Static pressure testing has finally become fashionable across the HVAC industry. There is still a lack of understanding about how, where, and why to take pressure readings and why each test location is critical to accurately interpret the performance of an HVAC system. Let’s take a look beyond the newest fad that require static pressure testing and learn how, where, and why to test correctly.
As quality assurance programs are jumping on the static pressure wagon, you have an emerging problem to contend with. Although these programs require static pressure measurement, they don’t explain how to test correctly or what the readings actually mean. If you follow their sketchy test methods and report forms, your tests will be inaccurate most of the time and real system performance will not be measured. To help you move forward, read on.
Most of us in the HVAC and energy industries test because we genuinely want to know what’s wrong with the operation of the system. Once you know what’s wrong, you can figure out the problem and improve it. If you focus on improvement more than testing, your success in measuring static pressure will skyrocket. To blindly measure and report just to meet the requirements of another program, really doesn’t accomplish much, does it?
How to Measure Total External Static Pressure
This is the big static pressure measurement that you can do the most with. In just a few minutes, you can understand more about airflow and system performance with total external static pressure than with any other test. The needed test equipment is cheap and available everywhere. As a matter of fact, a tech that doesn’t measure static pressure might as well be servicing systems half blind, because there’s no other way to see and interpret what the system is actually doing without it.
Let’s look at the basic steps required to effectively measure total external static pressure. We can’t cover every system here, but if you’re a technician or contractor you can email me for more detailed instructions.
Drill test holes. Drill 3/8-in. test holes where air enters the air-moving equipment and where air exits the equipment. Here are two tips: first, cover your drill bit with a sheath. This is a 1-in. x 4-in. dowel with a ½-in. hole drilled through its center. Think of a broom handle. Second, remove panels and inspect before you drill to avoid those awful hissing and electronic sizzling noises.
Where to drill test holes. Let’s use an upflow gas furnace with a remote cooling coil as our primary example: (More info on other systems will follow.)
Step One: Drill a test hole at the spot where airflow enters the gas furnace after the air filter.
Step Two: Drill another test hole before the coil as the air exits the gas furnace.
Step Three: Turn the system ON with the fan set to deliver the highest required airflow. This is cooling mode for most of the country.
Step Four: Measure and record the pressures at these two test sites.
Step Five: Add the two pressures together to find the operating total external static pressure of the system.
Step Six: Compare the measured total external static pressure to the manufacturer’s rated total external static pressure listed on the equipment nameplate. If the measured total external static pressure is lower than the rated pressure and your fan speed is set correctly, your fan airflow is probably adequate. Congratulations, you are in the minority. If total external static pressure is more than 10% to 20% too high, you may have an airflow problem degrading the performance of your system. Congratulations, you have work to do and you can improve the performance of this system.
Where and Why to Test Pressure
Where you test is critical. If you test in the wrong spots, you will not be measuring total external static pressure correctly and your conclusions and diagnostics will be useless. Furnace manufacturer’s rate equipment airflow and static pressure in the lab without a coil attached and often without a filter installed. Check individual equipment airflow specs to verify that fact.
For this reason, you measure the discharge pressure, or positive pressure of the furnace, before the coil. Your goal in to measure the same way the manufacturers do so you can compare our measurement to their published specifications. If you measure like they measure, you can accurately interpret the pressure and airflow the same way they do.
Likewise, if the manufacturer measures in that lab without a filter, measure after the filter to match lab conditions. If the lab measured with a filter, the field pressure measurement with the clean new factory filter will only be accurate. So, to field measure the equipment’s inlet or suction pressure for total external static pressure, measure after the filter because filters are normally dirty or have been replaced with more restrictive ones.
Air Handler Static Pressure Measurement
If you are measuring an air handler, there normally is an internal cooling coil. So measure the equipment discharge air pressure where the air leaves the equipment and use that as your positive pressure reading. Add this to the suction or negative pressure measurement taken after the filter and add together with the discharge pressure to find the operating total external static pressure.
The test procedure is the same with packaged equipment or electric fan coils. Measure as air leaves the equipment and after the air passes through the filter. Add these two readings together to find operating total external static pressure.
There are a half dozen other test tips needed to assure total accuracy, but this is enough to get you started.
What You can Do with Static Pressure
By measuring total external static pressure you can quickly and easily see the invisible and greatly increase your ability to understand, diagnose and solve system performance problems related to airflow and BTU delivery.
Compare measured pressure to rated pressure. The manufacturer lists maximum rated total external static pressure right on the nameplate. Measure the pressure and compare to the rated pressure. If total external static pressure is more than 10% to 20% too high, you probably have an airflow problem.
Find which side of the fan has the most restriction. If the discharge pressure is higher than the suction pressure, the system has more restriction to airflow on the discharge side of the fan. That’s where you look for problems and is most likely where you can make the repair that will increase performance the most. Of course, the opposite scenario is also true.
Plot Fan Airflow. By knowing a system’s operating total external static pressure and its fan speed setting you can easily plot fan airflow using the airflow tables each manufacturer provides for each piece of equipment. Imagine being able to determine the fan airflow. Without this knowledge, you’ve been unable to correctly interpret the equipment temperature readings you’ve been taking your whole career. I hope that gets you to measure and use total external static pressure in your daily work. Sorry for the bad news, but we can all do better.
Total External Static Pressure is one of dozens of essential measurements that are needed to effectively interpret the operating efficiency of a forced air HVAC system. Each reading is connected and interdependent on each other. Don’t worry about the others yet. Learn how to use and interpret total external static pressure, then you’ll be ready for the next one.
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 Static Pressure Information Package, 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.