Manual J is a procedure for estimating the heating and cooling load of a residence (single-family detached homes and mobile homes). This procedure is published and maintained by the Air Conditioning Contractors of America (ACCA). It is now in its eighth edition; older versions should not be used.
ACCA Manual J should be used anytime the heating and or cooling system is provided new or replaced in existing. It can also be used to evaluate an existing system. Manual J is required by code. “… building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies. “ IRC 2009 M1401.3
Each code jurisdiction will determine the scope and method of determining compliance. Also some energy rebate/incentive programs require Manual J calculations.
Manual J was written by experienced engineers within the Heating, Air Conditioning and Ventilating (HVAC) field. The method follows industry recognized procedures by the American Society of Heating, Air Conditioning and Refrigeration Engineers (ASHRAE) for all building types.
Manual J is a procedure; a set of rules, equations and many tables. The procedure may be performed in any manner. ACCA nor building codes specify how the procedure is to be completed. You can make all of the necessary calculations by hand using a calculator and forms. There are computer spreadsheets which increases the speed and accuracy of the results. This procedure can also be programmed into a computer. Software can ideally improve accuracy and time to complete. ACCA does approve software as following the Manual J procedure. Look for the “Powered by ACCA Manual J” logo. A list of these programs is at www.acca.org/industry/system-design/software.
Manual J can be performed in-house by HVAC Contractor using whichever method desired. This provides for the greatest control over completing the procedure. Changes can be made quickly and results can be checked when home is completed. The disadvantage is that at least one employee must take the time to learn the procedure. Even the software requires learning. The cost appears to be low, but can be quite high if the employee spends a huge amount of time completing.
The greatest variable in obtaining good accurate results is the accuracy and thoroughness of the input data. The old saying “garbage in is garbage out” applies here. Slight inaccuracies in some input data can yield large inaccuracies in the results. While other input data has minimal impact. The professional performing the calculations needs to know which data is critical and which data not to spend a lot of time on. For example, air infiltration is often incorrectly estimated. This can lead to large inaccuracy in the load estimate. On the other hand, spending much time on trying to get a detailed evaluation of internal loads is not necessary. Small electrical loads have little impact on equipment size. Usage is so variable and based on individual tastes; no estimate will be completely accurate.
For new homes, Manual J can be performed from a good set of drawings. For existing homes, the house needs to be surveyed and measured. Information on dimensions, building materials and the effects of age must be obtained. A blower door test for evaluating air infiltration is an absolute must. This information should be collected by an experienced energy auditor, preferably with Building Performance Institute (BPI) or Residential Energy Services Network (RESNET) certification.
Hire it done. While it may be good to have the expertise in-house, hiring an expert greatly shortens the time to complete the analysis. The expert may have years of experience and understands how all of the input parameters impact the results. Some energy rebate/incentive programs may require performance by independent party. Seeking a professional can be easy and low cost.
The old fashion method of sizing HVAC equipment based on house square footage is no longer valid or allowed by building code. Also keep in mind the litigious nature of modern society. This rule of thumb may be adequate some of the time, but what about the other times? Do you really have the time and money to fight off lawsuits?