HVAC Commercial HVAC Systems 5 — Questions and Answers
Question 1: What is 'static pressure' in the context of a commercial duct system, and why does it matter?
- The weight of ductwork pressing on supports
- The pressure of air in the duct perpendicular to flow, used to overcome resistance and size fans and ducts (Correct answer)
- The pressure difference between supply and return air temperatures
- The maximum refrigerant pressure in the system
Correct answer: The pressure of air in the duct perpendicular to flow, used to overcome resistance and size fans and ducts
Static pressure is the outward force of air on duct walls and is critical for sizing fans and duct systems to ensure adequate airflow against system resistance.
Question 2: In a commercial Variable Refrigerant Flow (VRF) system, what distinguishes a 'heat recovery' system from a 'heat pump' system?
- Heat recovery uses a cooling tower; heat pump is air-cooled
- Heat recovery allows simultaneous heating and cooling in different zones; heat pump can only heat or cool the entire system at once (Correct answer)
- Heat recovery uses two compressors; heat pump uses one
- Heat recovery operates on R-410A; heat pump on R-22
Correct answer: Heat recovery allows simultaneous heating and cooling in different zones; heat pump can only heat or cool the entire system at once
VRF heat recovery systems use a branch circuit controller to simultaneously deliver heating refrigerant to some indoor units and cooling to others, maximizing efficiency.
Question 3: Under ASHRAE 62.1, what is 'ventilation effectiveness' and how does it affect outdoor air calculations?
- A measure of filter efficiency for particulates
- A factor accounting for how well supply air mixes with zone air; poor mixing requires more outdoor air (Correct answer)
- The ratio of supply fan energy to total HVAC energy
- The percentage of time outdoor air dampers are open
Correct answer: A factor accounting for how well supply air mixes with zone air; poor mixing requires more outdoor air
ASHRAE 62.1 uses ventilation effectiveness (Ez) to adjust outdoor air quantities — ceiling diffusers typically achieve Ez=1.0, while floor diffusers may require more OA if Ez<1.
Question 4: What is the role of a 'building pressure differential' control strategy in a commercial HVAC system?
- Controlling the pressure difference across chiller tubes
- Maintaining slight positive building pressure relative to outdoors to prevent infiltration of unconditioned or contaminated air (Correct answer)
- Balancing pressure between floors of a high-rise
- Equalizing refrigerant pressure during off-cycle
Correct answer: Maintaining slight positive building pressure relative to outdoors to prevent infiltration of unconditioned or contaminated air
Slightly positive building pressure prevents unconditioned outdoor air, pollutants, and moisture from infiltrating through cracks and openings in the building envelope.
Question 5: What does 'integrated part load value (IPLV)' measure for commercial chillers?
- Maximum chiller capacity at design conditions
- Weighted average efficiency across multiple part-load operating points, reflecting real-world performance better than full-load ratings (Correct answer)
- The chiller's minimum operable capacity percentage
- Annual energy cost at full-load conditions
Correct answer: Weighted average efficiency across multiple part-load operating points, reflecting real-world performance better than full-load ratings
IPLV weights chiller efficiency at 100%, 75%, 50%, and 25% load to reflect actual seasonal performance, since chillers rarely operate at full design load.
Question 6: In commercial HVAC commissioning, what is a 'functional performance test (FPT)'?
- A visual inspection of equipment installation quality
- A dynamic test verifying that HVAC components and systems operate correctly through all modes and sequences of operation (Correct answer)
- An energy audit comparing actual vs. design energy use
- A test of emergency shutoff and life-safety systems only
Correct answer: A dynamic test verifying that HVAC components and systems operate correctly through all modes and sequences of operation
FPTs simulate all operating modes (occupied, unoccupied, economizer, heating, cooling, alarms) to verify the system performs as designed before occupancy.
Question 7: A commercial building's HVAC design specifies a supply air temperature of 55°F. If the return air is 75°F, what is the temperature difference (ΔT) used in airflow calculations?
- 10°F
- 15°F
- 20°F (Correct answer)
- 30°F
Correct answer: 20°F
The supply-to-return temperature difference (ΔT) is 75°F − 55°F = 20°F, which is used in the sensible heat formula Q = 1.08 × CFM × ΔT to size airflow.
HVAC Commercial HVAC Systems 5
What is 'static pressure' in the context of a commercial duct system, and why does it matter?