Issue
Installing the right size equipment to heat and cool buildings is essential to getting the best performance and comfort. A system that is too large will not keep the building comfortable because of frequent ‘on/off’ cycling. Incorrect sizing can also put stress on system components, shorten the equipment’s life, and lead to wasted energy and money.
View DOE resources about the commercial mechanical requirements of the 2009 IECC, 2012 IECC, and 2015 IECC.
Overview
HVAC systems need to be properly sized so the building will be comfortable amid the harshest outdoor conditions. If a unit is oversized, energy costs may increase and the home may experience high temperature swings during milder weather. Oversizing also increases the equipment’s initial cost, savings better spent on purchasing more efficient equipment. Determining the proper size for the HVAC system requires estimating heat loss (known as the design heat load) through the roof, walls, windows, basement, and other building components bordering unfavorable outdoor weather conditions. The system size is then matched to the design heat load. For many combustion appliances, the size of the unit selected will be 10 to 30 percent above design heat load. This oversizing allows the unit to recover quickly from night setback.
Tightly sealed ducting is also crucial to ensure high energy efficiency of HVAC equipment.
Commercial Policy Options
Design loads shall be determined in accordance with the procedures described in ASHRAE/ACCA Standard 183. The design loads shall account for the building envelope, lighting, ventilation and occupancy loads based on the project design.
Resources
- Article: Evaluating low-temperature water-heating options October 27, 2016
