For decades, we’ve known that the installation of an HVAC system determines its true operating efficiency more than the equipment’s laboratory-rated efficiency. The installation also determines how many tons of cooling make it into the conditioned space of a building.

While the equipment tonnage is often quoted on proposals, how can you be sure the equipment’s potential is really being achieved? Is it possible your installation practices are contributing to lost cooling efficiency? Let’s look at what causes lost tonnage and how you can ensure it’s not happening because of your company.



The typical installation many of us encounter in the field isn’t pretty. While we can see many of the obvious flaws in workmanship, the defects leading to lost tons of cooling often aren’t as noticeable. It requires measurement and diagnostics to reveal these capacity thieves.

National Comfort Institute (NCI) studies have documented typical HVAC systems only deliver 57 percent of their rated capacity into buildings. This means that, on average, a nominal rated 3-ton system only delivers 1.7-ton of true capacity for customers to use. Do you ever get complaints from customers about needing a larger capacity unit to cool their homes correctly when you know their systems are sized correctly?



Multiple factors contribute to lost tons of cooling. Discovering the cause of capacity loss often requires looking beyond the equipment and considering the impact of what it’s attached to. The five most common contributing factors are:

  1. Airflow;
  2. Excessive static pressure;
  3. Incorrect refrigerant charge;
  4. Duct leakage; and
  5. Duct thermal losses.

Factor No. 1: Airflow — First, airflow must be right at the equipment, and second, it must be properly delivered into individual rooms. If either of these aspects are off, the laboratory-rated equipment efficiency won’t be realized. If you’re assuming airflow at the equipment and registers is correct, you might be contributing to lost tons of cooling.

Factor No. 2: Excessive Static Pressure — This contributes to low airflow on constant-speed fans and excessive watt draw on variable-speed fans.

The reasons for high static pressure are plentiful. They range from restrictive coils and filters to undersized and poorly installed duct systems. If you assume proper static pressure due to confidence in your design, you might be contributing to lost tons of cooling.

Factor No. 3: Incorrect Refrigerant Charge — This third factor is the one typically looked at first. When there is a system issue, technicians often grab a set of refrigeration gauges and hook up. Sound familiar? By doing this; however, they assume many system characteristics.

The major assumption is regarding fan airflow. No amount of refrigerant will overcome improper airflow across an evaporator. If airflow isn’t set correctly, there is no way to properly charge the refrigeration side of the system and capacity decreases. So, if you’re attempting to charge a system without first verifying fan airflow, you might be contributing to lost tons of cooling.

Factor No. 4: Duct Leakage — There are systems out there operating with proper fan airflow and acceptable static pressure, yet they still lose tons of cooling due to duct leakage. For a system to deliver its true potential, airflow from the fan must be delivered by the duct system into the conditioned space.

Any air lost between the equipment and intended distribution point isn’t available for delivering cooling tons. If you assume your ducts are tight because you seal the connections, you might be contributing to lost tons of cooling.

Factor No. 5: Duct Thermal Losses — The fact that we are often forced to place duct systems in poor locations amplifies this problem. Our industry is in a “catch 22” — no duct system should ever be placed in an attic where temperatures can reach 140°F and higher. Yet, it’s often the only location given. When you try to move 55° conditioned air through ducts set in a space that hot, the conditioned air will gain heat.

We insulate ceilings with R-38 and higher, yet only place a maximum of R-8 on ducts carrying conditioned air located in the same space. So, for our friends — the architects, builders, and general contractors — if you design or construct a building where the only spaces allowed for duct installation are those that can reach 140°, you are contributing to lost tons of cooling.



There is hope. A system optimized to perform as intended can deliver greater than 90 percent of its rated capacity into the building. It takes attention to detail and a willingness to admit the way you’ve done things in the past might need some improvement.

Not everyone can or is willing to take the time to properly optimize duct systems. Many will choose to keep doing things the way they’ve always done them and continue to throw away tons of cooling. That’s good news for those who make the choice to advance and offer solutions for issues many will walk past. Let your competition continue to offer larger equipment on that undersized duct system. They will be your greatest source of opportunity.

To begin restoring lost tons of cooling, you’ll need to address and correct the five factors mentioned above. Those who are most successful start at the equipment and move to the duct system from there.

Step one is to measure and diagnose static pressure. From there, you can transition to plotting and verifying airflow through the equipment. Once the fan airflow is correctly set, then you can verify proper refrigerant charge.

If everything is corrected at the equipment level, move to the duct system by assuring duct leakage and thermal losses are kept to a minimum. It’s important to note that you might not be able to get things right at the equipment until the ducts are corrected.



The hardest part is admitting you need to change. If you believe you’re contributing to lost tons of cooling, figure out where and work to change it. If you don’t believe in this, you won’t do it.

Once you’re committed to addressing these issues, change the game by moving past the equipment laboratory rating. Focus on what is truly happening once the equipment is placed into a homeowner’s attic or crawlspace and attached to the structure’s duct system.

Imagine if you provided a rating of the system instead of the equipment by focusing on the result: recovered tons of cooling. You can verify that lost tons were truly restored through your skills in system optimization. Instead of selling the rating on a yellow sticker, you can finally sell your craftsmanship, and that provides true differentiation when it comes to HVACR contracting.

Publication date: 09/25/2017

David Richardson has served as a featured author for The NEWS since January 2014. His articles cover many aspects of the forced-air business, including duct design, air leakage, duct insulation, and more. For more information, or to read more Duct Dynasty articles, visit /keywords/10959-duct-dynasty.

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