Economizers 101

Why Do I Need an Economizer?

Economizer Control Options ⁠— Which Type is Best?

Dry Bulb

Thermostat or BAS sends cooling call (Y1) to RTU
RTU checks with economizer logic model to see if “free cooling” is available
If ambient temperature is equal to or less than setpoint (55 degrees F), the economizer will open and will begin to provide free cooling
Logic module monitors mixed air temperature and will modulate the return and outside air dampers to maintain 55 degrees F
Barometric relief will provide building pressure relief as needed

Single Enthalpy

Thermostat or BAS sends cooling call (Y1) to RTU
RTU checks with economizer logic model to see if “free cooling” is available
If ambient temperature and Rh are below the set curve, the economizer will open and will begin to provide free cooling
Logic module monitors mixed air temperature and will modulate the return and outside air dampers to maintain 55 degrees F
If the mixed air temperature is above 55, then stage one compressor will be energized
If the thermostat calls for stage 2, the economizer closes to minimum position and ends “free cooling” mode
Barometric relief will provide building pressure relief as needed
*Make sure the unit is set up to allow compressor operation during economizer mode
*Need two stage thermostat for this to work properly

Dual Enthalpy

Thermostat or BAS sends cooling call (Y1) to RTU
RTU checks with economizer logic model to see if “free cooling” is available
If ambient enthalpy is less than return air enthalpy, the economizer will open and will begin to provide free cooling
Logic module monitors mixed air temperature and will modulate the return and outside air dampers to maintain 55 degrees F
If the mixed air temperature is above 55, then stage one compressor will be energized
If the thermostat calls for stage 2, the economizer closes to minimum position and ends “free cooling” mode
Barometric relief will provide building pressure relief as needed
*Make sure the unit is set up to allow compressor operation during economizer mode
*Need two stage thermostat for this to work properly

Sensor Error

Dry bulb control sensors will be the most accurate and reliable, typically around +/- 0.33 degree accuracy
Enthalpy sensors typically average +/- 3% accuracy
Dual enthalpy control reliability will be around +/- 6%

Are Economizers Required by Code?

2006/2009/2015/2018 IECC: Economizers are required by code for rooftops and splits systems that are greater than 54,000 BTU/H (i.e. 5 tons and larger)
2012 IECC: Economizers are required by code for rooftops and splits systems that are greater than 33,000 BTU/H (i.e. 3 tons and larger)

Economizer Exceptions (where NOT required in 2015 IECC)

Systems expected to operate less than 20 hours/week
Where greater than 25% of air designed to be supplied by the system is to spaces that are designed to be humidified
Systems that serve residential spaces where system capacity is less than 5 times requirement
Where use of outdoor air for cooling will affect supermarket open refrigerated casework systems
Where cooling efficiency meets or exceeds efficiency requirements

What is the ROI of an Economizer?