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The First Step In The HVAC Design Process – Accurate Heating and Cooling ACCA Manual J8 Load Calculations
9th September, 2014

The first step of HVAC system design for a builder, HVAC contractor, and other trade partners is to understand the importance of properly calculating peak heating and cooling loads. Energy efficiency, occupant comfort, indoor air quality, and building durability are dependent on accurate load calculations. While a complete HVAC design involves much more than just the load calculation, the load calculation is the first step for a successful heating and cooling load. The loads acquired by the heating and cooling load calculation process dictates not only the equipment selection, but also the duct design to deliver conditioned air to the rooms of the house. Air Conditioning Contractors of America (ACCA) publication Manual J Residential Load Calculation Eighth Edition (ACCA MJ8) also incorporates American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) rules in its methodology. Single family detached dwellings, low-rise condominiums, and townhouses use ACCA MJ8 analysis. The accurate heating and cooling loads are used to right-size the equipment with ACCA Manual S Residential Equipment Selection, then to design the air distribution system and ductwork with ACCA Manual T Air Distribution Basics for Residential and Small Commercial Buildings and ACCA Manual D Residential Duct System Procedure. There are key parameters required to create accurate heating and cooling load calculations. Often inaccurate adjustments are applied to the HVAC design process. These various defaults and arbitrary safety factors can lead to significant increases in the load estimate. Inaccurate adjustments or ignoring critical inputs of the load calculation will result in inaccurate results. ACCA MJ8 view of safety factors: "Manual J calculations should be aggressive, which means that the designer should take full advantage of legitimate opportunities to minimize the size of estimated loads. In this regard, the practice of manipulating the outdoor design temperature, not taking full credit for efficient construction features, ignoring external window shading devices and then applying an arbitrary "safety factor" is indefensible.” “No additional safety factors are required when load estimates are based on accurate information pertaining to the envelope construction and duct system efficiency. Large errors are possible if there is uncertainty about insulation levels, fenestration performance, envelope tightness or the efficiency of the duct runs installed in the unconditioned space. What happens when inaccurate adjustments are applied to the heating and cooling load calculation process? Something which seems quite minor such as changing the outdoor/indoor design conditions can result in exaggerated loads. Making more than one adjustment only increases the inaccuracy of the calculation results. The results of the combined manipulations to outdoor/indoor design conditions, building components, ductwork conditions, and ventilation/infiltration conditions produce significantly oversized calculated loads. Not only will this oversizing impact the heating and cooling equipment costs, but duct sizes and numbers of runs must also be increased to account for the significantly increased system airflow. Oversizing the HVAC system is detrimental to energy use, comfort, indoor air quality, building and equipment durability. All of these impacts derive from the fact that the system will be “short cycling” in both heating and cooling modes. To reach peak operational efficiency and effectiveness, a heating and cooling system should run for as long as possible to address the loads. Short cycling limits the total amount of air circulating through each room, and can lead to rooms that do not receive adequate duration of airflow. In the cooling season in humid climates, cold clammy conditions can occur due to reduced dehumidification caused by the short cycling of the equipment. The system must run long enough for the coil to reach the temperature for condensation to occur and an oversized system that short cycles may not run long enough to sufficiently condense moisture from the air. Excess humidity in the conditioned air delivered to a space may lead to mold growth within the house.

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Tags:  Acca Manual J Load Calculations - Acca Manual S - Acca Manual D Duct Designs

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