CCP CCP Chimney Draft & Combustion Principles 1 — Questions and Answers
Question 1: What is the primary driving force behind chimney draft?
- Temperature differential between flue gases and outside air (Correct answer)
- Moisture content of the fuel
- Chimney height above ground
- Barometric pressure at sea level
Correct answer: Temperature differential between flue gases and outside air
Chimney draft is primarily driven by the temperature differential between hot flue gases and cooler outside air, creating a buoyancy-based pressure difference.
Question 2: What unit is typically used to measure chimney draft pressure?
- Pascals or inches of water column (Correct answer)
- Pounds per square inch (PSI)
- Decibels (dB)
- BTUs per hour
Correct answer: Pascals or inches of water column
Chimney draft pressure is measured in Pascals or inches of water column (in. WC) because draft is a very small pressure difference requiring sensitive units.
Question 3: Which condition is most likely to cause a chimney to backdraft?
- Negative indoor air pressure relative to outside (Correct answer)
- Excessive flue gas temperature
- Oversized chimney flue
- High outdoor wind speed
Correct answer: Negative indoor air pressure relative to outside
Backdrafting occurs when the home has negative indoor pressure — often from exhaust fans or tight construction — pulling flue gases back into the living space.
Question 4: What is the minimum natural draft typically required for proper residential appliance operation?
- 0.01 to 0.05 inches of water column (Correct answer)
- 0.5 to 1.0 inches of water column
- 2.0 to 5.0 inches of water column
- 10 to 15 inches of water column
Correct answer: 0.01 to 0.05 inches of water column
Most residential appliances require a minimum natural draft of 0.01 to 0.05 inches of water column for proper combustion and safe venting.
Question 5: How does chimney height affect draft performance?
- Taller chimneys generally produce stronger draft (Correct answer)
- Shorter chimneys always produce better draft
- Height has no effect on draft
- Taller chimneys only improve draft in winter
Correct answer: Taller chimneys generally produce stronger draft
Taller chimneys create a greater column of buoyant hot gases, increasing the pressure difference and strengthening natural draft.
Question 6: What effect does a cold chimney flue have on initial draft when an appliance is first lit?
- It reduces draft until the flue warms up (Correct answer)
- It immediately increases draft
- It has no effect on draft
- It permanently damages the appliance
Correct answer: It reduces draft until the flue warms up
A cold flue creates poor initial draft because the temperature differential between flue gases and the flue walls is insufficient to drive airflow upward.
CCP CCP Chimney Draft & Combustion Principles 1
What is the primary driving force behind chimney draft?