308A HVAC Ductwork Systems 2 — Questions and Answers
Question 1: What is the equal friction method of duct sizing?
- Making all ducts the same size
- A design method that maintains a constant friction loss rate per unit of duct length throughout the system (Correct answer)
- Using equal numbers of fittings in each branch
- Installing equal amounts of insulation on all ducts
Correct answer: A design method that maintains a constant friction loss rate per unit of duct length throughout the system
The equal friction method sizes each duct section to maintain the same friction loss per unit length (typically inches of water gauge per 100 feet). This helps balance the system naturally.
Question 2: What is the function of a fire damper in a duct system?
- To regulate temperature
- To automatically close and prevent the spread of fire and smoke through duct penetrations in fire-rated walls and floors (Correct answer)
- To control airflow volume
- To reduce noise
Correct answer: To automatically close and prevent the spread of fire and smoke through duct penetrations in fire-rated walls and floors
Fire dampers are installed where ducts penetrate fire-rated walls and floors. They contain fusible links that melt at a specific temperature, causing the damper to close and maintain the fire barrier.
Question 3: What is the recommended maximum air velocity in a main supply duct for a commercial HVAC system to minimize noise?
- 500 FPM
- 1,200 to 1,500 FPM (Correct answer)
- 3,000 FPM
- 5,000 FPM
Correct answer: 1,200 to 1,500 FPM
Main supply ducts in commercial systems are typically designed for 1,200 to 1,500 FPM to balance energy efficiency with acceptable noise levels. Higher velocities increase noise and friction losses.
Question 4: What is the purpose of a balancing damper in an HVAC duct system?
- To prevent backdraft
- To adjust airflow to individual branches or outlets to achieve the designed air distribution (Correct answer)
- To filter the air
- To reduce vibration
Correct answer: To adjust airflow to individual branches or outlets to achieve the designed air distribution
Balancing dampers are adjustable dampers installed in branch ducts to regulate airflow. They allow technicians to fine-tune the air distribution so each zone receives its designed volume of air.
Question 5: What is a 'flexible duct' commonly used for in HVAC systems?
- As the main trunk line
- To connect rigid duct branches to diffusers and registers in locations where rigid duct is impractical (Correct answer)
- As exhaust duct for kitchen hoods
- As underground ductwork
Correct answer: To connect rigid duct branches to diffusers and registers in locations where rigid duct is impractical
Flexible duct is primarily used for short final connections between rigid branch ducts and supply outlets (diffusers/registers). It accommodates misalignment and vibration but should be kept short and straight.
Question 6: What SMACNA duct pressure class is typically specified for a standard commercial supply air system?
- 1/2 inch WG
- 2 inch WG (Correct answer)
- 6 inch WG
- 10 inch WG
Correct answer: 2 inch WG
Standard commercial supply air systems typically operate at 2 inch water gauge (WG) pressure class. This determines the construction standards including gauge, reinforcement, and sealing requirements.
308A HVAC Ductwork Systems 2
What is the equal friction method of duct sizing?