HVAC Load Calculations

Duct-EZ® offers Manuals J, D and S calculations through its sister company Tacoma Energy®. Tacoma Energy® is an ACCA Accredited QA Contractor who currently provides services for customers in Ohio, Indiana, Kentucky, Michigan, Pennsylvania, and Massachusetts. In order to provide the highest level of service for the homeowners, Duct-EZ® and Tacoma Energy® work with the builder and the HVAC contractor as a unified team. The ultimate goal is in ensure the home is working efficiently and the homeowner is comfortable.

Manual calculations are prepared to determine how much heating and cooling air a specific home needs. Many factors are taken into consideration in order to prepare the Manuals and to ensure the proper sizing. There are three different manuals that must be calculated:

Manual J

This is the first step in the design process for the HVAC system in a new home. Manual J is the calculation of the heating and cooling loads for the home. Based upon statistics which are entered about the home, (orientation, square footage, number of windows, geographical location, number of people, etc.) designers are able to determine the amount of heat that is lost during the cooler months, and how much heat is gained during the warmer months.

There are two ways of calculating the Manual J:

  1. Whole House (Block Load), or
  2. Room by Room (code required)

Duct-EZ® uses the Room by Room method to ensure that each room is properly designed to achieve the required amount of air that the room needs. Once each room’s air flow is determined the HVAC contractor is then able to adequately balance each room to ensure homeowner comfort.

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Manual D

The second step in the calculations is the Manual D. This is the process of determining the correct size of each duct supply run and returns based upon the amount of air that a room requires. If the duct size is not big enough it is impossible to get the proper amount of air to the room.

The designer must take into consideration the following items to ensure the sizing is correct:

  • Friction Rate

  • Static Pressures

  • Distribution Losses

Each of the above factors play a significant role in determining the correct size of the supply runs and the returns. If any of these factors are too high not only may the home not be comfortable, but it may also become a health and safety concern. Therefore, working with an experienced HVAC designer is critical and not to work with one that uses the “old rule of thumb” methodology.

Manual J,D,and S

Manual S

Manual S is the determination of the correct size, or selection, of the equipment. When selecting the right size the designer must take the following items into consideration:

  • Outdoor Temperature

  • Indoor Temperature

  • Indoor Humidity

  • Airflow over the Coil

  • Sensible Capacity

  • Latent Capacity

The equipment selection is based on performance criteria such as the equipment’s total capacity to remove heat and moisture from the air, how much total air is needed, and at what pressure the system can produce. It is important to note because one manufacturer’s size can perform significantly different than another, and they can perform differently based on the geographical location of the home.

It is extremely important that each of the manual calculations be completed prior to installing the HVAC system. Using the “old rule of thumb” approach is downright silly now that we know so much more about how the homes, people and the systems affect each other. So make sure you work with an experienced HVAC designer.

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