Sizing an HVAC system is never an easy job. Even for relatively small structures, such as residential houses, there are a variety of factors that need to be considered. Air conditioning units that are too small will struggle to cool a given space, run inefficiently and use more energy, and ultimately fail much sooner. On the other hand, systems which are overmatched will cost drastically more than is necessary upfront and will still operate inefficiently due to more rapid cycling. Sizing an AC system correctly—finding the goldilocks zone that is neither too big nor too small—requires consideration of a large number of factors.

Square Footage Isn't the Whole Story

It's tempting to look at the overall square footage of the area that needs to be cooled as the determining factor for the HVAC system capacity, but this is nothing more than a starting point. Instead, the primary concern for any HVAC system is load. Keep in mind that an air conditioning unit works by removing warm air and ejecting it to the outside environment. The load on the system is essentially the amount of heat inside the building that the system's refrigerant will be forced to absorb.

Where does this heat loading come from? For an average commercial structure, the majority of this is environmental. If the outside air is warmer, then the interior will eventually heat up. There are often compounding factors, however. For buildings with a large amount of computer equipment, for example, interior heat load can sometimes greatly exceed the amount of heat imparted by the outside environment. Smaller internal sources, such as light bulbs or even people, can contribute to the load placed on the system as well.

There's No One-Size-Fits-All Rule

Now that you have a basic understanding of what the average commercial system is up against, it should be clear that there's no simple rule that can tell you the exact size of the system you need. Instead, it depends largely on the unique factors of your particular building. You can use a basic BTU calculator to arrive at a starting point, but keep in mind that this is only a starting point. It will give you a rough approximation of how much cooling is required if only the most basic factors are taken into consideration.

Once you have this initial estimate, ask if any of these questions are true for your facility:

• Are commercial kitchens present?
• Is the load from occupants highly variable (i.e., customers entering and leaving the building)?
• Is there significant thermal loading from equipment, such as computers or manufacturing gear?
• Are your fresh air circulation requirements higher than normal?
• Is there an unusually large load from lighting, such as in a retail space or a warehouse?

It's easy to underestimate the large effect that these loads can have on an HVAC system. In fact, they can easily move your requirements up by several tons.

Don't Neglect a Proper Heat Load Evaluation

As should be obvious by this point, cooling a large commercial space is a challenging technical problem. If you are installing or upgrading a new commercial air conditioning system, then consulting with a professional commercial AC technician is an absolute necessity. The technician will be able to evaluate the unique circumstances of your business and perform a detailed thermal load calculation. This will allow you to select components for your AC system that will land right in the middle of the goldilocks zone, keeping your building cool without overheating your budget.