Building Codes Assistance Project

Installing the right size equipment to heat and cool a home is essential to getting the best performance and comfort. A system that is too large will not keep the home comfortable because of frequent 'on/off' cycling. Incorrect sizing can also put stress on system components, shorten the equipment's life, and lead to wasted energy and money. To ensure proper sizing, builders should provide the homeowner a copy of the home's heat gain/loss calculations.

Residential HVAC systems need to be properly sized so the home will be comfortable amid the harshest outdoor conditions. If a unit is oversized, energy costs may increase and the home may experience high temperature swings during milder weather. Oversizing also increases the equipment's initial cost, savings better spent on purchasing more efficient equipment. Determining the proper size for the HVAC system requires estimating heat loss (known as the design heat load) through the roof, walls, windows, basement, and other home components bordering unfavorable outdoor weather conditions. The system size is then matched to the design heat load. For many combustion appliances, the size of the unit selected will be 10 to 30 percent above design heat load. This oversizing allows the unit to recover quickly from night setback.

Tightly sealed ducting is also crucial to ensure high energy efficiency of HVAC equipment.

There are several new and innovative products and techniques regarding HVAC equipment - like programmable thermostats and duct and equipment insulation -- that are not specified by the model codes that can save energy, money, and enhance thermal comfort. For more information, visit www.BuildingGreen.com.

The Air-Conditioning Contractors of America (ACCA) Manual D (Residential Duct Systems) and Manual J (Residential Load Calculation, Eighth Edition," 2003) also provide advanced code language for HVAC system installation. ACCA has published a free online checklist to verify Manual D compliance.

The following section provides examples of these types of code improvement language or methodologies in these areas.

Policy Options 

• Incorporate ACCA Manual D and Manual J into HVAC Design and Installation Procedures
  

  • California State Energy Office - Quality Homes: Procedures For HVAC System Design and Installation
    Criteria for a Quality HVAC System
    

    An HVAC system should:

    1. Be properly sized to provide correct air flow, and meet room-by-room calculated heating and cooling loads.
    2. Be installed so that the static air pressure drop across the handler is within manufacturer and design specifications to have the capacity to meet the calculated loads.
    3. Have sealed supply ductwork that will provide proper air flow.
    4. Be installed with a return system sized to provide correct return air flow.
    5. Have sealed return ductwork that will provide proper air flow to the fan, and avoid air entering the HVAC system from polluted zones (e.g., fumes from autos and stored chemicals, and attic particulates).
    6. Have balanced air flows between supply and return systems to maintain neutral pressure in the home.
    7. Minimize duct air temperature gain or loss between the air handler and room registers, and between return registers and the air handler.
    8. Be properly charged with refrigerant.
    9. Have proper burner operation and proper draft.

    Procedures to Design and Install an Air Distribution System
    

    The following steps should be followed in the design and installation of the HVAC system to ensure efficiency and comfort (for details, see Appendix 1):

    1. Determine room-by-room loads and air-flows using ACCA Manual J calculation procedures (or substantially equivalent).
    2. Layout duct system on floor plan, accounting for the direction of joists, roof hips, fire-walls, and other potential obstructions. Determine register locations and types, duct lengths, and connections required to produce layout given construction constraints.
    3. Size duct system according to ACCA Manual D calculation procedures (or substantially equivalent).
    4. Size HVAC equipment to sensible load using ACCA Manual S procedures (or substantially equivalent).
    5. Install equipment and ducts according to design specifications, using installation requirements and procedures from the Uniform Mechanical Code, the Air Diffusion Council, SMACNA, California Residential Energy Efficiency Standards, and manufacturers' specifications (Title 24); Using these procedures and those in Appendix A, the duct system should be substantially air tight.
    6. Charge the system appropriately, and verify charge with the evaporator superheat method or subcooling method (or substantially equivalent).
    7. Check for proper furnace burner operation and fire-box drafting.
    8. Test the system to ensure that it performs properly by determining (1) that the system is properly sized, (2) it does not leak substantially, and has either (3a) proper air handler fan flow, and proper plenum static pressures, or (3b) proper room and return air flows, and proper plenum static pressures. (Procedures are detailed in Appendix A.)

• Require Submittal of HVAC Sizing Calculations

  • City of Austin (Texas) Energy Code amending 2006 IECC (applicable to all climates):
    • 403.6.1 Documentation of Heating and Cooling Equipment Sizing. Documentation verifying the methodology and accuracy of heating and cooling equipment sizing shall be submitted with the final mechanical code compliance package on the jobsite. Documentation shall include the following information:

      a. Address of residence
      b. Name of individual performing load calculations.
      c. Name and version of load calculation software.
      d. Design temperatures (outdoor and indoor) according to the Air Conditioning Contractors of America's (ACCA) Manual J, ACCA Manual N, American Society of Heating, Refrigeration and Air-Conditioning Engineers, U.S Department of Energy standards, or other methodology approved by the City of Austin.
      e. Area of walls, windows, skylights and doors within +/- 10% of architectural plans or actual building.
      f. Orientation of windows and glass doors, infiltration rate, duct loads, internal gains, insulation values, and Solar Heat Gain Coefficient of windows.
      g. Heating and cooling load calculations.

• Require Testing of Air Balancing of HVAC Systems

  • City of Austin (Texas) Energy Code amending 2006 IECC (applicable to all climates):
    403.8 Air Balancing of Ventilation System. Volumetric airflow in cubic feet per minute (CFM) shall meet the design/application requirements. Airflow testing shall be performed by an independent third party technician approved by the building official, with all interior doors closed and all blowers turned on at cooling speed. Documentation shall be provided verifying that: individual room supply airflows are within +/-20% of the design/application requirements; that the pressure difference between each room and open area adjacent to that room does not exceed 3 Pascals; and that the pressure difference between the open area and the outside does not exceed -3 Pascals. Documentation shall also verify that total system CFM airflow is within +/- 10% of 360 CFM airflow per ton of air conditioning installed.

    All documentation shall be submitted with the final mechanical code compliance package on the jobsite.

    Batch testing shall be allowed per attachment 1.

    Documentation shall include the following:
    

    a. Address of building
    b. Name and company of technician performing the testing
    c. Date of final test
    d. Procedure used for the test
    e. Results of room-by-room airflow tests, including design/application CFM airflow required, design/application CFM airflow required as a percentage of total CFM airflow required, actual measured CFM airflow, actual percentage of total CFM airflow measured, and percentage of design/application CFM airflow required actually attained.
    f. Results of room-by- room pressure tests, including Pascal difference between room and open area adjacent to room and between open area and the outside.
    g. Results of static pressure test (See 403.8.1) and manufacturers' blower data table identifying total rated CFM airflow.

    Measurement of room airflow may be by one of the following procedures:
    

    a. Flow hood used per the manufacturer's instructions
    b. Traverse with anemometer (hotwire or rotary) used per manufacturer's instructions.
    c. Pitot tube and slant manometer procedure as specified by the Associated Air Balance Council, National Environmental Balancing Bureau, or the American Society of Heating, Refrigeration and Air Conditioning Engineers.

    Exception:
    Existing construction with no modification of or addition to the existing ductwork.

**Image 1 and Image 2 courtesy of Building Energy Codes Resource Center. Image 3 courtesy of U.S. EPA Energy Star.