Cooling Economizer Controls

Many commercial buildings use air conditioning almost year-round. For example, a restaurant produces so much heat that even when it is cold outside, the building may need cooling. An economizer uses the cool air outside to save cooling energy inside.

How It Works

An economizer control is a mechanical device that uses cool outside air to cool inside a building, thus reducing the need for electrical cooling. The control is installed as a part of the ventilation/cooling system. When it senses that outdoor conditions are correct, it shuts the air conditioner compressor off and opens the outside air damper. The fan then draws in outside air to provide cooling. When the control senses that the outside air is unsuitable for cooling, the damper closes and the air conditioning compressor turns on again.

An air conditioner uses electricity at three major components: the compressor; the condenser fan(s); and the evaporator or supply air fan(s). Energy used by the compressor accounts for most of the total electricity consumed. As the economizer replaces air cooled mechanically by the cooling unit with cool outdoor air, the compressor and condenser fan(s) do not have to run, which results in considerable energy savings.

The economizer can also work with the compressor to improve the overall efficiency of the system by drawing in cooler outdoor air to replace warm indoor air. The most effective economizers will operate in the following manner:

  1. When outdoor air is cool enough to satisfy air conditioning requirements, the economizer turns off the air conditioner and brings cool outdoor air into the facility.
  2. When the outdoor air is cooler than indoor air, but not cool enough to satisfy temperature requirements without further cooling, the economizer closes the return air damper, drawing in outdoor air through the air conditioner where it is mechanically cooled to the proper temperature. The air conditioner doesn't use as much energy to satisfying cooling requirements as it would without the economizer.
  3. If the outside air temperature is warmer than the inside return air, the outside air damper closes so that only return air is used for cooling.

Sometimes during installation the compressor is wired in such a way that it "locks out", or shuts off, whenever the economizer turns on, yet one of the ways the economizer helps save is by working with the compressor when it is the most efficient mode. Make sure that the contractor wires the economizer and compressor in such a way that they can work together.

Types of Economizers

There are two major types of economizers; one is a refinement of the other. The simple dry bulb economizer senses air temperature only, is comparatively inexpensive to install and easy to maintain, and is practical for most commercial buildings. Because it does not take latent heat (i.e., moisture) into consideration it will not provide as high a comfort level and will not save as much energy. The wet bulb or enthalpy economizer is a more sophisticated control that senses both the moisture content and temperature of the air. Its higher cost and maintenance requirements sometimes outweigh the advantage gained in comfort. If in doubt about which type is best for your situation, check with a mechanical contractor or consulting engineer.

Maintenance

An economizer that operates improperly may save little or no energy. Routinely check dampers, linkages, lever arms and other moving parts to ensure that they move freely. Clean and lubricate them as often as needed, following the manufacturer's instructions. The enthalpy control device is fragile and should be checked by a contractor frequently; replacement cost is far less than the price of extra energy used when the economizer is improperly controlled.