A load calculation, also known as a “Manual J” is a methodology developed by ACCA (Air-Conditioning Contractors of America). The method is used to calculate the estimated amount of heat your home gains in the summer and loses in the winter. The movement of heat into and out of a building determines the size, or capacity, of a heating and cooling system. Without a Manual J, there is no way to correctly size a furnace or air-conditioner, and the contractor would simply be guessing. As a paying client, you should always ask for a heating and cooling load calculation be performed.
Our Director of Training Explains:
In order for occupants to feel comfortable, a furnace or heat pump must be able to produce an equal amount of heat as the home loses. The same is true for air-conditioners and heat pumps but in reverse (heat pumps heat and cool) during the sweltering summer months. Every home will gain and loss heat at different rates and in different amounts. Many factors contribute to movement of heat into and out of a building. The orientation of the home relative to the sun, the number and size of windows, performance of windows including insulative value, solar heat gain, and air-tightness, type and amount of insulation on all six sides of a building, building surface to volume ratio, the ventilation rate of the home, and the amount of air-tightness.
Changing one factor, such as windows, can have a significant impact: compare the amount of heat gained during the summer in a home with several large windows oriented predominately to the west, versus a home across the street with windows even distributed around the home. The first home will pick up much more heat in the summer, particularly in the afternoons and evenings as compared to a neighbor. The differences can be significant and easily felt by occupants. The heating and cooling loads for each of these homes would therefore be significantly different requiring different sized equipment.
Benefits of a Correct Manual J
- Accurately identify the unique heat movement fingerprint of the home
- Correctly size heating and cooling equipment to prevent oversized and undersized systems
- Identify low load buildings and corresponding challenges and unique design requirements
The only method to correctly estimate heat gains or losses in a structure is by completing an ACCA compliant Manual J load calculation for each individual home. In fact, current building codes require heating and cooling load calculations as the single approach to properly size HVAC equipment, specifically, 2018 IRC (International Residential Code) section N1103.7, and the 2018 IECC (International Energy Conservation Code) section R403.7. In an effort to enforce energy conservation and affect climate change in a positive way, some jurisdictions in the Kansas City region are beginning to require a Manual J as part of the permitting process, but not all.
Some reasons why load calculations are not performed
- The permitting jurisdiction does not require a Manual J, therefore the contractor chooses not to perform the calculation although load calculations are a best practice for the homeowner.
- The contractor does not understand building science and the numerous factors that contribute to correct equipment sizing.
- Ignorance: the contractor simply does not have the training to perform the necessary calculations.
- Lack of Resources: the contractor does not own the software or have the personnel or time to perform the site assessments and calculations.
- The contractor has used rules of thumb and no one has complained before (unless the situation is serious)
- Contractors are not pulling permits in jurisdictions that require a Manual J.
Home Performance Group Heating and Cooling Services in Kansas City
If you are in the market for a replacement air conditioner, heat pump, furnace, or whole system, make sure your contractor is trained and experienced in performing Manual J load calculations. Be prepared if they ask you to pay for it: Manual J Load Calculations are time-intensive and require several hours or more to properly document all the necessary information and complete the calculation and analysis.
At Home Performance Group, we continue to invest in technical training and up-to-date software so that we can correctly size our customer heating and cooling systems. We have performed hundreds of calculations for our clients and prospective clients. Our in-home consultations are no cost and include a copy of the load calculation with a system purchase. Alternatively, if you choose not to buy from us, we can provide you the load calculation report for a nominal fee.
If you are interested in a no cost in-home consultation, schedule with a Solutions Advisor today.
Article by Larry L. Motley Jr., 24 May 2021
Larry is a graduate of both Wentworth Military Academy and Missouri Western State University earning a double bachelor’s degree in Economics and Finance. Additionally, he maintains six professional tradesman licenses in two states and advanced credentialing in green technology, project and program management, and process improvement. Larry is a three-time combat veteran having served in Operation Iraqi Freedom, Operation New Dawn, and Operation Inherent Resolve. He continues to serve through a value-based building science company focused on providing clients the best design, highest quality installation, and most honest repair services in the community.
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