Who Needs A Load Calculation?

JADE Learning HVAC load calculations

By: Reggie Hucks | Jul 02, 2019

There is an old joke among HVAC contractors. When bidding on a replacement system for a customer, step back 100 feet from the house and hold up your hand. If three fingers will block the view of the house, install a three-ton unit. If it takes four fingers to block the view, then install a four-ton unit. That meager attempt at humor does highlight an issue that has been kicked around since the beginning of the industry. Are load calculations necessary? If so, how do you properly size a heating and air conditioning system for a house or commercial building?

First, let’s discuss the necessity of a load calculation. A building that has an air-conditioning system sized by a “guestimate” or a rule of thumb method can cause indoor discomfort and air quality problems. For example, a grossly undersized system will never remove the latent and sensible heat properly. This can lead to undesirable indoor conditions. The system will run continuously but never reach the appropriate comfort conditions and temperature. An oversized system will turn on intermittently but never remove the moisture in the air. This leads to indoor discomfort and possible mold issues. Of course, the mold problems depend on where the building is located. For example, this could be a bigger problem in Florida than Arizona.

Accurately sizing an air conditioning system requires a load calculation. A load calculation is a method of determining the heat gain and loss of a building. Over the years many approaches have been taken. Most contractors, however, have historically referenced the Air Conditioning Contractors of America (ACCA) Manuel J as a basis for residential calculations. Back in the day, a calculating form was used with values out of the manual to create a mathematical calculation. All values were looked up and written on the form. This form was a precursor to today’s spreadsheets. Obviously, this took a lot of time when bidding on jobs. Later, forms that only considered a block load (whole House) were tried as a shortcut. This made the process somewhat easier but still time-consuming.

In the 90s, the personal computer started to rule the world. As the widespread use of PC’s gained acceptance, programmers developed solutions that made load calculations less laborious. The HVAC estimator could size a system in half the time it had been taking. Nevertheless, it took a while to enter all the factors and measurements to get an accurate result. Times were changing, and a fast-paced world wanted something even quicker.

In the 2000s, the dawn of the tablet arrived. Apps were developed to make the load calculations faster, but sometimes a little less accurate. Whenever shortcuts are taken, the results can swing widely. Over time, the apps became more sophisticated due to the integration of systems and data. It’s important to note that a good load calculation is about entering good data. The old phrase “garbage in equals garbage out” applies here too.

Today whole house load calculations can be done in less than five minutes, giving the contractor better profit and cost knowledge when they close a sale. Additionally, it can be demonstrated to the customer that properly sized equipment is what they are paying for.

As more applications are developed, systems that reference municipal tax data, google maps, and similar sites could make this process even easier. Simply highlighting a house on a google map could offer a contractor the information he or she needs to deliver an estimate. As we grow ever more cyber sophisticated, technology is empowering contractors with tools that are quick and much more accurate than the hand and finger method!


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