Calculations Guide to Manual J, S, T, and D

Guide to Manual J, S, T and D

Ah, Manuals J, S, T and D; measures and calculations developed by the Air Conditioning Contractors of America (ACCA) for designing and installing HVAC equipment and duct work. They are necessary protocols and standards for properly designed HVAC systems, and yet nearly every HVAC pro has been frustrated by them at one time or another. What is the difference between the protocols, why are there four protocols and how do they relate to each other?

A quick rundown:

  • Manual J determines the proper HVAC load for residential buildings
  • Manual S helps pros choose the right equipment for the job
  • Manual T guides the size of air distribution units like registers and grilles within the home
  • Manual D deals with duct systems

Many permit offices nationwide require new buildings to comply with these protocols. Beyond those basics, here's a deeper look into everything you need to know about Manuals J, S, T and D.


As HVAC pros know, Manual J is a calculation formula used to determine the proper size of the HVAC unit in any given residential building, from condos to single family mansions. The idea is to fit the dwelling with the perfect size unit that will do the job optimally, without using excessive energy. This calculation is required by national and local business codes, and helps HVAC pros determine the right size unit for the job. And it is the foundation for the other three protocols.

Here's a simple formula for the Manual J calculation that you don't have to be a mathematician to use.
(Building surface in square feet) x (ceiling height)
(Number of occupants) x 100 BTU
(Number of exterior doors) x 1,000 BTU
Add those totals together to get your optimal HVAC load.

However, some HVAC pros point out that these aren't the only variables in terms of things that affect heating and cooling. Humidity and other environmental factors, which direction the home faces, how each room is used and whether the building has adequate insulation all play a part. 

Manual S 

Building on the Manual J calculation, Manual S guides you in selecting the right unit for the job. Using the original equipment manufacturer (OEM) data plus the Manual J calculation, Manual S determines how large or small the HVAC unit needs to be.  

Highly useful for HVAC installers, it is designed to prevent the issues and headaches that can come from getting to the job site and finding the unit isn't the right size. It prevents headaches for homeowners, too. 

Here's how you arrive at that result, according to Mr. Cool:

  • Verify that the Manual J calculation is correct
  • Check the design conditions
  • Make sure the Manual J calculation and the performance data from the OEM match
  • Confirm your equipment's total heating capacity is equal to or less than 140% of the total heating load and 115% of the total cooling load. If it doesn't fall within those perimeters, the unit size needs to be reduced
  • Confirm the heat pump balance point

Manual T 

Even if you've calculated everything correctly with Manual J and selected the proper size HVAC unit with Manual S, there is still potential for headaches if the registers and grilles aren't right for the job. You can move all the warm or cool air you want from the properly sized HVAC units, but if it's not distributed properly, that room just isn't going to feel good. Simply put, Manual T ensures proper airflow and circulation.

Here's how you use Manual T:

  • Inspect the direction of the jet
  • Understand the pressure drop from the registers and grilles
  • Confirm the velocities of the return grille and supply registers


Manual D 

Manual D is the industry standard for sizing HVAC supply and return ducts. Using Manual J as the foundation, Manual D helps you size the ductwork properly to provide the optimal distribution of heating and cooling into any given room. It allows for increased efficiency of the HVAC unit itself, even temps throughout the house (some rooms too hot, others too cold), and it even guards against mold or coils freezing.
This protocol also helps the HVAC unit itself function optimally. If the ductwork is improperly sized, the unit needs to overcompensate and work harder. It helps determine:
  • How big the actual grilles should be
  • The optimal places for them in each room
  • How many you need
To use Manual D:
  • Start with Manual J
  • Determine the layout of the home
  • Ensure you have the right sized supply registers and return grilles based on Manual T
  • Ensure you have the properly sized supply and return main plenum based on velocity and friction

All of these ACCA manuals are necessary, useful and build on each other, resulting in the optimal size, capacity and design of an HVAC system. Using Manuals J, S, T and D, the system will be high performing, last a long time and keep your clients cool in the summer and cozy in the winter, in every room in their home. And that's the whole point of HVAC, isn't it?

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