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Manual J, S, and D

What are Manual J, S and D?

The Air Conditioning Contractors of America (ACCA) has developed standards and protocols for designing and installing HVAC equipment and duct work. Properly designed HVAC systems must go through the process of each of the four protocols — J, S, T and D.

A correct Manual Calculation leads to a well-designed HVAC system that improves overall performance, comfort and efficiency. Each manual plays a critical and unique role in the process.


A "Manual J" should be performed before the installation of any new HVAC system to determine the proper load requirements needed for the space in which it is to be installed. The "Manual S" utilizes the information from the Manual J to ensure that the proper equipment is selected. The "Manual D" refers to the ductwork in a residential building and is typically only done on a new construction, unless the entire ductwork system is being replaced in a retrofit.

In addition to new construction or before installation, there may be instances when your local government or an inspector will require you to get a Manual J and or Manual S completed after a job has been finished. We can help.

The technicians at Divine Energy Solutions can provide you with a complete and thorough Manual J or a Manual S that will satisfy all inspection requirements.

Contact us today. 


*At Divine Energy Solutions we typically deal with Manual J,S and D as the Manual T deals entirely with the placement of airflow vents in a new construction.

Manual J

Residential Load Calculation

The Manual J calculation is used to determine what the correct size is for an HVAC unit without using excessive energy. Calculating the peak heating and cooling loads, or the heat loss and heat gain, is crucial for designing a residential HVAC system. HVAC contractors and designers use this calculation for every home and building they work on.

Manual J can be used to determine the heating and cooling needs for a specific home based on:

  • The home’s location.

  • The humidity of the climate.

  • The direction the home faces.

  • The insulation R-values of the walls, ceiling and floor.

The loads dictate both the equipment selection and the duct design used to deliver conditioned air throughout the house. As a result, Manual J impacts the processes of Manual S, Manual D and Manual T. When homeowners need to replace an existing furnace or A/C, they may simply select the same size as the latest model. However, if the original system wasn’t sized properly, the new system will also be improperly sized.


Manual S

Equipment Selection

Manual S outlines specific procedures for choosing HVAC equipment based on design conditions and Manual J loads. Manual S utilizes original equipment manufacturer (OEM) data rather than the Air Conditioning, Heating and Refrigeration Institute certificate to size HVAC equipment. It specifies how small or large the capacity of the HVAC equipment can be when you compare it to the Manual J calculation.

When it comes to minimum rated efficiency value requirements like energy efficiency ratio, heating seasonal performance factor, seasonal energy efficiency ratio (SEER) and coefficient of performance, Manual S defers to the authority that has jurisdiction, such as LEED, Energy Star or a local permit office.


Manual D

Residential Duct System

Manual D is the standard in the industry for sizing HVAC returns in the home, along with supply duct systems and registers. When a homeowner is ready to replace their old HVAC system or build their dream home, the residential Manual D duct design system is essential. Before HVAC equipment is purchased, this manual is followed to ensure optimal comfort in the home for years to come.

The A/C and furnace selected during the Manual S process determine the duct CFM that’s ideal for the humidity and summer and winter design temperatures. Similar to Manual J and Manual S, many permit offices require a Manual D duct design prior to issuing an HVAC permit, as this manual has a track record for being reliable.

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