Article #137 - How to Size HVAC Systems Correctly

• The most recent version of Manual J should be used, since it incorporates large impacts on efficiency, such as duct leakage and duct heat gains and losses.
• Insect screening can have a significant impact on peak cooling heat gain and should be considered with the new procedures in the latest version of Manual J.
• If drapery or blind positions are not known, they should be assumed to be in place. Otherwise, sensible loads will often be overestimated.
• Infiltration assumptions should be approached with caution. In lieu of other information, an infiltration rate of 0.35 ACH is recommended.
• Do not alter the suggested design temperatures for your location. Using the annual extreme temperatures will lead to oversizing. For Manual J, the 97.5 percent design temperatures should be used for cooling, and the 2.5 percent design temperature should be used for heating.
• Be aware that exaggerating temperatures for indoor heating and cooling set points can have large impacts on required equipment size. Generally, Manual J recommends 75 degrees Fahrenheit for cooling and 70 degrees Fahrenheit for heating. Use judgment based on homeowner comfort preferences before altering these recommendations.

Standard programmable thermostat.

Select a programmable thermostat so that the home can be conditioned before occupants arrive, thus avoiding the temptation to oversize equipment for rapid cool-downs or warm-ups. This will improve sizing-related performance and potentially save energy associated with more reliable thermostat set-back and set-ups.

Select variable-speed air handlers to gain important advantages for both hot humid and hot dry climates. In hot dry climates, the air handlers can be set to provide high airflow rates (for instance 450 to 500 cfm per ton) so that the air conditioner provides the sensible air cooling that is needed. For humid climates, many variable speed systems ramp up blowers slowly, providing energy savings and improved dehumidification.

Selecting variable-speed air handlers also allows proper adjustment of the air conditioner and heat pump airflow. Maintaining the proper airflow is essential for heat pumps, which use supplemental strip heat if the achieved heating capacity is deficient. Inadequate airflow can rob systems of 5 percent or more of their potential performance.

To learn more about Manual J and Manual S , go to the Air Conditioning Contractors of America (ACCA) website.

This article is an excerpt from the Right-Size Heating and Cooling Equipment .