August 12, 2019

How to Expertly Size an Air Conditioner

It’s the height of summer and most of North America is sweating away under intense heat, so let’s talk about cooling things down. Way, way down. More specifically, let’s talk about an essential skill every HVAC specialist likely doesn’t think twice about: His or her ability to properly size an air conditioning system.

Before reading any further, ask yourself this question: When you visit a site to assess how much cooling the space requires, do you rely on intuition or a “rule of thumb” to guide your decision?

If the answer is anything short of no, you could be creating wildly inefficient air conditioning systems and, by extension, wildly unhappy clients. So, let’s turn that around and look at some of the hidden factors that go into properly sizing an air conditioner.

Ditch the “rules of thumb”

The most common misconception in the industry is that every 1000 square feet requires one ton of cooling. Even if you only take it as a ballpark or starting point for your assessment, this rule of thumb can lead to serious miscalculation.

Why? Because space is far from the only factor that impacts cooling. It’s not even the most significant.

For every person who occupies the space, expect to add another 400 BTUs to your calculation. For every window that faces sunlight, expect to add another 1000 BTUs. And for every kitchen, expect to add an additional 1200 BTUs.

Environmental factors also play an enormous part in AC sizing and performance. This includes everything from geography and building materials to furniture and electronic devices. Even the ducting, roof vents, and HRVs a building uses will impact the sizing needs and performance of an air conditioning system.

When assessing the cooling needs of a site, HVAC technicians need to account for unexpected sources of heat, such as pilot lights, fireplaces, major appliances, TVs, and lighting. Taken individually, they may not seem like much. But the heat these devices contribute to a space adds up.

Take the time to do a manual J calculation

As HVAC specialists, technicians, and installers, we tend to get very busy. That’s why the urge to rely on intuition and rules of thumb is understandable. And while this can save time, it won’t lead to happy clients and it won’t lead to efficiency.

The best and only really accurate way to assess the cooling needs of a site is to perform a manual J heat load calculation. This takes time, to be sure, but it allows you as the specialist to make a highly accurate and precise recommendation to your client.

A manual J calculation takes into account a long list of factors, including the building’s orientation to the sun, its geographic location, and local climate. It also calculates the thermal properties of each wall, floor, ceiling, door, and window, as well as assesses the building envelope, duct leakage, and more.

In addition to calculating the amount of heat gained in a building during the summer months, a manual J calculation also estimates the amount of heat lost in a building during the winter months, making this a valuable tool to engage with the client on their heating needs later in the year.

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