308A HVAC Ductwork Systems 4 — Questions and Answers
Question 1: What is a 'VAV' (Variable Air Volume) box and what does it do?
- A type of exhaust fan
- A terminal unit that regulates the volume of conditioned air delivered to a zone based on temperature demand (Correct answer)
- A ventilation filter
- A vibration dampener
Correct answer: A terminal unit that regulates the volume of conditioned air delivered to a zone based on temperature demand
A VAV box is a terminal unit with a motorized damper controlled by a zone thermostat. It varies the volume of supply air to maintain the desired temperature in individual zones.
Question 2: What is the purpose of a 'backdraft damper' on an exhaust duct?
- To increase exhaust velocity
- To prevent outside air from entering the building through the exhaust opening when the fan is off (Correct answer)
- To filter exhaust air
- To reduce noise from the fan
Correct answer: To prevent outside air from entering the building through the exhaust opening when the fan is off
A backdraft damper prevents wind-driven rain and unconditioned outside air from entering the building through the exhaust duct when the exhaust fan is not running. It opens only when airflow pushes it.
Question 3: What is the standard material used for kitchen exhaust hood ductwork and why?
- PVC plastic for corrosion resistance
- Minimum 16-gauge welded carbon steel or stainless steel for fire protection and grease containment (Correct answer)
- Fiberglass for insulation
- Aluminum for light weight
Correct answer: Minimum 16-gauge welded carbon steel or stainless steel for fire protection and grease containment
Kitchen exhaust ductwork must be welded carbon steel (minimum 16 gauge) or stainless steel per fire codes. This prevents grease-laden air from leaking through seams and withstands the heat from potential grease fires.
Question 4: What does 'CFM' stand for and what does it measure in HVAC?
- Celsius Fahrenheit Metric — temperature scale
- Cubic Feet per Minute — the volume of air flowing through the system (Correct answer)
- Central Fan Motor — the main blower unit
- Copper Fitting Measurement — pipe sizing
Correct answer: Cubic Feet per Minute — the volume of air flowing through the system
CFM (Cubic Feet per Minute) is the standard unit for measuring airflow volume in HVAC systems. It quantifies the amount of air being moved by fans or delivered to spaces.
Question 5: What is 'duct leakage testing' and why is it important?
- Testing ducts for water leaks
- Pressurizing the duct system and measuring air leakage to verify it meets specified tightness standards (Correct answer)
- Testing the structural strength of ducts
- Checking for asbestos contamination
Correct answer: Pressurizing the duct system and measuring air leakage to verify it meets specified tightness standards
Duct leakage testing pressurizes the sealed duct system and measures how much air escapes. Excessive leakage wastes energy, reduces system performance, and can cause comfort and air quality problems.
Question 6: What is the purpose of 'access doors' in HVAC ductwork?
- To allow air into the duct
- To provide access for inspection, cleaning, and maintenance of internal duct components like dampers and coils (Correct answer)
- To let light into the duct
- To serve as emergency exits
Correct answer: To provide access for inspection, cleaning, and maintenance of internal duct components like dampers and coils
Access doors are installed at strategic locations in ductwork to allow maintenance personnel to inspect, clean, and service internal components such as fire dampers, turning vanes, and coils.
308A HVAC Ductwork Systems 4
What is a 'VAV' (Variable Air Volume) box and what does it do?