Article #1468 - Residential Heating and Cooling Load Calculation Requirements - Code Notes

[2003 IRC, 2003 IECC, 2006 IECC]

It is common for mechanical systems in residential construction to be oversized which increases installation costs, wastes energy, and reduces comfort and moisture control. Properly sized equipment will last longer, provide greater comfort, reduce noise, and save homeowners money. Yet both builders and code officials are uncertain how to evaluate such calculations to assure they meet the intent of the code plus the sizing methodology approved in Air Conditioning Contractors of America (ACCA) Manual J (or equivalent).

Major mechanical systems covered by the IECC.

The 2003 IECC requires sizing calculations to be performed on every home using the procedures described in the ASHAE Fundamentals Handbook - 2001. Chapter 14 of the 2003 IRC requires heating and cooling systems to be sized to ACCA Manual J - Seventh Addition or other approved heating and cooling load calculations. Both methods have sufficient built-in safety factors to accommodate most conditioning needs. Therefore, it is important to follow all instructions in ASHRAE Fundamentals or Manual J using precise area measurements and specific data.

Heating and cooling loads can be determined using a whole house approach or performing a room-by-room load calculation. The room-by-room approach provides the information needed to determine the number of cubic feet per minute (cfm) needed of conditioned air to satisfy the heating and cooling load for the room. This information can then be use to determine the duct size necessary to deliver space heating and cooling for the space.

The IECC regulates the outdoor design temperature for use in performing load calculations. Table 302.1 requires that the outdoor winter and summer design temperatures be based on the 97 ½ percent value for winter and 2 ½ percent value for summer based on weather data from the ASHRAE Fundamentals Handbook 2001. It is important to note that the 97 ½ percent and 2 ½ percent values have been replaced by 99.6 percent and 1.0 percent respectively in the 2001 ASHRAE Fundamentals Handbook referenced above. This reference has been changed in the 2004 version of the IECC.

There are no requirements for the indoor design temperatures for heating or cooling. ACCA recommends an indoor winter design temperature of 70 °F and a summer indoor temperature of 75 °F drybulb and 63 °F wetbulb.

To select the appropriate system based on the heating and cooling load calculations, ACCA Manual S should be used. Excessive oversizing equipment causes short-cycling and creates unnecessary stress on the equipment. Also larger systems require larger duct sizes which increases the installed cost. In areas where humidity is an issue, an oversized system will degrade the humidity control. A properly sized system will run almost continuously at design conditions and provide the proper level of dehumidification during the cooling season.

Plan Review:

1. Verify that the correct outdoor design temperatures are used for the heating and cooling load calculations and that they are consistent with values in Table 302.1
2. Verify that the building geometry and glass area match what is shown on the plans and compliance documentation. Glazing orientation is important to verify for cooling load calculations but has no effect on heat loss calculations.
3. Verify that the levels of efficiency shown in the load calculations are consistent with the energy code compliance documentation. Insulation R-values, glazing U-factor and SHGC are important to verify.
4. Verify that the make, model number, and equipment size is specified on the plans or compliance.

Field Inspection:

1. Verify the make and model number for the heating and cooling system installed for the building and compare that to system specified on the building plans or documentation.
2. If a different system is specified, verify that the system has the same output capacity and also the same level of efficiency as specified in the plans.
3. If a larger system or an additional system is installed, require the responsible party to verify that the systems installed complies with the IECC.
4. Verify that the efficiency levels of insulation and window efficiency (U-factors and SHGC) meet or exceed those that are called out on the plans or documentation. Levels that are too low can cause the system to be undersized. If cooling is installed, verify that the glazing area and orientation is installed per the approved building plans.
5. Verify that the energy features of the house are installed per manufacturers installation instructions.
6. Verify that the refrigerant charge level was tested by the installer.

Code Citations*