Economizers for Packaged Air Conditioning Units

Overview

Packaged Rooftop Air Conditioning Units (RTUs) can consume a significant amount of electricity, especially when they operate for long hours. Using an "economizer" can reduce their energy use.

An economizer consists of dampers, sensors, actuators, controls and linkages that work together to bring outside air into a building.

Economizer systems take advantage of favorable weather conditions to reduce the amount of mechanical cooling required, by introducing cool outdoor air into a building.

Free Cooling

The term "free cooling" is used in the HVAC industry to describe savings achieved from a properly working economizer. In mild climates, economizers save energy by using outside air instead of refrigeration equipment to cool the building.

Energy Savings

Estimating the savings associated with a given economizer depends on many factors including building location, construction materials, building usage, occupancy, the nature of the HVAC equipment, HVAC parameter settings and control systems. When economizers are properly installed and maintained, they can reduce mechanical cooling by up to 75% in certain climates.

Principle of Operation

For most common applications, a sensor will monitor either the outdoor air "dry-bulb" temperature or the enthalpy (a measure of the total heat content of air based upon both the air's temperature and its humidity) to determine whether it is below a certain predetermined setpoint. This will cause an actuator to open the outdoor air damper and alter the path of the return air so that it is exhausted from the building. This is called "single sensing". Facility operators need to determine which setpoints are best for their location, building type and building internal loads.

Adding a second sensor in the return air is called "differential sensing". With this method, instead of determining an optimized outdoor air setpoint, the control system simply chooses the source of air (outdoor air or return air) that has the lowest temperature or enthalpy.

Cost

The extra costs for an optional economizer when purchasing a rooftop unit are summarized in the following table. An installation cost of $1000 will be added when retrofitting an existing system with an economizer.

Compressor Capacity (ton) Average Cost ($)
Less than 10 $500
Greater than 10 up to 20 $900
Greater than 20 up to 30 $1,100
Greater than 30 up to 60 $2,500
Greater than 60 up to 100 $4,000

Comparison

The chart below provides percentage electrical savings achieved for RTUs equipped with temperature-based economizers in seven cities across Canada for different types of buildings and systems. The energy performance models are derived from Natural Resources Canada's (NRCan's) set of prototype buildings developed in support of the "Technical Guidelines" produced for the Commercial Building Incentive Program (CBIP). The models all provide for a dry bulb limit of 74ºF, above which the economizer control is disabled and the outside air returns to minimum. All systems are heated with Natural Gas.

 

For more information refer to the Fact Sheet "Economizers for Packaged Air Conditioning Units". Developed for NRCan by Prism Engineering.