CEM HVAC and Building Envelope 4 — Questions and Answers
Question 1: What energy efficiency measure does 'supply air temperature reset' provide in a VAV system?
- Lowers supply air temperature during peak load to improve dehumidification
- Raises supply air temperature at part load to reduce chiller and reheat energy (Correct answer)
- Resets the thermostat setpoints based on outdoor temperature
- Adjusts fan speed based on supply air temperature feedback
Correct answer: Raises supply air temperature at part load to reduce chiller and reheat energy
Resetting supply air temperature upward during part-load conditions reduces chiller lift energy and minimizes terminal reheat, saving significant energy.
Question 2: What is the primary function of a cooling tower in a water-cooled chiller plant?
- Precool outdoor air before it enters the AHU
- Reject condenser heat from the chiller to the atmosphere via evaporation (Correct answer)
- Provide chilled water for air handling units
- Control chilled water supply temperature
Correct answer: Reject condenser heat from the chiller to the atmosphere via evaporation
A cooling tower rejects heat from the chiller's condenser water loop to the atmosphere primarily through evaporative cooling, completing the refrigeration cycle.
Question 3: Which building envelope parameter describes the fraction of incident solar radiation that passes through a window as heat?
- Visible light transmittance (VT)
- U-factor
- Solar heat gain coefficient (SHGC) (Correct answer)
- Emissivity (ε)
Correct answer: Solar heat gain coefficient (SHGC)
SHGC is a dimensionless value from 0 to 1 representing the fraction of solar radiation admitted through a window, both directly transmitted and absorbed then re-radiated inward.
Question 4: A facility engineer wants to reduce pump energy in a chilled water system. Which measure has the greatest single impact?
- Insulating chilled water pipes
- Installing variable frequency drives (VFDs) on pumps (Correct answer)
- Increasing chilled water supply temperature
- Adding parallel pump capacity
Correct answer: Installing variable frequency drives (VFDs) on pumps
VFDs reduce pump speed at part load, and because pump power varies with the cube of speed, even small speed reductions yield large energy savings.
Question 5: What is 'sensible heat ratio' (SHR) in HVAC system design?
- Ratio of latent cooling capacity to total cooling capacity
- Ratio of sensible cooling load to total (sensible + latent) cooling load (Correct answer)
- Ratio of supply air temperature difference to return air temperature difference
- Ratio of outdoor air enthalpy to return air enthalpy
Correct answer: Ratio of sensible cooling load to total (sensible + latent) cooling load
SHR is the fraction of a zone's total cooling load attributable to sensible heat; it guides coil selection and determines how much dehumidification the system must provide.
Question 6: Which standard provides minimum energy efficiency requirements for commercial HVAC systems in the U.S.?
- ASHRAE 62.1
- ASHRAE 55
- ASHRAE 90.1 (Correct answer)
- ASHRAE 189.1
Correct answer: ASHRAE 90.1
ASHRAE 90.1 (Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings) sets minimum efficiency requirements for HVAC, lighting, and envelope in commercial buildings.
Question 7: What is the benefit of using a heat wheel (rotary energy recovery ventilator) in an AHU?
- It provides additional dehumidification capacity independent of the cooling coil
- It transfers both sensible and latent energy between exhaust and supply airstreams (Correct answer)
- It increases supply air pressure without increasing fan energy
- It filters particulates from outdoor air before entering the system
Correct answer: It transfers both sensible and latent energy between exhaust and supply airstreams
A heat wheel rotates between exhaust and supply airstreams, recovering both sensible (temperature) and latent (moisture) energy and significantly reducing conditioning loads on outdoor air.
CEM HVAC and Building Envelope 4
What energy efficiency measure does 'supply air temperature reset' provide in a VAV system?