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ConsultAir Heating, Air Conditioning and Energy Efficiency Knowledge

Specializing in Manual J Load Calculations & Manual D Duct Designs

To properly design a complete HVAC system, there are three fundamental procedures that should always be followed:

HVAC design
  • ACCA Manual J Load Calculation
  • ACCA Manual S Equipment Selection
  • ACCA Manual D Duct Design
Due to the ever growing presence of new building materials, advanced insulation systems, and efficient ventilation systems, it's impossible to use rule-of-thumb sizing methods and consistently achieve accurate and energy saving results.

*(ACCA) Air Conditioning Contractors of America

Manual J Load Calculations

ACCA's Manual J is the first step in the design process of a new heating and air conditioning system. By following the Manual J methodology, HVAC designers are able to accurately determine the total amount of heat that is lost through the exterior of a home during the cooler months, and the total amount of heat that is gained through the exterior of a home during the warmer months. Through a complex series of calculations and inputs, the HVAC designer is able to analyze all aspects of the thermal characteristics of every wall, floor, ceiling, door and window. In addition, an HVAC load calculation also takes into consideration other factors such as the home's geographic location, orientation to the sun, envelope tightness, duct leakage, lights and appliances. ACCA's Manual J even calculates the amount of heat and humidity that each occupant of the house will add to the interior of the home.

There are two types of Manual J load calculations:

  • Whole House (Block) HVAC Load Calculations
    Whole House or Block HVAC Load Calculations provide the heating and cooling loads for the entire home. This type of load calculation is used when there is no need to design or modify an existing duct system. Whole house load calculations are commonly used to determine the correct HVAC equipment size and match-up that is required when replacing the HVAC system in an existing home.
  • Room-by-Room Load Calculations
    Room-by-Room Load Calculations provide the heating and cooling loads for each individual room within the home. In addition to the information produced by a block load calculation, the Room-by-Room method also determines the amount of air that is required to heat and cool each individual room. This information is critical when determining the individual duct sizes as well as the size and overall layout of the duct system.

*According to the International Code Council (ICC), “Heating and cooling equipment shall be sized based on building loads calculated in accordance with ACCA Manual J”.

Manual S Equipment Selections

Once a Manual J load calculation has been completed, the HVAC designer will have the information required to accurately select the proper HVAC equipment. The equipment selection is based on performance criteria such as the equipment's total capacity to remove heat and moisture from the air as well as how much total air, and at what pressure, the system can produce. This is important to note because one manufacturer's 3 ton HVAC system can perform significantly different than another manufacturer's 3 ton system. In addition, a 3 ton system that is installed in Maryland is going to perform differently than the identical 3 ton system would perform if it was installed in Houston.

Manual D Duct Designs

Manual D is the ACCA method used to determine the overall duct lay-out including the individual duct sizes. To design a duct system, the HVAC system designer must have completed a Room-by-Room Manual J load calculation as well as a Manual S equipment selection. All to often, duct systems are created using rule-of-thumb methods in lieu of using Manual J, Manual S and Manual D. This practice is the predominant reason for complaints of temperature differentials throughout a home as well as complaints of excessive noise caused by air velocity that exceeds the maximum allowed by Manual D.

Energy Sheets (REScheck & EnergyGauge Energy Code Compliance Reports)

Most states require that energy code compliance of a residential construction project must be reported in order to qualify for the appropriate construction permits. In addition, most state energy codes require that an ACCA Manual J heat load calculation be performed on the construction project, and that the heating and cooling equipment must be sized per ACCA Manual S.

While energy codes vary from state to state, in general there are 2 different paths that can be followed to determine if a construction project meets your state’s energy codes.

  • Prescriptive Method
    The Prescriptive Method has pre-assigned minimum values such as; thermal resistance (R-value), thermal transmittance (u-value) and solar transmittance (shgc), for each component of the building. This approach is quick and easy to use, but many users find it somewhat restrictive because the requirements are typically based on worst-case assumptions and all requirements must be met exactly as specified. Energy code compliance using the prescriptive method is usually not the most cost effective path to follow to achieve energy code compliance. 
  • Performance Method
    The Performance Method allows more flexibility by allowing one energy saving measure to be traded for another. Each energy saving measure is assigned points or credits. The total points/credits for each area must meet the minimum total points required to qualify for the building permit. Typically, this method is less restrictive than prescriptive approaches because components that exceed the requirements can compensate for those that do not meet the code.

ConsultAir can provide you with the energy code compliance certificates and reports, also known as energy sheets, that may be required to qualify for your next building permit.

Gas Piping Design & Schematics

A properly designed gas piping system will insure that the system will provide sufficient gas to meet the maximum demand of the gas equipment and that it will supply enough fuel for all appliances to operate at the same time.

When designing a gas piping system, one must consider:

  • Loss of pressure from point of delivery to appliance
  • Max gas demand
  • Length of piping and number of fittings
  • Specific gravity of the gas
  • Diversity factor

Gas piping systems are designed based on the gas pressure at the point of delivery (gas meter).
The three most common pressures that are used in residential gas piping systems are:

  • Low Pressure
    6 to 7 inches water column (equivalent to 4 ounces or ¼ pound) is the standard pressure supplied by natural gas utilities in the USA and Canada.
  • Medium Pressure
    1/2 PSI (12 to 14 inches water column) is available from many natural gas utilities as an alternate pressure supply. The increase in pressure provides for reductions in pipe size and does not require a pressure regulator. Most natural gas appliances manufactured for use in the US and Canada are designed to operate up to a maximum of 14 inches water column.
  • Elevated Pressure
    2 PSI is the highest natural gas pressure usually supplied within residential buildings in North America. This pressure always requires the installation of a gas pressure regulator between the gas meter set and the appliances.

Propane (LP Gas) is also used in residential gas piping systems. Propane is typically supplied to residential buildings at 11–13 inches of water column. For 2 PSI propane elevated pressure piping systems, use a line gas pressure regulator that is set for 11 inches water column outlet pressure.

1/4 PSI = 7" w.c. = 4 oz.
1/2 PSI = 14" w.c. = 8 oz.
1 PSI = 28" w.c. = 16 oz.
2 PSI = 56" w.c. = 32 oz.

Hybrid Gas Piping Systems
Flexible gas pipe and rigid black pipe (galvanized pipe if near the coast) combinations.
It is often advantageous to use both corrugated stainless steel tubing (flexible gas pipe) and rigid pipe in the same system when elevated gas pressure is available. Using this type of system can often give you a distinct advantage over your competition. For more information, contact ConsultAir.

The information provided above is not related to any specific gas appliance. Always follow the manufacturer's installation instructions when installing gas piping and or gas appliances.

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