Is your air conditioning bill running wild? Get your utility costs under control by adding an air-side economizer. This device draws in outside air as a form of free cooling.
While the potential for savings is significant, facility managers can lose all of an economizer’s benefits if they fail to specify the right features and neglect maintenance. Follow these tips to lock in your economizer’s potential.
1) Compatible Climate
Conditioning recirculated air can be a significant expense, an average of 10% of a building’s energy load. Depending on climate, square footage, and building type, some properties may spend up to 20% on cooling, according to the latest data from the Commercial Building Energy Consumption Survey (CBECS). An economizer can reduce this load just by taking advantage of outside air in lieu of mechanical cooling.
The most important consideration for an economizer retrofit is your location. Climates that are humid and hot aren’t typically a good fit as the outside air is rarely cool or dry enough to bring inside, explains Dave Moser, senior engineer with CLEAResult, an energy consulting firm.
If you’re not sure if your climate is compatible, take a cue from requirements in the International Energy Conservation Code (IECC), recommends David Callan, vice president of McGuire Engineers. For new construction and major renovations, economizers are required in cooling systems with a capacity exceeding 33,000 BTU/h.
They are exempt, however, from Zones 1A and 1B, which apply to the southern tip of Florida and islands like Hawaii and Puerto Rico. In these hot and muggy environments, energy savings are minimal and won’t yield a solid return on investment. Air that is too moist also increases your risk for mold and mildew, adds Callan.
Keep in mind that humid air contains heat, what humans register as relative humidity. Excess heat not only undermines your economizer cooling potential but it also decreases your occupants’ thermal comfort, Callan cautions.
Even buildings in colder environments can add an economizer. The 2012 update of the IECC added Zone 7, which covers northern parts of Maine, Michigan, Wisconsin, Minnesota, and North Dakota, as well as Alaska (Zone 8).
“In the winter, an economizer is a no-brainer because the air is generally cold and dry,” notes Callan. “A little bit of cold air goes a long way so you’re not necessarily introducing a ton of moisture into the ventilation even when it’s snowing.”
Apart from climate, economizers are suitable for facilities of any size and type. Those with mission critical loads, such as data centers, will experience significant benefits, as well as those with year-round cooling needs. Buildings with a high number of internal rooms are also good candidates, says Mark Walsh-Cooke, principal and mechanical engineer with Arup, a consulting firm. The only building environment that wouldn’t be ideal are those with humidified spaces in a dry climate, such as hospitals or museums, he adds.
2) Proper Installation
Retrofitting an economizer doesn’t require major structural or mechanical changes. The units are a modest size and should easily fit within your existing HVAC footprint, says Walsh-Cooke. They are typically added along the outside wall or roof so the air handling unit’s air intake plenum is connected to the outside wall and motorized dampers, adds Callan. Depending on your existing equipment, you can have the economizer custom-installed by a contractor or packaged with a new air handling unit.
“Adding air-side economizers to packaged HVAC equipment such as rooftop units is typically simpler than adding them to interior air handlers, but there are still aspects that need to be considered, such as controls and relief air,” Moser notes. “A thermostat with two-stage cooling capability may be required to fully take advantage of the economizer cooling.”
If you’re adding an air-side economizer to an existing HVAC system, you need to plan carefully for a larger outside air intake, a larger relief air opening, potentially a relief air fan, proper damper sizes, and appropriate and reliable controls, he adds.
You should also review your existing relief air and ventilation components. Because economizers bring in large amounts of outside air, you need a subsequent way to relieve air in order to maintain acceptable building pressure, says Moser. While an economizer adds fresh air during its cycle, you will still need to ensure your HVAC maintains appropriate ventilation in your building when it’s not running.
3) Quality Parts
Because economizers have a number of continuously moving parts that are exposed to weather conditions, it’s critical to specify robust materials that can withstand the climate and time.
“Good quality, low leakage dampers are very important. Even though a significant amount of surface area is required to bring in air during the economizer cycle, you don’t want it to leak when not in use,” Callan explains.
For dampers, aluminum is a good choice because it won’t rust. Galvanized steel is another option, though buildings in marine environments might need to upgrade to stainless, notes Walsh-Cooke.
Another overlooked component is the enthalpy sensor, which measures the combination of temperature and humidity. This is done so the economizer can compensate for the differential between the total heat content of the outside air and inside air.
“If the enthalpy sensors aren’t calibrated properly, it doesn’t take much on either side to turn your benefit upside down and increase the load of your air conditioning equipment rather than decreasing it,” warns Callan.
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.