How do you size a commercial HVAC system?

Purchasing a new heating and cooling unit for a home is one thing, but purchasing one for a commercial building is quite another. Just like with buying an air conditioner for a house/residential property, there are certain things that are similar, such as brand and price.
But when purchasing a commercial unit there are other things like the efficiency, equipment type, and of course, size that are equally, if not even more important in the decision making process.

While I could write volumes on the difference that brand, price, and efficiency make for HVAC systems, I’ll narrow my focus and discuss specifically about what I believe to be the most critical – commercial HVAC sizing.

With most of the country on stay-at-home orders, it is more important than ever to make sure that our immediate environments are safe.

First, you may be asking what do I mean by “size” and “sizing” anyway? Well, in the heating and air conditioning world, we measure size by tons or tonnage. This doesn’t mean how much the air conditioner or heater actually weighs (though they can weigh quite a lot!), but rather how many BTU’s or British Thermal Units the machine is capable of producing.

Just as a refresher from our high school science classes, a BTU is the amount of energy that is required to raise or lower the temperature of one pound of water by 1 degree fahrenheit. 12,000 BTUs equals one ton of heating or cooling.

While most residential HVAC units are between 1.5 to 5 tons in size, commercial systems can range from 6 tons to 50 tons or more! With such a wide range of sizes available, we need to know what unit to select for our building and there are a few ways we can do this.

The simplest method to sizing commercial HVAC systems is to simply replace the existing equipment “like for like”, meaning that if a 7.5 ton gas pack was installed on the roof of the building for the clothing store, then we install a new 7.5 ton gas pack for the store.
Those are definitely the easiest ways to size commercial HVAC equipment, but by no means are they the most precise, as a lot of assumptions are made. The main assumptions are:

  • Was the load sized right to begin with?
  • Has the building always been a clothing store?
  • Is 600 square feet to the ton right for this application?
  • Fortunately, just like thermodynamics are a firm science, so too is load calculation. Enter ACCA.

ACCA, The Air Conditioning Contractors of America have published more than a dozen Manuals detailing the sizing and design of all manner of air conditioning systems. There are even courses taught on the various “Manuals” like Manual N for commercial load sizing and Manual Q for commercial duct design.

What Manual N teaches us is that there are four main considerations to determining the right HVAC equipment for any given commercial building. They are:

  • Application – Whether the space is a restaurant, office space, retail outlet or grocery store
  • Building Type – Is the space a warehouse, a single or multi-story building or some other construction type?
  • Square Footage – How big is the space that is going to be air conditioned or an amount of heat added to it?
  • HVAC Equipment Type – Is the building going to have gas heat or electric heat? Will it be an air cooled or water cooled system?

But wait, there’s more! Manual N also takes all the building construction materials into the equation too. Everything is looked at, and it all adds up. Things like:

  • What are the outside walls are made of?
  • What are the inside walls are made of?
  • What is the level of insulation between the walls? (measured in “R” value.)
  • What are the floors are made of?
  • What kind of windows are installed?
  • What is the roof made of?
  • How many people are in the space?
  • How many, and what types of lights, electronics and plants are present? Yes, even plants add up!
properly working ac cooling process

Once all of the above items are factored into a Manual N calculator and the calculation is done correctly, the program will tell us what the heating and cooling BTU requirement is for the building.

The actual calculation is very involved and labor intensive. While when Manual N first came out it was done by hand, now the HVAC industry uses one of several load calculation programs to do all of the many complex calculations and expedite the process. That said, the time involved to do a comprehensive load calculation for an entire office building can still take the better part of a day!

Based on that we are trying to size the load for a commercial HVAC system, we can see that 600 square feet to the ton is not the correct calculation we should be using. Instead, depending on the application, we could be between 50 to 400 square foot to the ton – quite a difference!

This is by far the most accurate of all the various methods for determining what size a commercial HVAC system should be.

The best way to know for sure if your building has the correct equipment installed, have an experienced HVAC technician perform an evaluation on your heating and cooling equipment. Carolina Comfort offers a full suite of commercial services including maintenance, repairs and replacements.

It doesn’t matter what time of day it is, or even what day it is. When your AC unit needs repairs right now, then our emergency HVAC technicians are ready 24/7 to help. If you have an AC emergency that just can’t wait, then call us to get the best emergency service in South Carolina. For more information, please call us at 803-794-5526


5636 Bush River Rd.
Columbia, SC 29212
SC License – #M104545
NC License – #32356

Phone: (803) 794-5526