How Do You Size a Commercial HVAC System?

Commercial HVAC / July 22, 2021

Did you inherit a legacy HVAC system during your last office move? Or is your current HVAC system subject to frequent temperature control issues and breakdowns? If so, it may be time for an upgrade. Selecting the right-size HVAC unit is the first step in preempting future performance issues. But how do you size a commercial HVAC system correctly?

How to properly size a commercial HVAC unit involves basic math skills and some knowledge of relevant HVAC principles. Today we’ll review which factors weigh into sizing your HVAC system, the calculations required to do so, and why correct sizing is crucial to the long-term functionality of your system.

How To Estimate the Commercial HVAC Size You Need

Size matters! Determining your HVAC sizing is critical for energy efficiency, performance, and controlling installation costs. When it comes to HVAC sizing, remember the Goldilocks rule: not too small and not too large. “Just right” is the goal of creating a comfortable environment.

HVAC systems are measured in tons or tonnage. This is not a reference to the system’s weight, but to how many Btus (British thermal units) a system can produce.

In short, a Btu is the amount of energy required to lower or raise the temperature of a pound of water by one degree Fahrenheit. Therefore, determining your unit’s tonnage involves calculating how many Btus are needed to generate the amount of energy required for heating and cooling.

Size Your Commercial HVAC System in Five Steps

Calculate the square footage of the space you want to cool. Note: If your ceilings are higher than eight feet tall, the following calculations may need to be adjusted.
Divide the square footage area by 500.
Multiply the result from Step 2 by 12,000.
Add 380 Btu for each building occupant, plus 1,200 Btu for each kitchen and 1,000 Btu for each window in the space.
Convert the result from Step 4 to tons by dividing it by 12,000. This final number will indicate the HVAC system size you will need for your building.

For example:

If you have a 4,500 square foot office space, divide 4,500 by 500 to get 9.
Multiply 9 by 12,000 to get 108,000.
If 12 people work there, add 4,560 (12 x 380 Btu).
If the space has eight windows, add 8,000 (8 x 1,000 Btu).
If the space has one kitchen, add 1,200 (1 x 1,200 Btu).
Divide 121,760 by 12,000 to get a 10 ton HVAC system.

Other HVAC Sizing Considerations

The Air Conditioning Contractors of America (ACCA) has published numerous manuals detailing the sizing and design of air conditioning systems. For example, one manual, Manual N, instructs that there are four considerations in determining the correct HVAC equipment for any commercial building:

Application: Is the space an office, restaurant, grocery store, or retail outlet?
Building Type: Is the space a single-story building, multi-story building, a warehouse, or another building type?
Square Footage: What is the size of the space to be heated or cooled?
HVAC Equipment Type: Does the building use gas or electric heat? Will it be a water-cooled or an air-cooled system?

The ACCA’s Manual N also considers building construction materials, including:

What are the inside and outside walls made of?
What is the insulation level between walls?
What are the roof and floors made of?
What kind of windows does the building have?
How many people will work in the space?
What types of electronics, lights, and plants are present, and how many of each?

What’s Load Got To Do With It?

In addition to these calculations and considerations, commercial HVAC system sizing also depends on the loads generated in the building. The “load” is the amount of heat your cooling system must remove (or the amount of cold your heating system must remove) to maintain a constant temperature.

Loads are divided into external loads and internal loads, and you should consider both when sizing your HVAC system.

External loads result from weather conditions that bring heat and cold directly into the interiors, from weatherization, and as a result of the building’s design.
Internal loads result from internal factors like people, lighting, equipment, and fresh air.

Load is balanced with your HVAC system’s capacity, which is the amount of cooling or heating a system can produce at maximum effort.

Why is it Important to Choose the Correct HVAC Size?

The size of a commercial HVAC system directly affects its cost, performance, and maintenance issues. Therefore, it’s vitally important that you choose the correct HVAC size when installing a new heating, ventilation, and cooling system to keep people comfortable and protect your investment.

What Happens When Your HVAC Is Too Large?

An HVAC unit that’s too large for your space can cause poor air quality and excess humidity, leading to mold generation, asthma risks, and general discomfort. Oversized HVAC units also contribute to frequent maintenance calls, energy waste, increased wear and tear, and higher installation costs, not to mention higher initial unit costs.

What Happens When Your HVAC Is Too Small?

Conversely, HVAC units that are too small obviously can’t cool the space efficiently. Higher energy costs, increased wear and tear, and slow cooling result from the compressor being pushed past its limits.

HVAC Estimation DOs and DON’Ts

DO ask about ductwork and components that will provide the most efficient setup.
DON’T assume you’ll be replacing an older HVAC unit with the same size unit. New energy efficiencies can mean you could get by with a smaller system.
DO ask a licensed professional if you’re unsure about what you need.
DON’T assume bigger -- or smaller -- is better. Every situation is different.

Need Help With Commercial HVAC? Call FASTECH

Whether you’re replacing an old unit or starting anew, a commercial HVAC unit is a significant business investment that should be taken seriously. If you’re not confident sizing your commercial HVAC system, you’re not alone.

The HVAC technicians at FASTECH are here to help. We provide expert commercial HVAC system installation, maintenance, and upgrades that save money and extend the lifespan of your HVAC system.

FASTECH is the leading provider of end-to-end commercial HVAC and refrigeration services. Over the last 25+ years, our expert team has provided cooling and refrigeration services for thousands of clients across nearly every industry, including retail, restaurants, pharmacies, service stations, and more.