Summer is coming and it's time to brush up on servicing rooftop packaged units with economizers. Better companies allow time to verify economizer performance during commercial spring service calls. Let's review the steps required to check economizer airflow and BTU intake.
Measuring airflow and BTUs through an economizer can be a challenging task. Many have walked away from many economizers shaking their heads without the confidence they have taken good readings.
Two test methods, when combined and practiced in the field, will give you an assurance that you have secured accurate readings. The first test is to perform an airflow traverse of the economizer inlet, and the second requires a series of temperature readings and calculations that can back up your airflow readings.
Prepare to measure the economizer performance by gathering the engineering data for the equipment and the economizer. Hopefully the O &M manuals are available in the building. Study the controls schematic to understand how to manipulate the controls and make required economizer adjustments. You'll also need to know the specified minimum and, if specified, the maximum airflow required through the economizer.
Prepare your reports for testing and bring the equipment into full operation. Normally, test holes are drilled in the economizer eyebrow in a vertical line next to the packaged unit or intake housing from 2-in to 6-in. on center.
A good place to start testing is to set your louvers about ½-in. open as a minimum CFM position and take your initial airflow readings by making an airflow traverse. Record initial readings and calculations, and then make necessary adjustments and test until you have met the minimum airflow requirements.
The second test method is to take temperature readings and use them to confirm economizer airflow. While your system is in minimum airflow setting, take dry bulb temperature readings. These include the temperature of the outside air (OAT), the return air (RAT) and the mixed air (MAT) inside the system where the return air and outside air are mixed together before the air reaches the coil.
Apply the following formula:
Here’s an example for how to work the math:
This will give you a percentage of outside air being introduced into the system.
You may need to take temperature traverse readings until you are assured you have accurate system temperature. Record temperatures to within 1/10th of a degree on your report and make temperature calculations.
Next, manipulate the controls to achieve full open or maximum airflow through the economizer. Repeat the tests listed above.
A couple of hints: when adjusting the economizer to full airflow, keep an eye on the amp draw of the blower motor. As the economizer opens, static pressure decreases, and airflow will increase dramatically. Remember fan law three, that Amps increase at the cube of airflow, so the increase in Amperes may over-amp the blower motor and cook it.
Also, be sure to permanently mark your louver and linkage settings. You can almost always be sure they will be tampered with on an unusually hot or cold day by an unknowing soul someday in the future.
Study the attached Economizer BTU Performance Table to see the BTU value of the airflow pulled through an economizer on a particularly hot or cold day.
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 one- page Economizer Balancing Procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and download.