Manual J D and S Calculations

Many HVAC contractors skip Manual J, S, D, and T calculations, preferring to rely on estimates rather than put in the effort to correctly size and design a new HVAC system. But e3 Power has seen enough poorly designed and improperly sized heating and cooling systems installed throughout Colorado to know the importance of taking the time to do the job right.

We work with HVAC contractors, builders, architects, homeowners, and more throughout the country to quickly and professionally complete Manual J, S, D, and T calculations to ensure that new HVAC systems are properly designed and tested to:

  • Maximize energy efficiency
  • Keep heating and cooling bills low
  • Deliver optimum comfort everywhere in a home or building

In fact, e3 Power has completed more than 2,000 Manual J, S, D, and T calculations for clients over the last decade—talk to us today about getting fast, professional help for your upcoming project.

What Are Manual J, S, D & T?

They are not just letters in the alphabet, but standards and a protocol developed by the Air Conditioning Contractors of America (ACCA) for the proper design and installation of HVAC equipment and ductwork.

Each standard is important and plays a part in the standard that follows, which is why they are usually done in a prescribed order to deliver the best results. Every house is different and a lot of the calculation depends upon the location of the home—there can be a significant 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 properly size boilers or furnaces—every system was always oversized. Energy was cheap and contractors determined that too much heat for a building was always better than not enough heat.

In addition, the benefits of insulation were not as well understood, leading to significant energy waste. HVAC equipment of the time was not as efficient as it is today either, so making houses and buildings comfortable was most easily accomplished by oversizing the systems. Today, oversized systems are inefficient and expensive to operate, may short cycle (reducing the equipment’s lifespan), and can actually perform worse than properly sized equipment.


Cooling is a bit more complex than heating but most contractors oversize cooling equipment as well, often using a rule of thumb method of 400-500 feet per ton of cooling (12,000 BTUs). This was established a long time back when we measured cooling with tons of ice!

Incorrect cooling system sizing can bring big troubles to homeowners. In extreme circumstances, oversized equipment can lead to ductwork condensation and eventually mold growth in the ducts and on the walls of a home. This is more common in humid climates like Colorado as humid air condenses on cold surfaces, causing condensation.

Manual J: Residential Load Calculation

Manual J is the first step in designing a new HVAC system and is used to calculate heating and cooling loads (how much heating or cooling a building needs). There are several software programs available that make it easier to compute these loads—here at e3 Power, we use Right Suite Universal by WrightSoft.

Manual J load calculations take into account a number of different parameters in a home, including:

  • Area of the home
  • Square footage
  • Size and placement of rooms
  • Number of doors and windows
  • Total amount of insulation

Appliances, number of human occupants in the building, and more also factor into the equation. The end result is a load calculation for the home. You need this heating and cooling load to properly size new heating and cooling equipment.

Advantages to using Manual J Load Calculations

  • Helps optimize HVAC system performance and efficiency
  • Takes outdated guesswork and “rule of thumb” HVAC sizing methods out of the equation
  • Reduces heating and cooling bills (larger systems use more energy and cost more than smaller systems to run)
  • Increases comfort—especially in the summer. If a system is oversized and does not run long enough, it is possible that indoor air could be cooled without removing humidity. If this happens, condensation can form on the registers and inside ductwork, leading to problems like mold. Conversely, if the system is undersized, it won’t be able to cool the home.

Manual S: Residential Equipment Selection

System selection is the next step for the proper design of the HVAC equipment. Each manufacturer has their equipment tested under laboratory conditions—these results are printed in the specification guide for each piece of equipment.

Manual S is of particular assistance when selecting air conditioners and heat pumps—it lays out the correct process for selecting HVAC equipment based on the design load calculations from Manual J.

Manual D: Residential Duct Design

Poor duct design can significantly undermine the operation of an otherwise well-designed HVAC system, leading to:

  • Higher energy bills
  • Uneven distribution of air
  • Poor indoor comfort
  • Excessive operating noise

Proper duct design delivers the correct amount of airflow per room (measured in CFM, or cubic feet per minute) as determined by the type, size, and location of each room in the home. Ducts and fittings come in many types and sizes to fit specific spaces, and each piece needs to be chosen according to the specifics of the house to ensure proper static pressure and airflow.

Manual T: Air Distribution Basics

The final piece of the puzzle is a room by room airflow. There are some variables that can be controlled by the designer. What type of registers will be installed? Where are the return vents located? Where are the supply registers located? These choices can affect how air moves in the rooms of a house and how efficiently the HVAC system operates.

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

After J, S, D, and T calculations are completed and the system is installed, e3 Power can also 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.

With quick turnaround times from experienced independent building science professionals, e3 Power makes Manual J/S/D/T calculations fast and simple for clients across the country.

Get professional independent assistance to ensure new HVAC system installation is done right. Call 303-292-1233 or contact us today to schedule Manual J, S, D, and T calculation services from e3 Power.

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