How do you calculate commercial HVAC?

You’ve been asked to design a new commercial building and you need to calculate its heating and cooling needs.

How to calculate the correct HVAC tons per square foot

In commercial and residential buildings alike, there is no single rule of thumb to follow.

Rather, your HVAC tons per square foot requirement will depend on the building’s size, design, weather, the stated thermal comfort needs, and other factors.

Before we opt for commercial HVAC installations, we offer a summary for finding the correct HVAC tons per square foot in commercial and residential buildings. We use British Thermal Units (BTU) as a reference point for identifying the heat removal requirements of the building.

What Does it Take to Remove Heat?

Your calculations should start with understanding how much heat typical AC units can remove.

Today’s industry standard is that it takes 1 ton of AC to remove 12,000 BTU of heat per hour.

Most single unit houses require 1 or 2 tons of AC, but in commercial buildings, you will require far greater AC infrastructure.

Determining how much is the tricky part.

If your AC system is too small, then it will take longer for it to remove heat from your building. In practical terms, this means that the AC will run longer and consume more energy. As a result, it will drive up your building’s operating costs.

However, if the AC system is too large, then it will never run at full efficiency and, as a result, will consume more energy than necessary. This too will raise your building’s operating costs.

As noted above, your specific AC needs will depend on multiple factors, such as the building’s design and its location’s weather. This is because buildings retain and lose the amount of heat and cold air at different levels (i.e., some more rapidly than others). Furthermore, you must also factor in how occupants will use the AC system throughout the year.

These are both complex calculations that will result in a peak heating load figure and a peak cooling load figure. You need these figures to ensure your HVAC system runs efficiently.

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How to Calculate HVAC Tons Per Square Foot in Commercial Buildings

You should not use average figures to determine the final size of your air conditioner and heating system, but rather, to get a rough idea of the scope.

Some HVAC designers will calculate the AC requirement by measuring the space they want to cool in square feet. They will then calculate the BTU by dividing the space (in square feet) by 500 and then multiplying its answer by 12,000. They will also add approximately 380 BTU for each person in the space, 1,000 BTU for each window, and 1,200 BTU for each kitchen.

Thus, a 2,000 square foot area with two windows, four regular occupants, and one kitchen would have a BTU of 52,720. This would require 4.39 tons of AC or 455 square feet per ton.

This is close to the 400 square feet per ton sizing of the HVAC industry in Chicago and Orlando, but you will notice the variance due to us adding a kitchen and windows.

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Conclusion: Don’t Restrict Yourself to a General Rule

The industry standard is a benchmark, but not a firm rule. You will have variances in your final calculation based on the building’s unique attributes.

In fact, even the thermal challenges buildings face differ.

For example, buildings in hot climates will get hotter (i.e., the peak heating load), so you might have to focus on removing more than 12,000 BTU an hour to keep it cool. However, buildings in cold environments have a different issue, which is to retain heat, not remove it.

Moreover, as noted earlier, failing to calculate the right size of your HVAC units can cause major operational issues. These include costlier operating costs and, potentially, more frequent HVAC maintenance and repair work.

Additional variables affecting your calculations include the type of HVAC system. For example, with an Underfloor Air Distribution (UFAD) system, you can diffuse cool air closer to people and, as a result, use less energy and require less time to cool the space.

AirFixture helps architects, engineers, and building owners design and implement UFAD HVAC systems to lower their long-term costs and increase their building’s value.

Use our FREE guide to see how UFAD systems will help you propose an HVAC system that meets your budget and timelines.

Whether you’re considering a new commercial HVAC unit or simply upgrading your existing one, it’s crucial that you take into consideration its size and suitability for your building. 

That’s because the size of a commercial HVAC system directly affects its performance, cost, and other maintenance factors. How to properly size a commercial HVAC unit involves intensive number crunching alongside knowledge of important HVAC principles.

How to Size Commercial HVAC

So, what’s the ideal method of how to determine the size of an HVAC unit in a commercial building or project? Let’s take a closer look.

First of all, you must consider factors such as:

• The difference between outside temperature and your desired indoor temperature range

• The building design and the amount of insulation in the ceiling and walls

• Position relative to the sun (for example, is it in a shaded area?)

