Static pressure measurement on commercial HVAC equipment can vary significantly from measuring static pressure on residential equipment. In addition to pressures being higher than most residential equipment, the number and type of pressures measured differ as well. Some commercial equipment is rated by fan pressure as opposed to the total external static pressure listed on smaller equipment.
Commercial rooftop equipment is probably used the most in commercial applications today in most regions of the country. Packaged equipment 7.5 tons and larger offer some unique challenges of their own that you should be aware of to avoid erroneous readings.
Generally, when measuring total external static pressure, measure the pressure at the point where airflow enters the equipment and where the airflow exits the equipment. The pressure entering the equipment is a suction or negative pressure. The pressure exiting the equipment is a discharge or positive pressure. Add these two pressures together to find the equipment measured total external static pressure.
Example: A 10 ton packaged unit in a rooftop application placed on a curb.
Equipment entering pressure -.46-in. w.c.
Equipment exiting pressure +.51-in. w.c.
Total External static pressure .97-in. w.c.
Compare the measured static pressure to the equipment rated maximum total external static pressure to assure the system is operating at less than the maximum rated total external static pressure as rated by the manufacturer. Disregard the + and – signs as these represent the type of pressure measured and are not numerical values.
You can also use the measured total external static pressure and the measured fan RPM to plot fan airflow on the manufacturer’s fan performance table or fan curve.
Notice that this packaged unit is shipped with the filter and coil included. So to measure total external static pressure of this unit, the coil and filter are included in the equipment total external static pressure measurement.
“As shipped” is a term being circulated around the industry recently that brings clarity to static pressure measurement. When considering how to measure total external static pressure and determining if a component of a system should be included or excluded in the total external static pressure reading, determine if the component was included with the equipment “as shipped”.
You may still need to check the filter and coil static pressure drops installed in the unit. This is done by inspecting and then drilling test holes on either side of these components through the cabinet of the equipment. Always be sure to remove panels and inspect before drilling. Then subtract the pressure on either side of the component to find its actual pressure drop.
Compare the actual pressure drop of these components to the rated pressure drop at current airflow. If actual pressure drop matches the rated pressure drop, the component is clean. If the actual pressure drop exceeds the rated pressure at the specified airflow, this is an indication the filter or coil may be dirty and need cleaning to improve the performance of the system.
Commercial Air Handlers
Larger commercial air handlers may be rated with a total external static pressure rating similar to packaged equipment. But built-up or custom made units may also be rated based on fan static pressure.
Equipment rated at a specified fan pressure is often done so because a variety of components may be added to the air handlers for specific applications.
For example, the basic air handler is built with a fan rated pressure of 3.0-in. w.c, and the manufacturer will publish fan capacities under the tested conditions. Yet as these units are custom built, a number of accessories may be added to the equipment that would significantly change the total external static pressure of the equipment. So to keep effective engineering data, fan pressure only is used.
Air handler accessories may include a DX or cold water coil, a hot water coil, pre-filters for the outside air or perhaps a series of standard and charcoal filters to capture pollutants or a series of HEPA filters for an operating room.
Each of these, or a total of all of these accessories will greatly affect the rated total external static pressure of the unit. But if the equipment is rated at fan pressure, fan static pressure can always be measured and interpreted regardless of the number of accessories added to the equipment.
Example: A 20 ton air handler unit in a horizontal application is placed in a mechanical room.
Fan entering pressure -.1.22-in. w.c.
Fan exiting pressure +1.18-in. w.c.
Fan operating pressure 2.40-in. w.c.
The fan airflow would then be plotted on the manufacturer’s fan curve as published for the fan pressure. No total external static pressure would be published or referred to in the engineering data since only the fan pressure would be used.
Be sure to check the nameplate pressure listing and the published engineering data for each piece of equipment’s pressure rating. You will still find some larger equipment rated using total external static pressure, and others using fan rated pressure.
Air Handler Accessory Pressure Drops
The pressure drop of each of the accessories (coils and filters) that are added to the air handler can then be measured and compared to the their respective pressure drops rated at the operating airflow.
Often static pressure profiles are used to report measured pressure of these commercial air handlers. A static pressure profile is a graphic illustration of the equipment and will include the location of each pressure test and the corresponding pressure reading taken at that point in the system. The pressure drop of each accessory is then calculated and shown on the pressure profile illustration.
Review Published Engineering Data
Since commercial equipment testing is more detailed and often requires increased amounts of field support, better manufacturers are beginning to see the benefits of providing improved engineering data and better tech support to ensure their equipment is installed according to their specifications and to operate as designed.
Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a detailed commercial equipment pressure testing procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.