HVAC Ventilation Design 5 — Questions and Answers
Question 1: What is the primary design consideration when locating outdoor air intakes for a ventilation system?
- Proximity to return air grilles for short-circuit prevention
- Distance from contamination sources such as exhaust outlets and loading docks (Correct answer)
- Height above ground to maximize wind-driven flow
- Orientation toward prevailing winds
Correct answer: Distance from contamination sources such as exhaust outlets and loading docks
Outdoor air intakes must be located away from sources of contamination (exhaust fans, cooling towers, loading docks) to prevent introducing pollutants into the building.
Question 2: In a variable air volume (VAV) system, what happens to the outdoor air fraction when supply airflow decreases at part-load conditions?
- The outdoor air fraction stays constant
- The outdoor air fraction decreases proportionally
- The outdoor air fraction increases if outdoor air is fixed in volume (Correct answer)
- The outdoor air fraction is controlled independently of supply flow
Correct answer: The outdoor air fraction increases if outdoor air is fixed in volume
If the outdoor air volume is fixed (not reset), reducing total supply airflow at part load increases the outdoor air fraction, which can cause overcooling or excess humidity issues.
Question 3: Which document provides the minimum ventilation requirements adopted by most US state and local building codes for commercial buildings?
- SMACNA HVAC Systems and Equipment
- ASHRAE 62.1 (Correct answer)
- ACCA Manual Q
- NFPA 90A
Correct answer: ASHRAE 62.1
ASHRAE 62.1 'Ventilation for Acceptable Indoor Air Quality' is the industry standard referenced by the International Mechanical Code and adopted by most US jurisdictions.
Question 4: A laboratory with high concentrations of hazardous chemicals requires which minimum number of air changes per hour per ASHRAE 62.1 guidelines?
- 4 ACH
- 6 ACH
- 10 ACH
- The rate is determined by contaminant analysis, not a fixed minimum (Correct answer)
Correct answer: The rate is determined by contaminant analysis, not a fixed minimum
Laboratory ventilation must be designed based on the specific hazards present using industrial hygiene principles; ASHRAE 62.1 does not set a single fixed ACH for all labs.
Question 5: What is 'short-circuiting' in the context of ventilation air distribution?
- When supply air bypasses the occupied zone and goes directly to the return (Correct answer)
- When exhaust air is drawn back into the outdoor air intake
- When a VAV box closes completely and stops airflow
- When ductwork leaks air into the ceiling plenum
Correct answer: When supply air bypasses the occupied zone and goes directly to the return
Short-circuiting occurs when supply air travels directly to the return without adequately mixing with or traversing the occupied breathing zone, reducing effective ventilation.
Question 6: According to ASHRAE 62.1, what is the minimum exhaust rate for a men's restroom with 10 water closets in a commercial building?
- 50 cfm total
- 75 cfm per water closet
- 50 cfm per water closet (Correct answer)
- The rate is based on cfm per sq ft, not fixtures
Correct answer: 50 cfm per water closet
ASHRAE 62.1 Table 6-4 specifies 50 cfm per water closet or urinal for toilet rooms to control odors at the source.
Question 7: Which type of ventilation system is most energy-efficient for a school gymnasium that is occupied only during daytime hours?
- Constant volume system with fixed outdoor air
- Dedicated outdoor air system (DOAS) with energy recovery (Correct answer)
- 100% outdoor air system without heat recovery
- Recirculating system with HEPA filtration
Correct answer: Dedicated outdoor air system (DOAS) with energy recovery
A DOAS with energy recovery ventilates precisely to code minimums while recovering energy from exhaust air, making it highly efficient for spaces with variable and intermittent occupancy.
HVAC Ventilation Design 5
What is the primary design consideration when locating outdoor air intakes for a ventilation system?