Air velocity at the registers is just as important as volume. Air delivered at low velocities will stratify, and excessive velocity will produce noise.

We heat it, cool it, humidify and dehumidify it, clean it, move it, supply it, return it, and monitor it. Somehow, however, we have forgotten, never learned, or perhaps never were taught how to properly measure and set proper airflow.

Billions of dollars are wasted each year on improperly commissioned and serviced HVAC equipment. Improper equipment performance, as well as pressurization and depressurization problems, can all be linked to design or performance issues caused by improper airflow.

Compressor failures, heat exchanger failures, component failures, high utility bills, allergies, CO poisonings, poor IAQ - that’s the short list of problems linked directly to this often unmeasured (or mismeasured) medium.

Correct airflow is the key to system performance. Without it, refrigerant charge cannot be verified, an accurate or final combustion analysis cannot be performed, system performance will be substandard, and all parts of the commissioning process will remain compromised.

However, the truth of the situation is stranger than fiction when you consider the following: a typical contractor will find that seven out of 10 systems their technicians install or service will be left with incorrect airflow.

This results in over seven out of 10 of the air conditioners or heat pumps you service having incorrect refrigerant charge. How much money must we leave on the table?

Measuring airflow is the first step in the commissioning process and a critical first step when servicing equipment. It is the key component for proper operation.

Measuring airflow is a critical part of all service and sales calls. Before any system commissioning is complete, any evaluation of existing equipment is made, or during routine service, airflow should be measured and verified at the equipment and in problem areas of the home or office.

When replacing existing equipment, a complete evaluation of the ducting system, including verification of proper airflow at the registers, is warranted.


Industry objectives have changed, yet the masses are still using methods of airflow estimation for processes that require an actual airflow measurement. We are no longer only trying to select a proper blower speed.

We are additionally trying to calculate duct leakage, measure equipment performance and de- livered BTUs, and determine if system performance is affecting operation of equipment in the combustion appliance zone.

Static pressure charts and blower curves will not - and cannot - provide the accuracy required to quantify measured performance. Simply what was good enough to get you in the ballpark yesterday will get you thrown out today if you are not careful.

You can spend days chasing a problem that doesn’t exist, and lose a lot of money and customer respect in the process.


The first step in any measurement process is one’s knowledge of the desired outcome. When making an airflow measurement, what should it be, and how are you going to measure it? Are you simply selecting the closest blower speed from four possible choices, or measuring to quantify equipment or system performance?

Measurements made without knowledge of their expected outcome are a valueless proposition, and measurements made with improper instrumentation or techniques are equally as bad.

For years, technicians have been using standard air equations and formulas, making noncorrected airflow measurements, using density-dependent techniques, or making no measurement at all, simply leaving the airflow at factory settings or selecting high for cooling and medium or low for heat. In reality, they were never really measuring airflow. They were simply estimating airflow at best. It’s not what we consider when measuring airflow that matters. It’s what we don’t consider that causes the problems.

A small vane anemometer, like this Testo 416, is designed to prove density-dependant, high-accuracy airflow measurement.


Air density plays a key role in accurate airflow measurement. Barometric pressure, absolute temperature, and humidity all directly impact air density. When striving for an accurate airflow measurement, you must consider the measurement technique, tools you use, and be mathematically correct for conditions outside of standard air.

Density has a large impact on many of the measurement processes you might be performing. Density-dependent methods of measurement include the temperature rise method; most capture hoods, pilot tubes, pressure drop across heat exchangers, filters or coils, many hotwire anemometers, and air handler flow meters. Each requires corrections to obtain an accurate result.

If air density is not considered and corrected for, there could be a substantial amount of error in the computation of the final results, especially at the grilles and registers if the air being measured is conditioned, heated, cooled, humidified, or dehumidified. Density simply must be considered and accounted for or a non-density dependent method of airflow measurement must be used for the commissioning or service process.


Simply understanding the instrumentation you are using plays a vital role in your success in completing a measurement process. Methods used to estimate airflow couldn’t necessarily be used to measure it. Airflow cannot be interpreted, extrapolated, or guessed. It must be measured.

Technicians should not make a habit of ever making estimations where true measurements can be made. With the cost-effective solutions in instrumentation that are available today, technicians not only need to make an investment in technology, but also in its application and proper use.

Measuring airflow is easy. However, measuring airflow accurately can be very difficult. The trend to measure delivered performance, quantify duct leakage, and guarantee performance is, in turn, requiring accurate airflow measurement.

An airflow measurement that is not repeatable, accurate, corrected (if required), and representative of mass flow will result in calculations of system operation that are not representative of the system’s efficiency, capacity, or latent/sensible split, and resulting humidity removal.

Airflow must be correct at the equipment and then delivery at the registers, or terminal outlets, must be verified to ensure proper capacity, velocity, and throw, ensuring proper mixing (no stratification) and quiet performance (minimal terminal noise). Without proper airflow, the system and/or equipment efficiency and operation are compromised, resulting in unsatisfactory equipment operation, customer dissatisfaction, and utility waste.

Programs and standards like ACCA Standard 5, “The Quality Installation Specification,” are addressing these very issues. Proper system evaluation and documentation will be paramount to their success, but even more so is the quality of the measurements made during the quality installation process.

Technicians, contractors, and trainers all bear responsibility in ensuring this program’s success. Implementation is only part of the solution. Training is the key. We cannot achieve success by simply telling technicians to measure. We have to teach them how to do it, what to expect, and how to repeat the results consistently. We need to abandon substandard estimation methods and teach them to measure.

Publication date:04/28/2008