Economizers used in rooftop units and air handlers are multipurpose devices that can improve both the energy efficiency and indoor environment in a wide range of commercial buildings. By increasing the outdoor air supply when the weather is mild, air-side economizers can provide “free cooling,” which reduces a building’s need for mechanical cooling and usually results in substantially lower utility bills.
As a key part of the ventilation system, economizers also improve the comfort and IAQ in a building, provided they are applied and used correctly. But, like any other HVAC component, they also need periodic maintenance to ensure they are operating as intended.
SIMPLE YET EFFECTIVE
An air-side economizer is a fairly simple device, consisting of a set of return and outside air dampers, as well as controls that sense when outside air is below a certain set point and can be used to cool a space. Conversely, when the controls determine that the outside temperature is above a certain set point, they close the outside air dampers to the minimum position and activate mechanical cooling to reach the desired temperature, explained Eric Taylor, marketing manager, Aaon.
Economizers are standard features in many air handlers, as energy codes, such as ASHRAE Standard 90.1-2016, require that they be installed in many geographic regions in the U.S. Some states also require heat recovery; however, HVAC equipment can be exempt from these regulations if it meets certain efficiency thresholds, said Bill Dietrich, product general manager — chillers, Daikin Applied.
The key benefits of economizers are saving energy and lowering operational costs, said Dietrich.
“Payback is generally less than five years, and economizers last for the life of the equipment,” he said.
The controls are also critical for maintaining energy efficiency. Daikin Applied’s DDC controller, for example, allows the system to control ventilation of CO2 on demand as well as monitor airflow when cooling to ensure the economizers operate at peak efficiency. Optional fault diagnostics in the control system alert building managers when the economizer requires maintenance.
There are various types of control configurations available for air-side economizers, including those based on sensible temperature or enthalpy (temperature and humidity). Local codes may also dictate what type of controls must be used, explained Taylor.
“Title 24 in California, for example, requires that fault detection and diagnostics (FDD) controls be included with the economizer,” he said. “These controls ensure that the dampers and controls are working as they are intended and include alarms for sensor failures, when the unit is not economizing properly, when the damper is not modulating or is stuck open, or when an actuator is mechanically disconnected. Aaon factory-installed economizers can include all of the necessary controls and components to meet this code.”
Most manufacturers offer a variety of control options that are dependent on the method of control the customer wishes to use. Economizers from Johnson Controls Inc. are capable of functioning off of single dry bulb, single enthalpy, and dual enthalpy readings with the application of field-installed sensors.
“Additionally, our economizers can be paired with multiple airflow options, such as barometric relief and powered exhaust fans,” said Matthew Schlegel, commercial product manager, Johnson Controls. “Contractors can determine which economizer configuration to use based on the building design and typical ambient conditions of the application. Our product selection tool offers detailed information on the application of each sensing option, and our sales team is highly trained to help customers in making these decisions.”
Economizers do add cost to the price of a rooftop unit or air handler, and depending on the manufacturer, they can either be installed in the factory or in the field. Carrier, for example, offers a variety of factory- and field-installed economizers and control options to satisfy the needs of most applications, said Chris Opie, director of marketing, Carrier Commercial Systems North America.
“From the most economical field-installed electromechanical economizers to factory-installed, ultra-low leak economizers that are compliant to various FDD requirements, all Carrier economizers are integrated for maximum energy savings and comfort,” he said. “To accommodate the specific requirements of each rooftop unit and its application, they are offered as optional upgrades and priced accordingly.”
Some economizers can be retrofitted into existing equipment or units made by other manufacturers, but it really depends on the manufacturer. At Carrier, economizers can be retrofitted to select older units, including those from other manufacturers, but Opie suggests that installers consult their local Carrier expert, as existing unit controls and cabinet construction can have a large impact on compatibility.
The best energy savings opportunities for an economizer are in dry climates, particularly applications that require cooling nearly year round, such as data centers or process applications, said Taylor.
