The function of the economizer is, as its name implies, to "economize" or save on cooling costs. Obviously, it costs money to operate the compressor. If the compressor can be shut down and the system still provide adequate cooling, energy savings can be realized.

Heat internal to the building, such as people, lights, computers, copy machines, motors, and other machines, causes the temperature inside a structure to increase. Heat soaked up by the building structure may also continue to heat the building long after the temperature outside the building has dropped. There are times when the temperature outside a building is lower than the temperature inside.

Whenever the HVAC system is calling for cooling and the temperature outside is cool enough, it is economical to shut off the compressor and bring in cool outside air to satisfy the cooling needs of the building. Such is the function of an air economizer system.

There is one drawback to this type of control system. Even though the thermostat acknowledges that the outside air temperature is low enough to cool the building, the outside air may be too humid to provide adequate comfort for the building occupants. The occupants will feel cool but clammy. The solution is an economizer that adds a second control, which works in harmony with the outdoor thermostat and measures the outdoor air humidity. Such a control is called an enthalpy control. The term enthalpy means total heat. The enthalpy control measures both sensible and latent heat in the air and only allows outside air to be used for cooling if the air is both cool and dry enough to satisfy the space conditions.

Shown is a rooftop economizer, the Carrier EconoMi$er+ OA hood design. (Photo courtesy of Carrier Corp.)
If the indoor thermostat calls for cooling and the outside air enthalpy (total heat) is low enough, then the economizer brings in this cooler and less humid air and uses it for cooling instead of operating the compressor. Using the outside air for cooling is less expensive than operating the compressor to provide cooling.

So an enthalpy control is a control which checks to see if both the temperature (sensible heat) and the humidity (latent heat) are low enough to be used for cooling. This combination provides for the greatest comfort at the least cost.

Not all economizers use enthalpy controls. Some just check the outside air temperature and do not check the outside air humidity. Those controls do not provide the same levels of comfort as enthalpy controlled economizers.

Economizers can save a great deal of energy. They can also waste energy if they are not operating properly or are improperly adjusted. For example, if the outside air dampers are not closing properly when the outside air temperature is high, then hot air is unnecessarily entering the building and causing the air conditioning compressor to operate longer and under higher loads, thus consuming a great deal more energy than necessary.

If the dampers are open too far during the heating season, the heating system must heat the extra outside air entering the structure. Such extra heating and cooling costs can be quite high. The cost of a service call to repair such a problem is often less than the cost of one or two months of energy wasted.

Many economizers are not functioning at all or are out of service because they are not well understood by some service technicians. In fact, some service technicians simply disable them. It is essential that economizers are working properly and saving energy rather than increasing costs.

Since air economizers control and vary the amount of outside (fresh) air brought into a structure, they play an integral role in maintaining the quality of indoor air. A properly operating economizer can greatly improve indoor air quality (IAQ) and reduce air quality-related illnesses. Therefore, it is important for the service technician to have at least some knowledge of indoor air quality and its relationship to heating and cooling system operation.

Air economizers are available for residential and commercial systems and can be retrofitted to most systems as energy conserving devices. Most packaged light commercial systems (rooftop systems) have an economizer add-on package as an option which can be installed when the system is new or added to the system later.

Economizer Maintenance

The following items should be checked at least annually to ensure the air economizer is operating properly:

  • Setting and operation of the outdoor thermostat or enthalpy control;

  • Condition of the outdoor thermostat or enthalpy control;

  • Proper setting and operation of the economizer mixed air thermostat;

  • Proper damper operation and lubrication;

  • Minimum damper position adjustment;

  • Correct operation of the system when a call for cooling comes from the thermostat;

  • Function and condition of the economizer damper motor; and

  • Condition of the wiring and electrical terminations.

    Since the enthalpy control is located in the outdoor air airstream and is a relatively sensitive control, it is not uncommon to have to replace it every few years depending upon the location of the equipment and the weather extremes in the area. The cost of a replacement control is usually recovered quickly through the energy saved. Economizer service should be a part of the scheduled maintenance performed at least on a yearly basis.

    Just as our automobiles need regular service, so do residential and commercial heating and cooling systems. Like automobiles, the frequency of service depends upon how it is operated, how often and long it operates, and the environment where it operates. Like automobiles, well-maintained systems operate more efficiently, last longer, and fail less often.

    Norm Christopherson is a former HVACR instructor, a technical writer, and a seminar presenter. He is currently seeking seminar and training opportunities. He can be contacted at nchristo@juno.com.

    Publication date: 05/03/2004