What is Manual J, S, and D Calculations?

They are not just letters in the alphabet, but standards and a protocol developed by Air Conditioning Contractors of America (ACCA) for the proper design and installation of HVAC equipment and ductwork for air conditioner efficiency. Each standard is important and plays a part in the standard that follows. They are usually done in a prescribed order to deliver the best results.  Each house is different and a lot of the calculation depends upon the location of the home. There is a difference between a home in Denver and a similar home in Pueblo or Breckenridge.


Under the old rule of thumb in Colorado, no one bothered to size boilers or furnaces because every system was oversized.  The energy was cheap and too much heat was much better than not enough heat. In addition, the benefits of insulation were not as well defined and it was better to oversize than undersize.  The equipment was also not as efficient and making houses comfortable was accomplished by oversizing the systems. Today oversized systems are inefficient to operate, may short cycle, and actually perform worse than properly sized equipment.


Cooling is a bit more complex than heating.  However, most contractors oversize cooing equipment.  They use a rule of thumb method of 400 -500 feet per ton of cooling (12,000Btu’s). This was established a long time back when we measured cooling with tons of ice!  Incorrect cooling sizing can bring big troubles to the home dweller.  In extreme circumstances, oversized equipment can result in condensation in the ductwork and eventually mold growth in the ducts and on the walls.  This is more common in humid climates when humid air condenses on cold surfaces causing condensation.

Manual J

Just in case you are interested in heat and cooling loads, there are several software programs available that make it easier to compute these loads. Here at e3 Power, we use a software program called Right Suite Universal by WrightSoft to design the HVAC system to obtain air conditioner energy efficiency. The building or envelope is constructed using blocks that are manipulated to the room size. Windows and doors are added to the drawing.  Once the home is built the wall, ceiling, window, and door attributes are entered into the program. Finally, the appliance load and human factors are entered.  The end result is a heat load calculation for the premises. You need the heat and cooling load to properly size the heating and cooling equipment. Once the equipment is sized it is time to layout the ductwork, turn on the system and balance the airflow. 

Advantages to using Manual J

·       Proper system sizing helps optimum system performance for air conditioner efficiency

·       Takes the guessing out of the equation

·       Reduces cost because larger systems cost more than smaller systems for air conditioner efficiency.

·       Increases comfort especially in the summer by cooling and dehumidifying air simultaneously. If a system does not run long enough it is possible that the air could be cool but retain its humidity. If this happens condensation can form on the registers and inside the ductwork possibly causing big problems like mold. Conversely, if the system is too small the home won’t stay cool.

Manual S

System selection is the next step for the proper design of the HVAC equipment. Each manufacturer has their equipment tested and under laboratory conditions.  The results are printed in the specification guide for each piece of equipment. Goodman is going to perform differently than a Carrier. Your HVAC contractor or HVAC designer may specify a manufacturer by their relationship with the distributor or may have a preference based on price, previous installations, past performance, warranties, or parts availability.   

Manual D

Duct design delivers the correct CFM per room as determined by the type, size, and location of the room in the home.  Ducts and fittings come in many types and sizes to fit each specific space. Each duct and fitting has a certain amount of air resistance (friction) that impedes airflow. Straight ducts lengths are usually a 1:1 ratio. Some fittings are a 1:90 ratio, meaning one fitting could have as much resistance to 90 feet of straight duct equivalence. The total duct equivalent length affects the static pressure which in turn control airflow. 

Manual T

The final piece of the puzzle is a room by room airflow. This calculation is done simultaneously with Manual D as the software determines the airflow and duct design at the same time. There are some variables that can be controlled by the designer. What type of register will be installed? Where is the return vent located?  Where is the supply located?  Some answers to these questions depend upon the layout of the room and the furniture in the room. An example of a bad design might be installing a floor register in the kitchen that is underneath a kitchen cabinet. 

Measurement and Verification

M & V. After J S D T is completed we test the final system for correct airflow to each register and compare it to the designed system.  The airflow can be adjusted at each register to complete the fine-tuning. If you’re searching for a manual J, D, and S calculation near me, this testing can be done by the HVAC Company or an Energy Star ™ rater, HERS Rater, or another individual familiar with this procedure.  

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  • Thank you for your work and the information and professionalism.

    Judy Denver, CO

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