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What is an Economizer?

Economizers were designed to save energy used for heating and cooling by lessening the total energy consumption. They are also referred to as free cooling because they can be used in addition to or in replace of rooftop and boiler units. Keep reading to see how an economizer can benefit you.

Choosing an economizer

Economizers are versatile as there is one for each HVAC setup. The following are the types of economizers.

Air-side economizer

An air-side economizer, the most commonly used, is a ventilation system installed in addition to or in replace of a rooftop unit. It brings outdoor air inside and disperses it to use for cooling through a series of dampers and fans. Then the collected warm air is exhausted outside instead of being recirculated. If the outside return air is considerably cooler than the set temperature, the economizer may mix the exhaust air with it to reach the desired temperature and humidity. A properly maintained control system is vital to the operation of this economizer so that it can alert you in the case of a mechanical problem. A well-maintained air-side economizer cuts the cost of energy consumption because the air conditioning system does not turn on as often.

The air-side economizer’s performance is as good as its environment, although maximum efficiency can be achieved through the proper settings. A couple things to consider are

  • Temperature: Since an economizer’s data center must run continuously, an air-side economizer can be beneficial in hot and cold climates. Hot climates can have cool nights. The economizer works with the established air conditioning system to maintain the desired temperature.
  • Humidity: At times, humidity can become a problem for an economizer’s equipment. If there is too much humidity, condensation can form. If there is not enough, the economizer will spend extra energy to humidify the air.

Water-side economizer

A water-side economizer eliminates the need of cooling through compressors and is installed between the chiller and data center of a boiler. The production of the cool air and the removal of waste heat are the only change made in the boiler room after the installation of a water-side economizer. When the conditions are right, the warm return water runs through the economizer. The condenser rejects the warm water into the atmosphere by way of a dry cooler or evaporative tower. The outside air cools the condenser water to allow for proper heat exchange. Once the water reaches the desired temperature, the cool water returns to the data center to be used for cooling.

Stack economizer

A stack economizer recovers some heat for pre-heating water. This water is either used for make-up water or another need within the boiler system. This type of economizer is an efficiency measure when large amounts of make-up water is involved or when large amounts of hot water is needed somewhere else.

Stack economizers are heat exchangers with hot flue gas on one side and water on the other. These economizers must be large enough for

  • The volume of flue gas that will pass through it
  • The flue gas temperature
  • Maximum pressure drop allowed through the stack
  • The kind of fuel used in the boiler
  • The amount of energy that needs to be recovered

How air-side economizers work

This type of economizer decides how much outdoor air comes into a space with the help of its components. In return, the workload of the cooling system is reduced. Dampers control the air introduced, recirculated or exhausted from a space. Logic controllers and outside temperature sensors determine if the conditions are right for the economizer to function.

The actuators open and close the dampers based on the commands from the logic controllers. When the outside temperature is cool enough, the outside air damper will open and the air conditioning compressors shut off.

When the economizer is running, an exhaust fan will always be on. The same amount of air taken in for the economizer to work properly should be exhausted. If the outdoor temperature rises too high to provide cooling, the damper is moved to a minimum position to maintain minimum ventilation. The return will open, the exhaust will close, and the compressors will take over the cooling process.

Common Problems

Some economizers can be defective from the very beginning whether from the installation process or the manufacturer. Keep reading to see how to fix some common problems.

Improper sensor

It is easy to assume that an air-side economizer with an enthalpy-based control is more energy efficient than temperature based; however, it can be more cost effective to use a temperature-based control when considering operation and maintenance costs and sensor as energy benefits. If you have an enthalpy-based control, evaluate the energy costs. Also, make sure all the sensors are in a good location to achieve optimum operation.

Oversized return air dampers

Return air dampers need around 1,500 fpm of air velocity flowing through them. If it is considerably less, consider blocking off some return air dampers to increase the velocity.

Insufficient building pressure relief

During its operation, there should be sufficient pressure relief available to avoid pressure issues. If the difference in outdoor and indoor pressure is greater than one-tenth of an inch during economizer operation, explore the relief air system.

Maximizing efficiency of an air-side economizer

Using an economizer will help you save on electricity. The more outside air that can be used for cooling, the less the air conditioner compressor is used.  Also, economizers can extend the life of an HVAC unit if it is being maintained properly. Here are additional tips to maximize the benefits of your economizer.


Economizers can be used anywhere; although cool, dry climates are the best fit. Hot and humid climates can increase the risk of bringing mold and mildew inside the building. Also, the economizer would not be used often because the compressors would have to turn on to cool the air.


If an economizer is being added to an existing HVAC unit, the size difference must be taken into consideration. The person installing it should plan for a larger air intake and relief opening, proper damper sizes, and appropriate controls. A relief air fan may be required. The HVAC system should continue to maintain appropriate ventilation when the economizer is not in use.


The components should be made of aluminum or galvanized steel to withstand the environment. The sensors must be calibrated to differentiate between the total heat content of the inside and outside air.

Economizers are an efficient way of cooling a building if it is well-maintained. If you have an existing economizer and need help with troubleshooting or would like to install a new one, a certified HVAC Expert is on hand to help. Ask an Expert for a customized answer to your specific problem.

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