Oversized furnaces, heat pumps, and air conditioners are the norm in most American homes. That’s a problem, since oversized equipment often provides lower levels of comfort and costs more than right-sized equipment.

Designing and installing a forced-air heating and cooling system is a multi-step process that should include heat-loss calculations, heat-gain calculations, equipment sizing, and duct system design. In a typical residential installation, however, some of these steps are performed sloppily or entirely omitted.

The purpose of a heat-loss calculation is to determine the design heating load, which is used to select the heating equipment needed during worst-case conditions: that is, during a very cold day in winter with no solar gain through the windows. Heat-gain calculations are used to determine the design cooling load, which is used to size the home’s air conditioner. These heat-gain calculations also assume worst-case conditions: a very hot day in summer with maximum solar gain through the windows.

What is Manual J?

According to most U.S. building codes, HVAC contractors are required to perform the system design steps listed above using calculation worksheets developed by the Air Conditioning Contractors of America (ACCA). Three of the most important of these worksheets are called Manual J, Manual S, and Manual D. For many years, these worksheets have been the standard methods for calculating residential heat loss, heat gain, equipment size, and duct sizes.

In the old days, heat-loss calculations were done with paper-and-pencil worksheets. For more information on old-school heat-loss calculations, see How to Perform a Heat-Loss Calculation — Part 1 and How to Perform a Heat-Loss Calculation — Part 2 .

Manual J, Manual S, and Manual D calculations are no longer performed with a pencil and paper; contractors now use software programs to complete these worksheets. Hank Rutkowski, the mechanical engineer who wrote (and continues to…