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
B. US DOE map of 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.