HVAC Energy Efficiency 5 — Questions and Answers
Question 1: What is 'heat recovery ventilation' (HRV) and how does it save energy?
- It transfers heat between exhaust and incoming fresh air streams, reducing the energy needed to condition ventilation air (Correct answer)
- It recovers waste heat from the compressor to heat domestic water
- It recirculates exhaust air back into the occupied space
- It uses outdoor air only when temperatures are within a setpoint range
Correct answer: It transfers heat between exhaust and incoming fresh air streams, reducing the energy needed to condition ventilation air
HRVs use a heat exchanger to pre-condition incoming fresh air with energy from the outgoing exhaust air, reducing the heating or cooling load on the HVAC system.
Question 2: Which action will most improve the seasonal efficiency of a split-system air conditioner?
- Ensuring the outdoor unit is shaded and has adequate airflow clearance (Correct answer)
- Installing a larger return air grille
- Increasing the refrigerant charge by 5%
- Replacing the thermostat with a programmable model only
Correct answer: Ensuring the outdoor unit is shaded and has adequate airflow clearance
Shading the condenser and ensuring unrestricted airflow lowers entering air temperature, reducing condensing pressure and improving efficiency throughout the season.
Question 3: What is the purpose of 'duct static pressure reset' control in a VAV system?
- To reduce duct static pressure setpoint when zone damands are low, saving fan energy (Correct answer)
- To increase static pressure during high-demand periods to overcome filter resistance
- To maintain constant airflow regardless of zone valve position
- To equalize pressure between supply and return plenums
Correct answer: To reduce duct static pressure setpoint when zone damands are low, saving fan energy
Static pressure reset lowers the duct pressure setpoint when VAV boxes are mostly open, allowing the supply fan VFD to slow down and save energy.
Question 4: A technician finds that a chilled water system has fouled heat exchangers. What is the energy impact?
- Increased chiller lift (higher condenser temperature or lower evaporator temperature), reducing chiller efficiency (Correct answer)
- Reduced chilled water flow rate with no efficiency penalty
- Higher evaporator pressure improving compressor efficiency
- Lower condenser entering water temperature reducing compressor work
Correct answer: Increased chiller lift (higher condenser temperature or lower evaporator temperature), reducing chiller efficiency
Fouling adds thermal resistance, forcing the chiller to operate at a higher lift (greater temperature difference between condenser and evaporator), which directly reduces COP.
Question 5: Which energy code compliance path allows a trade-off between building envelope and HVAC efficiency?
- Performance path (energy modeling to meet a total energy budget) (Correct answer)
- Prescriptive path (meeting fixed minimum values for each component)
- Climate zone exemption path
- Simplified prescriptive path for small buildings
Correct answer: Performance path (energy modeling to meet a total energy budget)
The performance compliance path uses whole-building energy modeling, allowing a less efficient envelope to be offset by a more efficient HVAC system as long as total energy use meets the budget.
Question 6: What effect does high indoor relative humidity have on HVAC energy efficiency?
- It increases latent load, requiring additional cooling capacity and energy to dehumidify the air (Correct answer)
- It reduces sensible cooling load, lowering energy consumption
- It has no effect on cooling efficiency if temperature is controlled
- It improves heat transfer in the evaporator coil
Correct answer: It increases latent load, requiring additional cooling capacity and energy to dehumidify the air
High humidity adds latent load to the system; removing moisture requires additional cooling energy, often requiring overcooling the space or supplemental dehumidification.
Question 7: What is 'setback' or 'setup' control in HVAC energy management?
- Temporarily relaxing temperature setpoints during unoccupied periods to reduce heating or cooling energy (Correct answer)
- Increasing airflow during peak hours to improve comfort
- Raising supply fan speed during unoccupied hours for air circulation
- Cycling the chiller off during periods of low demand
Correct answer: Temporarily relaxing temperature setpoints during unoccupied periods to reduce heating or cooling energy
Setback (heating) and setup (cooling) raise or lower thermostat setpoints when a building is unoccupied, reducing HVAC runtime and energy consumption significantly.
HVAC Energy Efficiency 5
What is 'heat recovery ventilation' (HRV) and how does it save energy?