• Average room size and total number of occupants

• Specific type and amount of lighting in the room (LED vs halogen)

• Activities in the room (cooking? hot baths?)

HVAC sizes are measured in tons - most commercial HVAC systems range between 2 tons and 30 tons. A 1 ton HVAC can remove around 12,000 Btu (British Thermal Units) of heat per hour. You will need to calculate the building's cooling load to know the appropriate capacity of the air conditioner to use.  

As discussed earlier, your building's cooling load depends on several factors such as average external temperature to building design, orientation, and its primary purpose. However, a general rule of thumb is that it takes approximately 25 Btu to cool one square foot.

Here are some steps you can take for the purpose: 

1. First, calculate the square footage of the building space you want to cool

2. Divide the space area by 500

3. Multiply the result by 12,000 to get the exact space cooling load

4. Add about 380 Btu per occupant in the building, and 1,200 Btu for every kitchen

5. Add 1000 Btu to account for solar radiation in windows

6. Convert the number back to tons by dividing by 12,000 to get the size of the HVAC you need for your commercial building

How to Determine the Size of the HVAC unit in a Commercial Building

Read this section to determine how to size commercial HVAC systems and save money. We’ve outlined a rough estimation of the cooling capacity that a commercial HVAC system would need based on data by EnergyStar.gov.

1,000 to 1,200

21,000

1,200 to 1,400

23,000

1,400 to 1,500

24,000

1,500 to 2,000

30,000

2,000 to 2,500

34,000

Air Conditioner Sizing is Critical for Performance, Energy Efficiency, and Low Installation Costs

Bigger isn’t always better - an oversized commercial HVAC has adverse effects on heating and cooling. For example, your office space cooled by an oversized HVAC might have poor humidity control that would lead to occupant discomfort, the risk of asthma, and building mold.  

The economic implications of oversized HVAC units are also significant because it contributes to energy wastage. This will lead to increased wear and tear, high installation costs, and frequent maintenance. 

Similarly, undersized HVAC units are unable to cool the space efficiently. They’ll work harder to produce the requisite amount of cooled air, which means the compressor will be pushing its limits. Expect wear and tear, longer wait times for desired temperatures, and much higher energy costs. 

How to Size Commercial HVAC System

Let’s make your job easier - when it comes to commercial systems, many HVAC professionals prefer to use 1 ton per 5,000 sq foot of floor area as a general rule of thumb. This estimation comes in handy when contractors need a quick reference point of HVAC equipment size. 

The estimation is, however, presumptive of the significant HVAC sizing factors mentioned earlier (from building design, to activity and type of lighting installed).

Heat load and cooling load calculations

Another thing you must take into consideration are heat load and cooling load calculations. That requires precise estimates for heat gain vs heat loss in your building. The procedures published by the Air Conditioning Contractors of America (ACCA) are useful for this purpose.

Heating load calculations help to estimate the amount of heat loss from the building during the coldest climates. In some cases internal heat sources such as occupants, lighting, and building activity can help to compensate for heat loss.

Cooling load calculations are for estimating the heat gain in a building during the hottest part of the day. Internal factors add to the cooling load, similar to what we outlined in the previous sections. 

Heat flow rate calculations

You can also get an accurate HVAC sizing estimation by calculating the space heat gain (the amount of air entering the building) and the space cooling load (the amount of air that needs to be removed).

The space heat gain is the rate at which heat enters the building or is generated within it at a given point in time. For accuracy, you will have to factor in solar radiation through the windows, conduction through the roofs and walls, and heat generated by occupants, equipment, lighting, and appliances.

The ASHRAE Task Group developed a standard procedure for these calculations, known as the transfer function method (TFM). This method simplifies the cooling load and heating load calculations, and factors in all the other determinants that increase or reduce heat gain and heat loss.

The formula is based on conduction transfer functions for the walls, roof, occupants, and glazing and room transfer functions for lights, appliances, and other radiant components. This formula involves complex calculations that require special computer applications, as shown here.

The bottom line: how to size commercial HVAC systems effectively

You can calculate HVAC sizing by following industry best practices for cooling and heating load calculations. For accurate predictions, one must factor in the building's design and orientation, lighting, and type of windows, among other aspects. 

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