You might also consider a fixed or differential dry bulb rather than enthalpy sensors: “Dry bulb-based economizer controls are more reliable due to the error potential of enthalpy sensors. Recent research shows that dry bulb-based controls are more suitable than enthalpy-based controls, even in humid climates,” says Moser.
4) Smart Control Sequences
Because an economizer’s performance is dependent on so many variables, you truly need dedicated BAS monitoring. This will allow you to keep tabs on performance and alert you to any mechanical problems. It also provides a degree of customization, which can be beneficial during the shoulder months or a specific weather event.
One critical element is having a high-limit cutoff, a setpoint that disables the economizer if the temperature is too hot or humid.
“You need to have high-limit cutoffs in place and working properly, particularly in the swing seasons. They’re the difference between wasting energy and saving it,” Callan cautions.
If you oversee a portfolio, keep in mind that the cutoff limit for one location may be unfavorable for a property in a different climate zone.
“For example, a building in a humid climate may have a lower economizer changeover temperature setpoint than one in a dry climate,” Moser explains.
If you want more control than your BAS, you can also install airflow monitoring stations, a device that is placed in the air stream to measure how much air is flowing through an opening.
“If your economizer is cycling back and forth, it’s easy to disrupt the pressure balance in the building,” says Callan. “You can have tremendous pressure inside the building if the economizer is full running and your exhaust or return system isn’t keeping up or isn’t properly designed. The airflow sensors allow you to track and calculate more directly how much air is coming in and how much needs to go out.”
You may also want to bypass the economizer on days when pollen counts or smog levels are excessive.
“If you have excellent outdoor air quality, an economizer will improve indoor air quality as it provides more oxygen to occupants. But if it’s poor air quality, you could be introducing pollutants and allergens at a higher volume than you would without an economizer,” Callan stresses. “In those cases, you need to increase filtration, which can be difficult in a one-pass system.”
5) Preventive Maintenance
If you’re in the bad habit of deferring maintenance, don’t bother with an economizer. While by no means a delicate piece of equipment, any number of issues can trigger a chain reaction that ultimately causes the unit to waste energy. The longer you ignore inspections and cleaning, the more likely you are to run into issues.
“Many air-side economizers aren’t working properly due to lack of maintenance. BOMA’s Preventive Maintenance Guidebook: Best Practices to Maintain Efficient and Sustainable Buildings lays out a plan for upkeep,” Moser says. “The mixed air plenum of an air handler or rooftop unit may be a dark and windy place, but it’s important to regularly test the operation of air-side economizers to ensure their long-term performance.”
“If you have a qualified engineering staff, they should evaluate the economizer and its dampers during their routine daily checks,” adds Callan. “You should also do a full inspection before and after economizer season, he adds, or every six months if you have year-round cooling.”
Pay special attention to all moving parts, including the actuator and linkages, as well as seals, recommends Walsh-Cooke. Properly calibrated sensors are another area to keep an eye on. This should be done at least once a year.
“You already have a number of sensors around your building that need to be maintained and inspected – an economizer is only adding a few more to the good maintenance practices you should already be doing,” Walsh-Cooke notes. “If you’re not calibrating your other building sensors and keeping up on equipment care, you’re already losing energy elsewhere.”
Jennie Morton firstname.lastname@example.org is senior editor of BUILDINGS.