CCS Ventilation & Draft Systems 3 — Questions and Answers
Question 1: A masonry chimney liner is found to be cracked and spalled. Which primary hazard does this pose to the ventilation system?
- Increased draft velocity
- Infiltration of combustion gases into living spaces (Correct answer)
- Reduced condensation in the flue
- Overfiring of connected appliances
Correct answer: Infiltration of combustion gases into living spaces
Cracks in the liner allow carbon monoxide and other combustion gases to leak through the chimney wall into the building structure or living spaces.
Question 2: What is 'wind-induced downdraft,' and which chimney feature most effectively prevents it?
- Draft caused by high interior pressure; prevented by a barometric damper
- Downward air movement caused by wind turbulence; prevented by a proper chimney cap design (Correct answer)
- Reverse flow from undersized flue; prevented by relining
- Stack effect reversal; prevented by exterior insulation
Correct answer: Downward air movement caused by wind turbulence; prevented by a proper chimney cap design
Wind-induced downdraft occurs when wind turbulence forces air down the chimney; a properly designed cap with wind-deflecting geometry redirects airflow to prevent this.
Question 3: When sizing a flue for a wood-burning fireplace, the cross-sectional area of the flue should be approximately what fraction of the fireplace opening area?
- 1/4 to 1/6
- 1/8 to 1/10
- 1/12 to 1/15 (Correct answer)
- 1/3 to 1/2
Correct answer: 1/12 to 1/15
For a wood-burning fireplace, the flue cross-sectional area should be approximately 1/12 to 1/15 of the fireplace opening area to maintain proper draft without oversizing.
Question 4: Which condition describes 'short cycling' in relation to chimney draft?
- Rapid alternation between positive and negative flue pressure
- Combustion gases cooling and falling before reaching the top of the chimney (Correct answer)
- An appliance turning on and off rapidly due to oversized output
- Wind repeatedly reversing draft direction at the chimney termination
Correct answer: Combustion gases cooling and falling before reaching the top of the chimney
Short cycling in draft context refers to cooled, heavy flue gases that lose buoyancy and descend before exiting, interrupting continuous upward flow.
Question 5: A gas appliance vent connector should not exceed what maximum horizontal run per NFPA 54 guidelines?
- 50% of the vertical height of the chimney or vent (Correct answer)
- 75% of the vertical height of the chimney or vent
- 100% of the vertical height of the chimney or vent
- 25% of the vertical height of the chimney or vent
Correct answer: 50% of the vertical height of the chimney or vent
NFPA 54 limits the horizontal vent connector run to no more than 75% of the vertical height; however, the common practical rule applied in many jurisdictions is no more than 50% for optimal draft.
Question 6: What effect does a tightly sealed, energy-efficient home have on chimney draft performance?
- It enhances draft by reducing turbulent air near the chimney
- It can depressurize the home, causing backdrafting (Correct answer)
- It has no measurable effect on draft
- It forces combustion gases upward more efficiently
Correct answer: It can depressurize the home, causing backdrafting
In tightly sealed homes, exhaust fans and HVAC systems can depressurize the building, causing combustion gases to be drawn back down the chimney rather than up.
Question 7: Which type of chimney liner is specifically recommended for high-efficiency oil-fired appliances with flue gas temperatures below 350°F?
- Aluminum Type B liner
- Stainless steel AL29-4C liner (Correct answer)
- Clay tile liner
- Cast-in-place pumice liner
Correct answer: Stainless steel AL29-4C liner
AL29-4C stainless steel is recommended for oil appliances producing low-temperature, condensate-generating flue gases due to its excellent acid and corrosion resistance.
CCS Ventilation & Draft Systems 3
A masonry chimney liner is found to be cracked and spalled.
Which primary hazard does this pose to the ventilation system?