“Climates that are humid will require mechanical cooling to remove the humidity from the outside air entering the building,” he said. “That is why the International Energy Conservation Code (IECC) exempts certain areas from economizer requirements, such as the southern tip of Florida, Hawaii, Guam, Puerto Rico, and the Virgin Islands.”
Using an economizer in humid conditions is often not ideal, as it can lead to discomfort for the building occupants, according to Schlegel. Additionally, it could increase a building’s cooling load in order to counter humidity levels.
“As with location, applications will differ in the level of benefit received from the use of an economizer,” he said. “Comfort cooling applications see the benefits of an economizer as the variable cooling load is offset by free cooling, a process that can see even greater energy reductions as favorable outdoor temperatures provide more opportunities for application.”
Even though economizers provide more efficiency when there is less moisture in the air, they can still be used in high-humidity locations, said Dietrich. Because economizers offer greater efficiency with higher load requirements, they can often provide a great deal of energy savings in buildings with large load requirements, such as an arena.
Still, there are certain applications that would not be a good fit for economizers, such as hospitals, which have strict requirements for low space temperatures and humidity. Corrosive ambient environments, such as coastal applications, may also not be a good application for economizers because the gears and dampers can corrode, said Taylor.
“The Aaon design is less susceptible to being stuck in position than a damper assembly with linkages, but the aluminum gears and dampers can still corrode in some environments,” he said.
Maintenance is also important, added Taylor, because if the damper is not operating correctly or the sensors are out of calibration, the economizer may not be saving the energy it was designed to save.
“Contractors should always inspect the damper and make sure it is not stuck in position,” he added. “Also, make sure the high-limit temperature and lock-out temperature settings make sense for the environment and application.”
Given that air-side economizers offer such benefits as energy savings and improved IAQ, they should be a part of just about every commercial HVAC design. And thanks to new building codes and standards, they often are. For those owners who do not see the benefits of investing in an economizer, it is likely that they aren’t fully aware of the short- and long-term benefits economizers provide, said Opie. That’s why he urges contractors to make use of the array of tools and literature that many manufacturers offer.
“For example, the Carrier Commercial Invest software generates a quick comparison between different units (up to seven at a time) to show the value and return on investment of various configurations and options, including economizers,” he said.
Showing building owners and managers just how much energy they can save by utilizing an air-side economizer may make the sale just a little bit easier.
Water-Side Economizers Also Save Energy
Similar to air-side economizers, water-side economizers can help make chillers more energy efficient by using outside air to cool return water. Water-side economizers operate with free cooling cycles and hot water heat recovery from the condenser side of the chiller, and Daikin Applied offers products that work with both concepts, said Bill Dietrich, product general manager - chillers, Daikin Applied.
Free cooling cycles use ambient air to cool water in a chiller. For water-cooled chiller plants, systems often have a water-to-water heat exchanger to use colder water from the cooling tower when outside temperatures permit and transfer this cold water via the heat exchanger to the chilled water loop, explained Dietrich.
“In air-cooled chillers, free cooling can be achieved with stand-alone fluid coolers or by adding additional water coils to the air-cooled chillers,” he said. “Even if an economizer is only able to reduce part of a load, it still saves on energy when the chiller is activated to deliver the target chilled water temperature.”
Daikin Applied chillers are designed to work in conjunction with these free cooling systems, including the transition from free cooling to mechanical cooling when the chillers are required to start and operate with cold condenser water temperatures, noted Dietrich.
“Some systems skip the use of the heat exchanger and just run cold condenser water from the cooling tower directly to an oil-free centrifugal chiller,” he said. “In this case, the chiller operates at a low kW/ton with the cold, entering condenser water and can rival the free cooling system efficiency.”
Geography has a smaller impact on water-side economizers, as the type of system will determine how much energy can be saved. For example, heat recovery in chillers will be less location-sensitive than free cooling applications. For chilled water free cooling, a higher water temperature used in the building will provide more opportunities for the application, said Dietrich.
Publication date: 6/4/2018