It can be challenging to know where to start when adding HVAC system performance to your company. Because of this, many don’t achieve the full potential available from upgrading HVAC systems. I recently read an observation by John C. Maxwell in “The 21 Indispensable Qualities of a Leader” that illustrates why this can happen.

Maxwell describes why lion trainers bring a stool with them as they step into a cage with a lion. I always assumed it was for the trainer’s protection, but it serves a more important purpose. When the trainer extends the stool legs toward the lion’s face, the animal tries to focus on all four legs at once, and it paralyzes him.

Let’s look at what drives this behavior and how it applies to you successfully adding HVAC system performance to your company.



The lion’s response is a survival instinct that originates in the limbic system — a part of the brain that controls subconscious behaviors. The first response it triggers is to “freeze” up. This defense mechanism helps us deal with predators and allows time to assess a situation to determine needed actions.

Humans have a more developed limbic system. You will see the freeze response in the first few rows of an audience watching the lion and trainer. The audience won’t move in the presence of a predator (the lion) to assure they don’t draw any unwanted attention.

For those new to HVAC system performance, it can feel just like those four stool legs as you work to master the principles. If you focus on all of them at once, paralysis sets in. It prevents you (the lion) from successful application in your business.

Your company team members may also freeze — they’re the audience. The new skills and test instruments may look like a predator to them if they don’t understand why they’re important. They may see all this new stuff as a threat instead of as an opportunity to better serve your customers.


The HVAC System Performance Stool

HVAC system performance has four legs, just like the stool. However, each leg represents a skill that should be mastered before you move to the next. For any stool to be stable, all four legs must be attached and strong. You’ll need to apply those same traits for performance testing to grow within your company.

The HVAC system performance stool legs are:

  1. Static Pressure
  2. Airflow
  3. Temperature
  4. Delivered Capacity

This is the order and how they should build upon each other. It starts with static pressure, moves to airflow, then temperature, and concludes with delivered capacity. For an HVAC system to perform as intended, these four legs must work together. Let’s break each leg down a little more to ensure your success.


Static Pressure

Static pressure is the first leg of the HVAC system performance stool. It is a foundation of airflow and indicates overall HVAC system health.

Total external static pressure (TESP) is the first test to master. It reveals hidden airflow issues that lead to premature failure and comfort issues. To diagnose TESP, compare your measured TESP to the maximum rated TESP found on the equipment nameplate. If measured TESP exceeds rated TESP, further investigation is needed to uncover why.

A simple blood pressure comparison will help techs and salespeople understand why to measure it. High static pressure — just like high blood pressure — means you’re getting ready to have major issues and are probably not healthy. To help with the explanations, you can use the NCI Static Pressure to Blood Pressure Tables. If you would like a PDF copy, send me an email request.

Common causes of high static pressure are the air filter, indoor coil, and duct system. You can add these measurements to diagnose where the highest airflow restriction is. Once your technicians see how static pressure makes their life easier, they most likely will then ask, “What’s next?”



Airflow is the second leg of the HVAC system performance stool. It has been said that airflow is king because it’s one of the mediums used to transfer heating and cooling from the equipment to the living space. You’ll often see airflow listed as cfm (cubic feet per minute).

Airflow is key to a well-performing and long-lasting system. Without it, you won’t achieve rated efficiency, and the system will suffer long-lasting negative impacts. Many customers have endured low airflow for years yet have never had it identified through measurement.

Once you measure TESP, add in the fan speed setting and a fan table to plot fan airflow. This helps your team see approximately how much air the fan is moving compared to what is required for proper operation. It also shows the relationship between elevated static pressure and airflow.

Use a fan table as an aid to visualize plotted fan airflow and required fan airflow. Your team can see how much a system needs to improve to get the airflow it needs. Let’s say required fan airflow is 1,200 cfm for a 3-ton system at .50” wc on high speed. Measured static pressure readings reveal the system operating at .80” wc and only moving 843 cfm in its current operating condition. To improve airflow, you’ll need to lower pressure near .50” wc.

A properly operating system should have fan airflow within +/- 10 percent of manufacturer specified airflow.



Temperature is the third leg of the HVAC system performance stool. It is one of the most overlooked aspects of system performance. You can have the right amount of airflow, but without proper temperatures, comfort and efficiency won’t happen.

To help overcome any intimidation of this leg, start with four temperature measurements. They will give your team a glimpse into how much influence the duct system has on the system’s comfort and efficiency. You’ll need to measure the following temperatures to show this impact with the system running in cooling mode:

  • Temperature entering the equipment
  • Temperature leaving the equipment
  • Temperature at the farthest supply register
  • Temperature at the farthest return grille

Once you have these four temperature readings, use them to estimate system capacity lost through duct system temperature losses.

First, subtract the equipment leaving temperature from equipment entering temperature. This is the equipment delta t (∆t). Next, subtract the farthest supply register temperature from farthest return grille temperature. This is your system ∆t.

Finally, compare equipment ∆t to system ∆t. The difference is the percentage of temperature gain you have through the duct system. Ideally, equipment ∆t and system ∆t should be the same.



Delivered capacity is the final leg of the HVAC system performance stool. It’s the proof that your work does what you said it would and that your customers received what they invested in. Delivered capacity is what defines real HVAC system performance.

Delivered capacity is measured in Btu/hr (British thermal units per hour) and is a measurement of how much heat is removed or added from the HVAC equipment and system. Btus are calculated using your airflow and temperature measurements. Depending on the mode of operation tested, you can determine sensible Btus (change in temperature), latent Btus (change in moisture), and total Btus (sensible and latent added together).

To lower the intimidation level, start with sensible Btus at the equipment. Airflow and temperature measurements must be mastered before you conquer this final leg. First, plot air-handling equipment fan airflow and record it. Next, measure temperature entering and leaving the air-handling equipment after the system has run for at least 15 minutes and record your ∆t.

Plug these readings into the sensible heat formula (cfm x rt x 1.08) and see how close you are to the manufacturer specifications. If you’re within +/- 10 percent, you’re doing great.

It’s important to note that equipment capacity and system capacity are very different measurements. To assume they are the same would be a huge mistake, since it leaves out the duct system impact.



Unfortunately, many company leaders freeze when staring at all four legs of the system performance stool. If you try to do everything at once, it results in paralysis. Don’t believe the multitasking myth. You can have success with one leg at a time.

The first few rows of the audience (your team of installers, service techs, salespeople, and office personnel) are watching. They will also freeze when faced with a threat of new skills and situations to master.

Recognize the signs that indicate your team suffers from the freeze response. If you consistently aim for your techs to test and they don’t, it might be a sign that they feel threatened or vulnerable. Work with them to uncover the cause so they can be successful.

Once a lion learns to focus on one leg at a time, it’s a bad day to be the lion tamer who gets in his way. If you’re working to add system performance to your service offerings, keep at it. Success is achieved in small, consistent steps and habits that move you forward each day.


If you’re an HVAC contractor or technician interested in learning more about measuring system performance, contact me at or call 800-633-7058. NCI’s website,, is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.

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