The Good News and Bad News about Economizers

By Dan Greenberg, E Source Associate Director, www.esource.com -- Foodservice Equipment & Supplies, 4/7/2008 12:05:00 PM

An economizer allows your HVAC system to take advantage of free cooling whenever the outside air drops below a predefined setpoint. An economizer consists of a collection of dampers, sensors, actuators and logic devices that control fresh air intake. When outside air is cool enough, the damper opens wide to allow that fresh, cool air into your building, reducing the load on the air conditioner. When outside air is too warm, the damper closes to the minimum position that still satisfies the need for fresh ventilation air. Depending upon an operator's climate and cooling loads, an economizer could cut their cooling bill by 10 percent or more.

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 breaks down when its damper is in a fairly wide-open position, peak loads shoot up as cooling or heating systems try to compensate for 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 percent to that building's summer peak load - assuming the building had enough cooling capacity to meet the much higher load resulting from cooling excessive outside air.

To increase the likelihood that an economizer will not turn from an energy saver to an energy waster:

1. Specify upgraded components, such as stainless-steel dampers, direct-drive actuators and enthalpy 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 are cheaper than the total cost (including labor) of removing and replacing a failed damper. Unless a building is near sources of marine or industrial corrosion, however, this extra cost is probably not justified.

. Direct-drive actuators 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.

. In a relatively dry, mild climate, a drybulb control system that simply measures outdoor air temperature is sufficient. In more humid climates, enthalpy-based controls, which account for both drybulb temperature and humidity, are best. Modern sensors that use solid-state electronics to measure enthalpy are much more reliable than older-technology sensors.

2. Test economizers right from the start and at least twice a year thereafter. Here are two testing techniques:

. 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.

The biggest choice economizer owners and operators face is whether it is 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 rooftop unit might save about $1,000 per year, and so it is 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. 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.