If you’re responsible for a cooling system that has a capacity of 7.5 tons or more, you probably have an air-side economizer—and chances are it could use some attention.
When the outdoor temperature and humidity are mild, economizers save energy by cooling buildings with outside air rather than using refrigeration equipment to cool recirculated air (figure 1). When paired with well-designed systems, economizers can reduce cooling loads up to 30%.
Economizers are designed to save energy, but about half of all newly installed economizers don’t work properly―and their problems increase as they age. In addition, there’s a good chance that malfunctioning economizers waste more energy than they were intended to save. If an economizer breaks down when its damper is in a wide-open position, peak loads shoot up as cooling or heating systems try to condition the excess air entering the building. A computer simulation of an office building in arid Phoenix shows that a damper permanently stuck in the wide-open position could add as much as 80% to that building’s summer peak load, assuming that the building had enough cooling capacity to condition the excessive outside air.
What are the options?
Step 1 Select upgraded components, such as stainless-steel dampers, direct-drive actuators, and dry-bulb high-limit control.
- Stainless-steel dampers resist corrosion much better than the galvanized-steel and aluminum dampers typically used in economizers. Though stainless-steel dampers cost about twice as much as galvanized-steel dampers, they’re cheaper than the total cost (including labor) of removing and replacing a failed damper. When justifying the costs, be sure to consider the climate conditions of the facility: If the building is near sources of marine or industrial corrosion, you may need to replace the dampers more often than you would in milder climates.
- Direct-drive actuators, which physically move the dampers open and closed, have fewer moving parts between actuator and damper, and therefore fewer parts that can fail. They’re also much easier to install than common linked actuators and come at a similar cost. Since their introduction in the 1980s, a company named Belimo has dominated the market. Belimo’s legacy direct-drive actuators used to come at a price premium compared to the more failure-prone linked actuator, but increased competition has narrowed the price gap. In many cases, direct-drive models now cost the same as or less than their linked counterparts.
- Multiple economizer-control methods are available. High-limit controllers measure outside air temperature only; differential controllers measure both outside air and return air streams. Measurements can be performed using air temperature only (dry-bulb control) or using temperature and humidity (enthalpy control). A 2010 journal article from ASHRAE, Economizer High Limit Controls and Why Enthalpy Economizers Don’t Work, describes how researchers modeled various combinations of high-limit and differential controllers performing dry-bulb and enthalpy control. The study found that fixed dry-bulb controls using specific setpoints based on climate zone were the preferred choice. Single high-limit dry-bulb sensors have the lowest initial costs, are highly energy efficient, and are prone to minimal sensor error and energy penalties when sensors did experience errors. Differential enthalpy control, which is often cited as the most advanced control strategy, was one of the least energy-efficient methods due to the energy penalties resulting from sensor errors.
What’s best for your facility will vary by climate zone (figure 2). You can determine your location’s climate zone and see a map of US climate zones in the US Department of Energy’s report Guide to Determining Climate Regions by County (PDF).
Step 2 Consider upgrading to a Western Premium Economizer (WPE) specification to increase the reliability of and savings achieved from conventional economizers. WPE requirements include:
- The economizer must have fully modulating dampers to properly control the amount of outside and return airflow, both of which are proportionally adjusted by the economizer controls as the outside temperature varies.
- Instead of engaging or disengaging the economizer based solely on the temperature of the outside air, the unit must have a control strategy that compares the outside air temperature with that of the return air. As the outside air becomes warmer than the return air, the outside air damper closes, and the compressor provides all the cooling.
- Dry-bulb sensors (rather than enthalpy sensors) are also required because they offer higher reliability and lower cost.
- Instead of using a single-stage thermostat that will trigger either the economizer or the compressor when cooling is required, WPE requirements specify a two-stage thermostat that allows the economizer to operate whenever the outside air is cool enough. When cooling is required, the two-stage thermostat will first trigger the economizer. If the thermostat senses that more cooling is needed after that first stage, it will activate the compressor to provide a second stage of cooling.
Step 3 Commission economizers periodically—once on installation and at least twice a year after. Here are three testing techniques:
- Observe the damper position. Stand next to the outside air damper with a handheld thermometer and compare the damper position with the lockout and high-limit settings. If the damper’s position is inconsistent with the settings on the controller, either the controls are malfunctioning or the damper is stuck. For the same reason that a broken clock tells the right time twice a day, it’s impossible to know from a single observation whether a damper is functioning properly or just happens to be frozen in a position that happens to be consistent with the controls. You must repeat this test under a range of outside air conditions for this test to be effective.
- Fool the economizer controls. To test dry-bulb economizers, wait for a cool day when the economizer damper is open, and then warm the outdoor temperature sensor with your hands or an electric hair dryer. When the measured temperature exceeds the lockout setting, the damper should move to its minimum position. If the economizer has enthalpy controls, lightly spraying the enthalpy sensor with water from a spray bottle will temporarily raise the humidity of the air, which should trigger a reaction from the system. If the system doesn’t behave according to its control settings, either the sensors are inaccurate or the economizer controller is malfunctioning.
- Install temperature data loggers. For a detailed look at how individual economizers operate over time, diagnosticians can install portable devices that measure and log temperature. Typically, these devices are installed in the outside, return, supply, and mixed airstreams for two weeks. Building technicians can then download and diagnose collected temperature data using simple spreadsheet software. One manufacturer of data logging equipment is Onset, which has published several reports on its White Papers page on the use of its sensors for testing HVAC equipment.
