Fresh air is essential to good indoor air quality in some homes and nearly all commercial buildings. Its primary use is to freshen the indoor air and provide a positive pressure to the building. The question is, once you determine how much fresh air the building needs, how can you verify what amount the system is delivering?
In most cases, and airflow hood is the quickest and most reliable method for measuring airflow. However, few fresh air inlets can accommodate an air balancing hood. When a hood can’t be used, we must turn to other time-proven methods of measurement to test fresh air volume. The first step is to measure airflow by employing an airflow traverse, if conditions permit this test. The back-up test is to measure temperature and apply some simple math.
What Not To Do
When testing fresh air inlets, measurement with a Pitot tube or by pressure is not recommended by National Comfort Institute. Velocities can fall below 100 fpm, and that means accurate measurement and interpretation of pressures at or below 0.003-in. w.c. Good luck. Even with the best instruments, these pressures may be inaccurate due to a pigeon flying by the rooftop unit.
Also, measuring the pressure drop over the inlet (a hood or louver) and comparing it to the manufacturer’s published data isn’t reliable in the field, especially when the system is operating at minimum airflow. A good learning experience may be to test the pressure variations that you’ll find in live field conditions; you’ll quickly see these test methods can’t be relied upon.
Get the Right Test Instruments
Good testing requires big-boy tools. A cheap anemometer or an elaborate scientific transducer can often render a test useless, if they’re not designed or applicable to the exact use that you need them for.
Top grade anemometers have come a long way in the last 20 years, and several have programs that calculate, store, and retrieve the test results. Talk to someone who has significant field experience and you’ll usually get some sound advice about which test instruments work best for this test. A good hot wire anemometer demands an experienced user, but is my personal favorite -- if you’re willing to put in the time to master the instrument.
Remember, just because you have bought the right test instrument doesn’t qualify you to use it. If you need training and air balancing certification, get it.
This timeless test method has been employed for decades in the air balance profession. It was used by Willis Carrier and Dave Lennox as they wrestled to understand ways to deliver comfort into homes and buildings. Thank goodness we have top quality reliable test instruments today that allow us to measure airflow by traversing a duct or louver with extreme accuracy and ease.
A traverse is an air balancing test method that employs a series of air velocity readings taken across a louver, hood, or duct. Here’s the short version: Air velocity readings are taken in a very specific pattern. Then the velocity readings are averaged. The average air velocity is then multiplied by the area of the duct in square feet. The answer to this formula is airflow in cfm.
Of course, there are a host of conditions that must be met in order for the traverse to be accurate. Ideally, 10 widths of straight duct are required to perform a traverse, but good air balancers can get away with as low as five diameters in a pinch. The testing must be done 80% downstream of the louver or intake hood, free from any motorized dampers or turning vanes.
With hundreds of economizers tested daily, it’s obvious that test methods using a traverse are effective and reliable. Most balancers drill test holes in the eyebrow of the economizer hood to take their fresh-air traverse readings. This creates a test site where a reliable traverse can be made.
If a louvered intake has no hood, each balancer must suffer the tedious experiment of developing his or her test method and over time develop correction factors custom made for their traverse methods. Correction factors are applied to test results for various types of measurements, and may differ between from one balancer to another due to each individual’s testing style. Most balancers end up with very similar correction factors with experience, but the commitment to developing accurate testing is the key to consistent readings.
Temperatures tests are often used to confirm or deny the accuracy of the airflow traverse of a fresh air inlet measurement. This testing can be short and sweet, or can be long and drawn out. An air balancer always stands in the position of being the judge and the jury on a project. Good balancers test until they can render a firm judgment of the system value that’s being read.
Click on the thumbnail at the bottom of this article for the fresh air measurement formula.
Measure the temperature of the fresh air entering the system. This may require a temperature traverse of the entering air to ensure accuracy. Measure the temperature to within one tenth of a degree Fahrenheit. Then, measure the return air temperature of the system before it mixes with the fresh air. Next, measure the mixed air temperature downstream of the fresh air inlet where the return air and outside air are mixed together. Finally, apply the formula listed above. This will give you a percentage of fresh air being introduced into the system.
Traverse the airflow exiting the equipment, if an appropriate test site exists. Multiply the total supply air exiting the equipment times the percent of fresh air as revealed by the fresh air formula to find the fresh air cfm.
Compare this test to the airflow traverse. This test is only reliable if the difference between the return air and the fresh air exceeds 20F.
Other Economizer Considerations
When testing economizer airflow, always measure and adjust minimum and maximum airflow. This is done by adjusting the control settings, and then repeating the tests described above.
As always, when I describe air balancing tests, there are usually a dozen or so related tests and procedures that could have a significant effect on the overall results that could be presented if space permitted. But these basic principles described here are the keystones to fresh air measurement. This test can be extremely accurate and effective if performed properly.
Once fresh air volume can be measured, the Btu gain or loss that the economizer brings to the system can also be determined. Click here to view a table that shows how large of an effect an economizer can have on system performance. It’s not uncommon on extremely hot or cold days for the economizer BTU to totally overcome the heating or cooling capacity of the equipment if it has never been measured and adjusted.
When employing an economizer in a system, the load of the economizer must be considered in the Manual J calculation, or the actual operation of the system could be dreadful. All of us have also seen the other end of the scale: an economizer with a few screws in it and the linkage or controls disabled.
So, measure economizer performance. Because so many economizers have never been tested or adjusted, the work is extremely gratifying. We all love to make a system work better and delight the building occupants with newfound comfort.
Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a free economizer test report, contact Doc email@example.com or call him at 800/633-7058. Go to NCI’s website at www.nationalcomfortinstitute.com for free information, technical articles and downloads.