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Manual J Load Calculations

Posted by AC Contractor on June 17, 2013 at 10:30 PM

If you're researching a new central AC installation, there is a good chance you heard someone mention the importance of having a manual J load calculation. Not only is it a good idea, but in many states like Florida, it is also the law (section So what is manual J? Manual J is a registered trademark belonging to ACCA. It is the specific methods for calculating heating and cooling requirements for single family and mobile homes. In other words, it is the recognized method in determining what size of equipment your house requires. 

So does size matter? The Proctor Engineering Group published a famous article titled Bigger is not Better. Contrary to what some may think, a bigger air conditioner may not provide better cooling. In fact, there a number of problems oversized air conditioners create. There will be increased wear and tear on the compressor. This is due to short-cycling or frequently turning on and off. A large system cools an area too quickly, forcing it to shut down often. Because of the short cycling, there will be increased humidity. While the system runs, moisture is removed from the air. It simply won't run long enough to dehumidify. In addition to higher installation costs, the larger equipment will also consume more energy, resulting in higher utility bills.

We've established the negatives with oversized equipment, but was it a big enough problem it needed to be included in the code? Yes. According to this study by the Florida State University, the state of Florida has had sloppy design and construction practice and as many as 50% of the homes have oversized air conditioners. When you multiply the costs (using our energy savings calculator) by all the homes in Florida, it ends up being a large number.

So how did this happen? Most often, energy efficient upgrades such as additional insulation or reflective windows are not considered, guesses are made on the load, mistakes are made on the calculation or assuming the original design is correct all lead to improper sizing. 

The combination of lower installation costs, lower operating costs, less service calls and better comfort makes equipment sizing a no brainer. We see the value and want to ensure it is passed onto our customers. Melo Air uses ACCA approved Wrightsoft for all our load calculation requirements.

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