Building codes, especially those related to energy efficiency, have improved a lot over the years. With building enclosures, this has made a big difference. We now have more insulation, less thermal bridging, and tested air barriers. On the mechanical side, the improvements are significant — reduced duct leakage and mechanical ventilation in airtight homes — but there’s still a gap between some code requirements and what’s being installed.

But that gap isn’t my focus today. Instead I’ll spell out what those HVAC design requirements are. But first, let me say a little about how building codes work. At the highest level are the model codes. The major player in this arena is the International Code Council (ICC), and I’ll focus on two of their codes that include HVAC design requirements for homes: the International Residential Code (IRC) and the International Energy Conservation Code (IECC). Of course, when you build a home, you’ll have to meet the requirements of your local building department, which probably uses one of the I-codes, perhaps with some modifications.

On the topic of HVAC design, the IRC has more detail than the IECC, as you might expect. Also, the IECC just repeats some of the major requirements from the IRC.

Equipment sizing

Both the IRC and the IECC say the same thing here:

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.

That’s section N1103.7 in the IRC and…