HVAC Economizers

Introduction

Economizers are simply vents to the outside in the return air side of the air handler section of HVAC units. A few years   ago economizers were rarely used. Today they are pretty much standard equipment and may even be required by local code. However, most manufactures still consider them ‘options’ that must be specified at the time of ordering for factory installation, or added on in the field by simple panel replacement and plug-in controls.

Economizers allow conditioned spaces to be cooled by outdoor air, rather than running the compressor of the air conditioning section. Basic economizer controls operate with a simple outdoor temperature setting that allows them to be either open or closed. Better units use ‘enthalpy controls’ which sense relative humidity in addition to sensible temperature. The most advanced units use modulating controls and ‘integrated operation’ that allows the compressor to run in combination with the economizer in a part open condition.

Although they are a source of ‘free cooling’ the amount of free cooling depends on the local weather conditions and how well the economizer functions and is controlled. Economizer often fail due to poor maintenance. SeeHVAC Rooftop Units. Economizers offer the most potential savings in relatively dry climates when serving spaces that have high internal heat gains when it is less than about 65F outside. After the outside temperature rises above about 70F and/or the relative humidity rises above about 60%, economizers offer very little to no savings and are shut off.

Identification

The HVAC unit to the right is equipped with an optional economizer section and a standard outdoor air intake vent (one each hood)

Note that most all HVAC units will have a small damper for fresh air intake to balance the system and meet building codes. These dampers are open all the time and are not the same as economizers that are much larger and open and close under control.

HVAC_RooftopUnit
 

Even small HVAC units can be equipped with economizers.

Look for the existence of air intake hoods. (Larger, not to be confused with fresh air vents.)

HVAC_Sm_WEcono
 

Note that this unit does NOT have an economizer, but there is only a fresh air intake vent (sticking out on round duct) on the return air duct.

Ducts_Outside

Controls

Facilities with Energy Management Systems and more sophisticated controls will likely have some sort of ‘weather station’ like the one below. This Novar weather station includes a sensible temperature sensor, a humidity sensor, and a photo eye for daylight control. The information from this station is feed to the EMS control station that controls the operation of the economizer according to actual weather conditions and the needs of the space.

Other economizer controls include fixed temperature settings (open/close at a set sensible temperature), variable control, and enthalpy controls – either in combination with an EMS or stand-alone on the HVAC unit.

 

EMS_WxStation

EMS_Panel

EMS Weather Station

EMS Central Control Panel

 

 

 

HVAC_EconomizerDampers

 

The economizer controls activate damper controls to open/close dampers to either a full open/closed position or some part-open setting, based on the capabilities of the controls. Dampers, linkages and controls must all function properly for proper temperature and energy management.

 

HVAC_EconomizerDamper_Draw1

HVAC_EconomizerDamper_Section1

Exterior drawing of economizer section

Cross section drawing of economizer section showing rain hoods, dampers (shown as circles w/louvers) and location of enthalpy control switch sensor.

 

More Information

HVAC Measures

HVAC Rooftop Units

 

 

 

 

 

Source: Text by Bob Fegan 1/2009; photos by Bob Fegan; Damper photos from CEC ‘Small HVAC System Design Guide’ October 2003; economizer section drawings from Carrier installation manual, typical economizer;


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