Part 1: Introduction
This is first of several discussions in a series on factors impacting residential HVAC design. Manuals J, D and S, the ACCA/ANSI Standards for Residential HVAC Design, are recognized by virtually all current Energy, Mechanical and Building Codes, DOE, RESNET, BPI, EnergyStar®, and numerous other energy efficiency focused organizations. The International Code Council codes adopted in Utah specifically identify these Standards for the design and installation of residential HVAC systems in Utah.
These Standards are introduced here, with future discussions addressing details in each Standard. With thermal envelope improvements found in current building codes and the demand for more efficient homes, the proper design and installation of HVACs is critical. Improved insulation and air sealing reduces the winter heat loss and summer heat gain, which directly lead to HVAC system size reductions. Unfortunately, this creates fears concerning system under-sizing, as too many continue to believe ‘bigger is better’. Also misunderstood is, as air leakage is reduced, whole house ventilation becomes a critical necessity.
Understanding and applying these HVAC design Standards is vital as homes are built many times more energy efficient than homes built in the past several decades, coupled with an ever-increasing demand for occupant comfort. Consideration must also be given to energy efficiency improvements made in existing homes. We must not look at a home as four independent components, i.e., structure, plumbing, electrical, and HVAC system, rather an integration of all into a safe, healthy, aesthetically pleasing, durable, efficient, and comfortable place we call home.
As you review this and future discussions. please reach out with questions or concerns you may have on this subject, so I can address those issues for all. Also note, this series of discussions is not intended to be a comprehensive training on J, D and S, rather focusing on current common questions and challenges.
The Air Conditioning Contractors Association of America (ACCA) introduces each Manual as follows:
ACCA MANUAL J – Residential Load Calculation
Achieving occupant satisfaction is the principal goal of any HVAC design. Primary factors impacting occupant satisfaction include filtration, temperature and humidity control, air motion in the room, adequate ventilation, interior zoning needs and energy efficient operation. Occupant satisfaction is maximized when the heating and cooling system and equipment are the correct type and size, and the air distribution system is properly designed and installed.
For residential applications, ACCA’s Manual J, Eighth Edition (MJ8™) is the only procedure recognized by the American National Standards Institute (ANSI) and specifically required by residential building codes. Methods not based on actual construction details, nor founded on relevant physical laws and engineering principles, are unlikely to result in correct equipment sizing.
ACCA MANUAL S – Residential Equipment Selection
Occupant satisfaction is maximized when the heating and cooling equipment are the correct type and size to meet the capacity requirements from the Manual J load calculation. For residential equipment selections, ACCA’s Manual S®, is the only procedure recognized by the American National Standards Institute (ANSI). If the Manual J load calculation is done, then the next step is to select the equipment that will deliver the necessary heating and cooling.
Undersized equipment will not meet the customer’s comfort requirements at the design specifications. Oversized equipment generally requires larger ducts, increased electrical circuit sizing and larger refrigeration tubing. These cause higher installed costs and increased operating expenses. The temperature may feel right at the thermostat but the temperature in other rooms will suffer from the oversized equipment going through short operation cycles. Short cycling can cause temperature swings as the equipment over-conditions, stops, then over-conditions, etc…
ACCA MANUAL D – Residential Duct Design
Achieving occupant satisfaction is the principal goal of any HVAC design. For residential air duct designs ACCA’s Manual D is the procedure recognized by the American National Standards Institute (ANSI) and specifically required by residential building codes. Air is the first word in air conditioning. If the network of ducts carrying the air is not properly designed then the health and safety of the occupant are at risk, the equipment could fail more quickly, the energy costs could rise, and occupant comfort might be sacrificed.
Please consider the following partial list of issues to be discussed. Notice how many are related to air flow issues.
- Design conditions based on location and climate historical data.
- Incorrect data is often used in the load calculation; specifically, window U-factors and insulation R-values.
- Builders along with subcontractors fail to build and insulate per the plans, energy code compliance methods including REScheck, or load calculations. Too often, the thermal performance values and factors are inconsistent across construction documents.
- Windows, doors, skylights – size, frame materials, performance ratings, orientation (solar gain), and shading.
- Internal gain – heat we produce internally.
- HVAC contractors might blame deficiencies in an HVAC system on the code ‘requiring a smaller system’, while in reality; the failure to correctly design and install the duct system is the single greatest issue with air conditioner performance. Poor fitting design and failure to account for all devices located in the airstream are the issue.
- Improper installation of flex duct products.
- Improper insulation of duct outside the thermal envelope.
- Leaking duct systems.
- Poor airflow remains the number one issue energy raters are finding as they test duct systems for EnergyStar® and other above code programs. Rarely do they find a system moving the minimum 400 CFM per ton required for efficient cooling in our climate at our altitudes.
- Manufacturers expanded performance data: Low airflow equals reduced sensible capacity, while increasing latent capacity, and conversely, high air flow increases sensible capacity and reduces latent capacity. In our dry climate, higher sensible capacity is our goal, where 450-500 CFM per ton provides improved performance.
- Pressure drops across filters and coils are often ignored, again producing lower airflows.
- Humidity, too little or too much. Moisture condensing in floors, walls, and roofs.
- Building tightness and code triggers for whole house ventilation.
- Whole house ventilation methods.
- Open combustion verses direct vent fuel burning appliances and the impact on whole house comfort and efficiency.
A final comment. Most residential HVAC contractors unfortunately lack experience and formal or in-house education on these subjects. The code is pushing them to comply with something they do not fully understand. Compounding the issue is the inspector, who along with trying to ensure compliance with the building, plumbing, electrical, and mechanical codes are attempting to understand the complexities of J, D and S. Please consider obtaining a complete set of J, D, and S, for personal reference and study.
Please reach out to me with your questions, concerns, or suggestions.