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HVAC Sizing: Why do a load calculation?

A whole new vocabulary greets those who want to purchase a new HVAC system. Instead of seeing the normal housing “square feet” metric, people are likely to see measurements in “tons.” These are not the same tons that semi-trucks are hauling on the freeway. It's a measurement of the cooling rate, and it's about equal to 12,000 Btu/hr. To properly size an HVAC system, the buyer needs to determine how the home or office building loses and maintains heat.

Since HVAC may seem like a complicated new world, new HVAC buyers might be tempted to use a 'rule of thumb' to determine the size of their HVAC system. These “rules” can be as simple as holding up a cut-out to the home. The hole that the house fits within is labeled with a ton amount. Other people will recommend a certain number of tons per square foot. This method is slightly better because it takes into account the weather conditions for the local area. However, there are disadvantages to using a rule of thumb.

Using a 'rule of thumb' is like going to the store and picking out a shirt in your size without trying it on. If you are usually a medium, it's a good bet that a medium shirt will fit you. But, in some stores, a medium can be too tight to fit around your chest or so loose that the shirtsleeves go down to your elbows. Without accounting for the unique contours of your body by trying it on, it's possible that you can buy the wrong size. An improperly sized HVAC system is like an ill-fitting shirt; it does the job, but it doesn't do it as well as it could. For example, an over-sized HVAC system doesn't de-humidify the air as efficiently during the summer.

The secret to purchasing the right size of HVAC system is to take more factors into account. Like the unique contours of your body, your home or business has unique characteristics. For a building, the unique characteristics are the materials used for the construction. Are the walls made of brick or siding? What type of insulation was used? How many vents are in the floor? The doors are made of what type of material? These characteristics (and others, such as your location) are used in a Manual J load calculation, an industry standard method to determine the HVAC size for a home or office building.

With the internet, it is possible to do a Manual J load calculation yourself. There are many sites dedicated to the task. However, they do assume that you have the basic information about the materials used in the construction of the building. Many homeowners don't know what type of siding or insulation they have (or where to find it). If you don't know what ceiling type you have in your home, it's probably worth asking a professional to complete a calculation (according to Manual J or another method). A trained technician can quickly assess the location.

An HVAC is a long-term investment, often heating and cooling a location for decades. Isn't it worth it to get the right size for maximum efficiency?

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