & explanationAnswer & explanation
Correct answer: B. To regulate draft by admitting room air to stabilize vent pressure
A barometric draft damper (draft regulator) hinges open to admit dilution room air into the vent connector, stabilizing draft under varying conditions such as wind or chimney height changes. This prevents excessive draft that could pull too much heat from the heat exchanger or cause flame disturbance, while also diluting flue gases before they enter the chimney.
Why the other options are wrong
- A. Condensation control on Category I vents relies on proper sizing and materials, not the damper.
- C. Combustion air for burning fuel is separate from the vent-side dilution air the damper admits.
- D. Barometric dampers do not filter particulates; they only regulate draft pressure.
Barometric Draft Damper
A hinged damper installed in a Category I vent connector that admits room air to regulate and stabilize draft pressure in the venting system.
- Used only on natural-draft, negative-pressure (Category I) appliances
- Swings open under excess draft to admit dilution air
- Not used on induced-draft or condensing (Category III/IV) systems
Memory trick: Swinging door lets air in to calm the draft.