HVAC Commercial HVAC Systems 3 — Questions and Answers
Question 1: What is the main advantage of a water-cooled chiller compared to an air-cooled chiller?
- Lower installation cost
- Higher efficiency (lower kW/ton) due to lower condensing temperatures (Correct answer)
- No need for refrigerant
- Simpler maintenance requirements
Correct answer: Higher efficiency (lower kW/ton) due to lower condensing temperatures
Water-cooled chillers achieve higher efficiency because evaporative cooling in the cooling tower produces lower condensing temperatures than ambient air cooling.
Question 2: In a commercial HVAC system, what does 'DOAS' stand for and what is its primary purpose?
- Dual Output Air System — splits heating and cooling
- Dedicated Outdoor Air System — provides ventilation air separately from sensible cooling (Correct answer)
- Direct Outdoor Air Supply — bypasses air handlers in summer
- Distributed Output Air System — serves multiple zones from one unit
Correct answer: Dedicated Outdoor Air System — provides ventilation air separately from sensible cooling
A DOAS handles ventilation (outdoor air) requirements separately, allowing zone equipment to focus on sensible load control.
Question 3: What is the typical chilled water supply temperature range in a commercial central plant?
- 32–38°F
- 44–48°F (Correct answer)
- 55–60°F
- 65–70°F
Correct answer: 44–48°F
Commercial chilled water systems typically supply water at 44–48°F to provide adequate dehumidification and cooling capacity at coils.
Question 4: Which control strategy best describes 'demand-controlled ventilation (DCV)'?
- Supplying maximum outdoor air at all times for safety
- Adjusting outdoor air intake based on CO₂ levels as a proxy for occupancy (Correct answer)
- Controlling supply fan speed based on duct static pressure
- Modulating heating coil output based on return air temperature
Correct answer: Adjusting outdoor air intake based on CO₂ levels as a proxy for occupancy
DCV uses CO₂ sensors to modulate outdoor air dampers proportionally to actual occupancy, reducing ventilation energy waste in partially occupied spaces.
Question 5: In a commercial building automation system (BAS), what is a 'setback' schedule used for?
- Reducing chiller plant capacity during peak demand
- Allowing space temperature to drift to a less conditioned setpoint during unoccupied hours (Correct answer)
- Increasing outdoor air during occupied hours
- Resetting hot water supply temperature in summer
Correct answer: Allowing space temperature to drift to a less conditioned setpoint during unoccupied hours
Setback schedules let space temperatures rise (cooling) or fall (heating) to unoccupied setpoints during nights and weekends, saving energy.
Question 6: What is 'bypass factor' of a cooling coil?
- The percentage of airflow that bypasses the chiller
- The fraction of air passing through the coil that does not contact the coil surface (Correct answer)
- The ratio of return air to outdoor air in the mixed air stream
- The efficiency loss due to dirty filters
Correct answer: The fraction of air passing through the coil that does not contact the coil surface
Bypass factor represents the portion of air that passes through the coil without making thermal contact, reducing the coil's effective heat transfer.
Question 7: A commercial building uses a '4-pipe' fan coil unit system. What does this mean?
- Four separate refrigerant circuits per unit
- Separate chilled water and hot water supply/return pipes allowing simultaneous heating and cooling in different zones (Correct answer)
- Four stages of compressor capacity
- Four duct connections per unit
Correct answer: Separate chilled water and hot water supply/return pipes allowing simultaneous heating and cooling in different zones
A 4-pipe FCU system has dedicated chilled water and hot water circuits, enabling independent heating or cooling in each zone simultaneously.
HVAC Commercial HVAC Systems 3
What is the main advantage of a water-cooled chiller compared to an air-cooled chiller?