Jeopardy Category: HVAC
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ANSWER: We heat it, cool it, humidify and dehumidify it, clean it, move it, supply it, return it, monitor it, AND breathe it — but sometimes it seems like we have forgotten, or never learned, how to properly measure and set it.
QUESTION: What is airflow?
Millions of dollars are wasted on improperly commissioned and serviced HVAC equipment. Comfort and equipment issues, as well as building pressurization/depressurization problems, can 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 just the short list of problems linked directly to this mis-measuredmedium.
Correct airflow is the key to system performance. Without it, refrigerant charge cannot be verified, an accurate combustion analysis cannot be performed, system performance readings will be substandard, and the commissioning process will remain compromised. Measuring airflow is the first step in the commissioning process and a critical first step when servicing equipment.
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 building. When replacing existing equipment, a complete evaluation of the ducting system, including verification of proper airflow at the supplies and returns, is warranted.
The Objective
Industry objectives have changed, yet the masses still estimate airflow instead of actually measuring. Static pressure charts and blower curves cannot provide the accuracy required to nail down measured performance. What was good enough to get you in the ballpark yesterday will get you ejected from the game today. You can spend days chasing a problem that doesn’t exist, and losing money and customer respect in the process.
Measurement Process
The first step in any measurement is to know where you want to end. When making an airflow measurement, what should the result be, and how will you measure it? Are you selecting the closest blower speed from four choices, or quantifying system performance?
Technicians have been using standard air formulas, making non-corrected 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. It’s not what we consider when measuring airflow that matters; it’s what we don’t consider.
Measurement Considerations
Air density plays a key role in accurate airflow measurement. Barometric pressure, temperature, and humidity all impact air density. To get an accurate airflow measurement, you must consider the measurement technique, tools, and correct for conditions outside of standard air.
Density has a large impact on many of the measurement processes you might be performing. Density-dependent measurement methods include: temperature rise method; capture hoods, Pitot tubes, static pressure tests across heat exchangers, coils or filters, hotwire anemometers, and air handler flow meters. Each requires corrections to obtain an accurate result. Many newer devices correct for some, if not all, of these factors.
If air density is not considered and corrected, there could be a substantial error in the fi nal results, especially at the grilles and registers if the air is conditioned. Density must be factored in or a non-density dependent method of airflow measurement must be used.
Technique Matters
Simply understanding the instrumentation you are using plays a vital role in your success in the measurement process. Methods used to estimate airflow can’t necessarily be used to measure it.
Technicians should never make estimations where true measurements can be made. Measuring airflow is easy, measuring airflow accurately can be difficult. The trends to measure delivered performance, quantify duct leakage, and guarantee performance is driving the need for accurate airflow measurement.
An airflow measurement that is not repeatable, accurate, density corrected, and representative of mass flow will result in calculations of system operation that will not properly represent system efficiency, capacity, or latent/sensible split, and resulting humidity removal.
Airflow must be set at the equipment and then at the registers/outlets, then verified to ensure capacity, velocity, and throw, ensuring proper mixing and quiet performance. Without proper airflow, the system and/or equipment efficiency and operation are compromised.
Many 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.
Posted In: ACCA Now, Building Performance
