Heating and cooling to maintain appropriate conditioning of indoor air consume a large portion of building energy use. Installing the right size equipment to heat and cool homes is essential to getting the best performance and comfort and reducing inefficiencies. 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. To ensure proper sizing, builders should provide the homeowner a copy of the home’s heat gain/loss calculations.
HVAC systems need to be properly sized to optimize occupant comfort even 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 home 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.
2015 IECC: R403.7 Equipment sizing and efficiency rating (Mandatory)
Heating and cooling equipment shall be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies. New or replacement heating and cooling equipment shall have an efficiency rating equal to or greater than the minimum required by federal law for the geographic location where the equipment is installed.
Identical requirements can be found in section R403.6 of the 2012 IECC.
The 2009 IECC Section 403.6 states that HVAC systems must be sized in accordance with Section M1401.3 of the International Residential Code. This requires the use of ACCA Manual S and J calculation procedures. HVAC sizing requirements in the 2006 and 2009 IECC editions are identical. Prior to the 2006 version, the requirements specified the use of the ASHRAE Handbook of Fundamentals.
Standard issues/concerns that arise include:
- HVAC contractor or subcontractor does not use the required standards and instead roughly estimates size
- Contractor does not know how to conduct a Manual S or J calculation
- HVAC contractor does not include IECC requirements in bid
- Contractors are unable to provide code officials with sizing calculations and other energy-related HVAC requirements
The Elephant in the Room: HVAC for High Performance Homes
2009 RESNET Conference | David Butler – Optimal Building System, LLC