ASHRAE HVAC System Design and Operation 2 — Questions and Answers
Question 1: In a VAV system, what primarily reduces supply fan energy as zone loads drop?
- Reducing fan speed with a VFD to maintain duct static pressure (Correct answer)
- Opening the outdoor air damper fully
- Raising the chilled water supply temperature
- Closing the return air damper
Correct answer: Reducing fan speed with a VFD to maintain duct static pressure
As VAV boxes close, a VFD slows the fan to hold the static pressure setpoint, cutting fan power sharply.
Question 2: According to the fan affinity laws, if fan speed drops to 50%, fan power drops to roughly what fraction?
- 50%
- 25%
- 12.5% (Correct answer)
- 75%
Correct answer: 12.5%
Fan power varies with the cube of speed, so 0.5³ = 0.125, or about 12.5%.
Question 3: What is the main goal of a trim-and-respond duct static pressure reset strategy?
- Keep the duct static pressure setpoint fixed at design
- Lower the static pressure setpoint until the most-open VAV damper is nearly fully open (Correct answer)
- Increase supply air temperature during occupied hours
- Balance the return fan against the exhaust fan
Correct answer: Lower the static pressure setpoint until the most-open VAV damper is nearly fully open
Resetting static pressure down to just meet the most demanding zone saves fan energy while still satisfying all zones.
Question 4: With a differential dry-bulb high-limit, when does an airside economizer stop using extra outdoor air?
- When outdoor air humidity goes above 50% RH
- When outdoor air temperature goes above return air temperature (Correct answer)
- When outdoor air temperature drops below 55°F
- When the building is unoccupied
Correct answer: When outdoor air temperature goes above return air temperature
Once outdoor air is warmer than return air, bringing in more of it would add to the cooling load.
Question 5: Which equation gives sensible heat in Btu/h for standard air?
- Q = 4.5 × CFM × Δh
- Q = 500 × GPM × ΔT
- Q = 1.08 × CFM × ΔT (Correct answer)
- Q = 0.68 × CFM × ΔW
Correct answer: Q = 1.08 × CFM × ΔT
For standard air, 1.08 combines density, specific heat, and 60 min/hr.
Question 6: In the latent heat equation Q = 0.68 × CFM × ΔW, what units does ΔW use?
- Pounds of water per pound of dry air
- Grains of moisture per pound of dry air (Correct answer)
- Percent relative humidity
- Degrees F dew point
Correct answer: Grains of moisture per pound of dry air
The 0.68 constant assumes the humidity ratio difference is given in grains per pound of dry air.
Question 7: What is a common result of 'low delta-T syndrome' in a chilled water plant?
- Fewer chillers need to run
- More pumping and extra chillers staged on at partly loaded conditions (Correct answer)
- Lower condenser water temperatures
- Less coil fouling
Correct answer: More pumping and extra chillers staged on at partly loaded conditions
A low coil ΔT means more flow is needed for the same load, which pushes up pump energy and forces extra chillers online.
