Why Do I Need an Economizer?
- Minimum fresh air ventilation requirements
- Energy code requirements
- FREE cooling
Economizer Control Options — Which Type is Best?
Dry Bulb
- Simple
- No additional sensors need to be installed
- Least expensive upfront option
- 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
- More energy savings than dry bulb
- Only one additional sensor to be installed
- Not too complicated
- 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
- More energy savings than dry bulb or 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 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?