Strategy Guideline: Accurate Heating and Cooling Load Calculations

Strategy Guideline: Accurate Heating and Cooling Load Calculations

Strategy Guideline: Accurate Heating and Cooling Load Calculations
Strategy Guideline: Accurate Heating and Cooling Load Calculations

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The Strategy Guideline: Accurate Heating and Cooling Load Calculations report provides information for the following groups:
• Heating Ventilation and Air Conditioning (HVAC) Mechanical Contractors
• HVAC System Designers
• Builders
• House Remodelers.
This guide can be used as a point of entry for collaborative discussions between a builder, HVAC contractor, and other trade partners to understand the importance of appropriately calculating peak heating and cooling loads, as the first step of HVAC system design.
Accurate load calculations have a direct impact on energy efficiency, occupant comfort, indoor air quality, and building durability. The load calculation is the first step of the iterative HVAC design procedure, as a full HVAC design involves much more than just the load calculation. The loads modeled by the heating and cooling load calculation process will dictate the equipment selection and duct design to deliver conditioned air to the rooms of the house. This guide references the methodologies of the Air Conditioning Contractors of America (ACCA) publication Manual J Residential Load Calculation Eighth Edition (ACCA MJ8), which in turn references information provided by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). ACCA MJ8 only applies to single family detached dwellings, low-rise condominiums, and townhouses.
This guide is not a new method for performing load calculations, a substitute for established methods of performing load calculations, or step-by-step instructions on how to perform the load calculations. This guide presents the key criteria required to create accurate heating and cooling load calculations and offers examples of the implications when inaccurate adjustments are applied to the HVAC design process. The guide shows, through realistic examples, how various defaults and arbitrary safety factors can lead to significant increases in the load estimate. Emphasis is placed on the risks incurred from inaccurate adjustments or ignoring critical inputs of the load calculation.

Contents

Forward …………………………………………………………………………………………………………………………………….. v
List of Figures …………………………………………………………………………………………………………………………. vii
List of Tables …………………………………………………………………………………………………………………………… vii
Definitions ………………………………………………………………………………………………………………………………. viii
Executive Summary ………………………………………………………………………………………………………………….. ix
Introduction ………………………………………………………………………………………………………………………………. 1
Baseline Load Calculation …………………………………………………………………………………………………………. 4
Load Components ……………………………………………………………………………………………………………………… 6
Peak Heating Load ………………………………………………………………………………………………………7
Peak Cooling Load ………………………………………………………………………………………………………8
Location – Design Conditions …………………………………………………………………………………….10
Orientation ……………………………………………………………………………………………………………….13
Building Components ………………………………………………………………………………………………..15
Heating and Cooling System Location and Duct Leakage ………………………………………………15
Ventilation/Infiltration ……………………………………………………………………………………………….16
Risks Associated with Oversizing …………………………………………………………………………………………….. 16
First Cost, Energy Costs and Durability ……………………………………………………………………….16
Comfort – Space Temperatures ……………………………………………………………………………………16
Comfort – Humidity Control ……………………………………………………………………………………….16
Load Dependencies …………………………………………………………………………………………………………………. 17
Outdoor/Indoor Design Condition Dependencies …………………………………………………………..17
Building Components Dependencies ……………………………………………………………………………20
Ductwork Conditions Dependencies …………………………………………………………………………….23
Ventilation/Infiltration Conditions Dependencies ………………………………………………………….25
Combined Dependencies ……………………………………………………………………………………………28
Conclusion ………………………………………………………………………………………………………………………………. 30
References ………………………………………………………………………………………………………………………………. 31
Appendix A: Enhancements to Scope for Mechanical Contractor/HVAC Manual J Designer ………. 32
Appendix B: ACCA Manual J8 Input Checklist ………………………………………………………………………….. 33

Strategy Guideline Accurate Heating and Cooling Load Calculations

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