Economizers

If you are 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). A properly operating economizer can cut energy costs by as much as 10 percent of a building’s total energy consumption (up to 20 percent in mild coastal climates), depending mostly on local climate and internal cooling loads.

Figure 1: The components of an economizer
An economizer is collection of dampers, sensors, actuators, and logic devices that work together to decide how much outside air to bring into a building.

So economizers are designed to save energy, and that’s good. The bad news is that probably about half of all newly installed economizers don’t work properly, and their problems increase as they age. To make matters worse, there’s a good chance that malfunctioning economizers waste much more energy than they were intended to save. If an economizer (which is actually a temperamental collection of parts including dampers, sensors, actuators, controls, and linkages) breaks down when its damper is in a fairly 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, Arizona, shows that a damper permanently stuck in the wide-open position could add as much as 80 percent to that building’s summer peak load—that is, assuming the building had enough cooling capacity to condition the excessive outside air.

What are the options?
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There are four steps you can take to increase the likelihood that an economizer will not turn from an energy saver to an energy waster.

Step 1. Specify upgraded components, such as stainless-steel dampers, direct-drive actuators, and drybulb high-limit control.

  • Stainless-steel dampers resist corrosion much better than the galvanized-steel and aluminum dampers typically used in economizers. And though stainless-steel dampers cost about twice as much as galvanized-steel dampers, they are 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, the dampers may need to be replaced more often than they 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 are also much easier to install than typical 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 the return air streams. Measurements can be performed using air temperature only—called drybulb control—or using temperature and humidity, called enthalpy control. A 2010 journal article from ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers), Economizer High Limit Controls and Why Enthalpy Economizers Don’t Work, describes how researchers modeled various combinations of high-limit and differential controllers performing drybulb and enthalpy control. The study found that fixed drybulb controls using specific setpoints based on climate zone were the preferred choice. Single high-limit drybulb sensors featured the lowest first costs, inherently high energy efficiency, minimal sensor error, and minimal 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, of course, 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).

Figure 2: Optimal economizer setpoints differ based on climate zone
High-limit drybulb economizer control is a strategy that uses measurements of outside air temperature only, disregarding humidity (enthalpy control) and return air characteristics (differential control). In 2010, researchers from ASHRAE modeled various combinations of controls and found that fixed drybulb controls were the preferred choice in all climate zones due to their simplicity and inherently high energy efficiency. Researchers also recommended optimal setpoints for each climate zone (A). The climate zones shown are delineated by the US Department of Energy (DOE) in its report, “Guide to Determining Climate Regions by County” (B).

A. Setpoints for turning off high-limit logic economizers, by climate zone

A table listing setpoints by climate zone

B. US DOE map of climate zones

A map from the DOE showing US climate zones

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.
  • Drybulb 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—on installation and at least twice a year thereafter. 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 is momentarily consistent with the controls. For this test to be effective, it must be repeated under a range of outside-air conditions.
  • Fool the economizer controls. To test drybulb 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 does not behave according to its control settings, either the sensors are inaccurate or the economizer controller is malfunctioning.
  • Install temperature dataloggers. 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-air, return-air, supply-air, and mixed-air streams for two weeks. The collected temperature data may then be downloaded and diagnosed using simple spreadsheet software. One manufacturer of datalogging equipment is Onset, which has published several white papers on the use of its sensors for testing HVAC equipment.

Step 4. When all else fails, lock the economizer in its minimum-outside-air position. Some economizers cannot be cost-effectively maintained in working order, for a variety of reasons, including:

  • The device is located in an especially corrosive environment
  • It’s made from inadequate materials
  • It’s only capable of producing 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 its minimum-outside-air position. Although you won’t get the benefits of the economizer’s potential energy savings, you will guard against its becoming a significant energy waster.

How to make the best choice
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The biggest choice faced by economizer owners and operators is whether or not it’s worthwhile to invest in upgraded components and testing for a particular economizer. To make this decision, first estimate how much energy an economizer is likely to save. Then, choose upgraded components and testing procedures accordingly. For example, a functioning economizer installed on a 30-ton packaged rooftop unit 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 are typically produced by economizers 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 located 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. There are a host of economizer controls on the market that are designed to simplify installation and configuration and add premium features and functionality. Stand-alone economizer controls, such as the Belimo Zip Economizer or 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 2013 California Energy Code 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:

  • Failure or faults in the air-temperature sensor
  • That the economizer not economizing when it should be
  • That the economizer is economizing when it shouldn’t be
  • That the damper isn’t modulating
  • When excess outdoor air is being utilized

More-advanced controls for single-zone packaged rooftop units add variable-speed drive (VSD) capabilities to the evaporator fan, in addition to DCV and FDD. The Transformative Wave Catalyst, Enerfit V2, and Bes-Tech Digi-RTU controls fall into this category. Some of these devices also add VSD functionality to the compressor. Devices in this category have been shown to save anywhere from 26 percent to 57 percent of the energy consumed by the HVAC units, mostly due to fan energy savings when operating in ventilation-only mode.

What’s on the horizon?
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As of 2014, any manufacturer can claim that its product features FDD, but there’s no minimum level of functionality that this claim ensures. ASHRAE Standard 207, expected to come out in January 2016, will define a minimum level of FDD functionality and provide a uniform testing methodology to ensure that the FDD functions are working correctly.

Researchers in the Pacific Northwest National Laboratory (PNNL) have been working on self-correcting FDD on and off for about a decade. The idea behind self-correcting FDD is that the device would not only be able to detect faults, but it could also fix them without human intervention when they occur. 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?
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Content last reviewed: 
09/15/2014
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