HVAC System Design 5 — Questions and Answers
Question 1: In a VAV system, a minimum airflow setpoint for VAV terminal boxes is required primarily to:
- Prevent the main supply fan from surging
- Maintain adequate ventilation and thermal comfort at low cooling loads (Correct answer)
- Protect the duct from negative pressure damage
- Meet fire code requirements for smoke control
Correct answer: Maintain adequate ventilation and thermal comfort at low cooling loads
Minimum airflow setpoints ensure each zone receives sufficient ventilation air and prevents 'dead air' conditions even when the cooling load is near zero.
Question 2: What is 'infiltration' as it applies to HVAC load calculations?
- Deliberate introduction of outdoor air through the air handler
- Uncontrolled air leakage into a building through cracks and gaps in the envelope (Correct answer)
- Air transferred between zones through interior partitions
- Air leakage out of the duct system
Correct answer: Uncontrolled air leakage into a building through cracks and gaps in the envelope
Infiltration is uncontrolled outdoor air that enters through gaps, cracks, and openings in the building envelope, adding to both heating and cooling loads.
Question 3: Which of the following best describes a 'dedicated outdoor air system' (DOAS)?
- An air handler that recirculates 100% return air
- A separate system that provides pre-conditioned outdoor air to each zone independent of sensible cooling/heating equipment (Correct answer)
- A rooftop unit with an integrated economizer
- A system that uses outdoor air only during unoccupied hours
Correct answer: A separate system that provides pre-conditioned outdoor air to each zone independent of sensible cooling/heating equipment
A DOAS handles ventilation by conditioning 100% outdoor air to neutral conditions before delivering it to zones, while separate equipment (fan-coils, radiant panels) handles sensible loads.
Question 4: When designing refrigerant piping for a split system with the outdoor unit above the indoor unit, what special design consideration is required?
- Increasing the liquid line diameter by 50%
- Adding oil traps in the suction line riser at regular intervals (Correct answer)
- Using only vertical suction line runs
- Insulating the liquid line to prevent subcooling loss
Correct answer: Adding oil traps in the suction line riser at regular intervals
When the compressor is above the evaporator, suction line risers need oil traps (P-traps) at the bottom and at every 20 feet of vertical rise to return lubricating oil to the compressor.
Question 5: In HVAC system design, 'diversity factor' is applied to:
- Reduce the total calculated peak load when all zones are unlikely to peak simultaneously (Correct answer)
- Increase equipment sizing to account for future loads
- Adjust for climate uncertainty in ASHRAE design temperatures
- Account for part-load efficiency degradation
Correct answer: Reduce the total calculated peak load when all zones are unlikely to peak simultaneously
A diversity factor reduces the sum of individual zone peak loads to reflect that zones rarely all peak at the same time, allowing smaller central plant equipment.
Question 6: What is the primary design advantage of a four-pipe hydronic fan-coil system over a two-pipe system?
- Lower installation cost due to less piping
- Simultaneous heating and cooling to different zones year-round (Correct answer)
- Higher water flow velocity for better heat transfer
- Ability to use domestic water as the heat transfer medium
Correct answer: Simultaneous heating and cooling to different zones year-round
Four-pipe systems have separate hot water and chilled water circuits, allowing individual fan-coil units to heat or cool independently based on each zone's demand at any time of year.
Question 7: Which ASHRAE standard governs energy efficiency requirements for commercial building HVAC systems in the US?
- ASHRAE 15
- ASHRAE 62.1
- ASHRAE 90.1 (Correct answer)
- ASHRAE 55
Correct answer: ASHRAE 90.1
ASHRAE 90.1 (Energy Standard for Buildings Except Low-Rise Residential) sets minimum energy efficiency requirements for HVAC, lighting, and envelope in commercial buildings.
HVAC System Design 5
In a VAV system, a minimum airflow setpoint for VAV terminal boxes is required primarily to: