design
methods for rooftop heating, ventilation, and air conditioning
(HVAC) systems, significant improvements in operational savings,
energy efficiency, and indoor comfort can be achieved.
Small HVAC systems are the workhorses of the light commercial
building market, which represents more than half of the annual
commercial new construction floor area in California. Design,
installation, and operations issues can prevent these systems from
performing up to their full potential. This design brief focuses on
actions that the architects, engineers, and design/build
contractors can take to improve the energy efficiency of small HVAC
systems, reduce operating costs, and improve indoor comfort and
environmental quality. These actions include:
- Practice energy-efficient design strategies such as reduced
lighting power, high-performance glass and skylights, cool roofs,
and improved roof insulation techniques in the overall building
design.
- Size units appropriately using American Society of Heating,
Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE)
approved methods that account for the energy efficiency strategies
implemented in the design, and use reasonable assumptions on plug
load power and ventilation air quantities when sizing
equipment.
- Select unit size and airflow based on calculated sensible loads
without oversizing. Consider increasing unit flow rate to improve
sensible capacity in dry climates.
- Specify units that meet the Consortium for Energy Efficiency
Tier 2 efficiency standards; and incorporate premium efficiency fan
motors, thermostatic expansion valves, and factory-installed and
run-tested economizers.
- Design distribution systems with lower velocities to reduce
pressure drop and noise. Seal and insulate duct systems located
outside the building thermal envelope.
- Operate ventilation systems continuously to provide adequate
ventilation air. Incorporate demand-controlled ventilation to
reduce heating and cooling loads.
- Specify commercial grade thermostats with the capability to
schedule fan operation and heating and cooling setpoints
independently.
- Commission the systems prior to occupancy through a combination
of checklists and functional testing of equipment control,
economizer operation, airflow rate, and fan power.
- Develop clear expectations of the services provided by HVAC
maintenance personnel.
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