There is evidence that many heating, ventilating & air conditioning (HVAC) systems, installed
in larger buildings, have more capacity than is ever required to keep the occupants
comfortable. This paper explores the reasons why this can occur, by examining a typical
brief/design/documentation process.
Over-sized HVAC systems cost more to install and operate and may not be able to control
thermal comfort as well as a “right-sized” system. These impacts are evaluated, where data
exists.
Finally, some suggestions are developed to minimise both the extent of, and the negative
impacts of, HVAC system over-sizing, for example:
• Challenge “rules of thumb” and/or brief requirements which may be out of date.
• Conduct an accurate load estimate, using AIRAH design data, specific to project
location, and then resist the temptation to apply “safety factors
• Use a load estimation program that accounts for thermal storage and diversification
of peak loads for each zone and air handling system.
• Select chiller sizes and staged or variable speed pumps and fans to ensure good part
load performance.
• Allow for unknown future tenancies by designing flexibility into the system, not by
over-sizing. For example, generous sizing of distribution pipework and ductwork will
allow available capacity to be redistributed.
• Provide an auxiliary tenant condenser water loop to handle high load areas.
• Consider using an Integrated Design Process, build an integrated load and energy
use simulation model and test different operational scenarios
• Use comprehensive Life Cycle Cost analysis for selection of the most optimal design
solutions.
This paper is an interim report on the findings of CRC-CI project 2002-051-B, Right-Sizing
HVAC Systems, which is due for completion in January 2006