Step 4 When all else fails, lock the economizer in a position that minimizes outside air intake. Be aware that you can’t cost-effectively keep some economizers in working order for a variety of reasons, including:
- It’s located in an especially corrosive environment
- It’s made from inadequate materials
- It’s capable of producing only inconsequential energy savings, even when in top condition
- It’s installed in a building with undersized outside and exhaust air openings
- The technicians servicing it aren’t motivated to keep it running well
Regardless of the reason, if an economizer repeatedly fails and it’s prohibitively expensive to repair or replace it, the best solution is to lock it into a position that minimizes outside air intake. Although you won’t get the benefits of the economizer’s potential energy savings, you will guard against it becoming a significant energy waster.
How to make the best choice
The biggest choice that economizer owners and operators face is whether it’s worthwhile to invest in upgraded components and testing for a particular economizer. To make this decision, estimate how much energy an economizer is likely to save, and then choose upgraded components and testing procedures accordingly. For example, a functioning economizer installed on a 30-ton packaged rooftop air conditioner might save about $1,000 per year, so it’s probably worth maintaining. An economizer in a unit one-tenth that size that saves only $100 a year might be better off locked in minimum position.
Determining savings The biggest challenge you’ll face is estimating the savings associated with a given economizer. Because those savings vary widely by location and building type, check with local sources to learn what savings economizers typically produce at similar buildings.
Water-based systems When involved with the design or retrofit of chilled-water-based cooling systems, keep in mind that it’s permissible to specify a water-side economizer (cooling tower) as a substitution for an air-side economizer in many energy codes. Under the right circumstances, such as when the building is in an arid climate and there’s available space to install a generously sized cooling tower, a water-side economizer will provide greater energy savings and less chance of equipment failure.
Advanced economizer controls Many economizer controls on the market simplify installation and configuration and add premium features and functionality. Stand-alone economizer controls, such as the Belimo Zip Economizer and the Honeywell Jade Economizer, offer premium functionality such as demand-controlled ventilation (DCV) and fault detection and diagnostics (FDD). In large spaces with intermittent occupancy, DCV is a strategy that uses carbon-dioxide sensors to determine the occupancy level of the conditioned zone and reduces the input of ventilation air at times of low occupancy. FDD systems monitor the sensors and controls of the economizer and notify the building manager if something isn’t working correctly. The 2016 Manufacturer Certification for Equipment, Products and Devices: Economizer Fault Detection and Diagnostics requires FDD for air-cooled unitary air-conditioning systems with more than 4.5 tons of cooling capacity; the FDD features must include the ability to detect when:
- A failure or faults in the air temperature sensor exist
- The economizer isn’t economizing when it should be
- The economizer is economizing when it shouldn’t be
- The damper isn’t modulating
- Excess outdoor air is being utilized
Advanced rooftop controls that include air-side economizers, DCV, and supply-fan speed controls can help retrofit rooftop units (RTUs) to improve their efficiency. When implemented, these controls deliver an average of 57% savings, but savings can vary drastically, from 22% to 90%. These controls also average a three-year payback, according to the report Advanced Rooftop Control Retrofit: Field-Test Results.
Maintaining your economizer
Dampers are notorious for falling into disrepair. According to the article Free Cooling: At What Cost?, a survey of California contractors found that economizers were disabled because their dampers wouldn’t move 30% to 40% of the time. This problem can have major energy consequences in regions that otherwise could take advantage of economizer operation; it can also have potentially serious impacts on indoor air quality in all climates.
Figure 3 shows outside air dampers on a packaged RTU. These dampers face a constant flow of dirty air that contaminates the pivot points and actuator mechanism, as the coarse prefilter only keeps larger things, such as leaves and birds, out of the unit. If the dampers operate properly, they can reduce the need to run the compressor when the outside air temperature is below about 60° Fahrenheit (F). However, unless you keep them clean and well lubricated, they can stick in place and rob the unit of free cooling potential (if closed) or overload the cooling coil with too much hot outside air (if open).
During damper servicing, clean and lubricate movable surfaces. Though you can clean the unit with a power washer, the high pressure can bend the fins and cause problems. A safer method is to scrub the surfaces with a brush, using a foam cleaner and a hose.
After cleaning and lubrication, test the damper. First, run it through its full range of motion. Tools can generate electrical control signals to drive the actuator, or you can manipulate the economizer setpoint at the control panel. Then check the economizer setpoint. Although many economizers are set at about 60°F, the setpoint can be as high as the return air temperature (about 74°F) to provide beneficial ventilation. However, in high-humidity climates (or where outside air is polluted), it may not make sense to maximize outside airflow at low dry-bulb temperatures.
As long as a service technician is already on the roof, this cleaning and testing should take about 15 minutes.
What’s on the horizon?
In the past decade, economizers have become an established technology, yielding guaranteed savings. Though there isn’t much in the way of upcoming advances in economizers, many building managers still have yet to implement them and reap the benefits that they provide.
Researchers in the Pacific Northwest National Laboratory (PNNL) have been working on self-correcting FDD for about a decade. The idea behind self-correcting FDD is that the device would be able to detect faults and also fix them without human intervention. PNNL has been working with an undisclosed manufacturer to develop and test this technology for air handlers, but it’s not clear when or if this technology will be commercially available.
Who are the manufacturers?
- Belimo (direct-drive actuators and controls)
- Bes-Tech (variable-speed drive [VSD] retrofit devices with economizer controls)
- Enerfit (VSD retrofit devices with economizer controls)
- Honeywell (dampers, actuators, and controls)
- Johnson Controls (dampers, actuators, and controls)
- Transformative Wave (VSD retrofit devices with economizer controls)
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