Economizers 101

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

1. Thermostat or BAS sends cooling call (Y1) to RTU
2. RTU checks with economizer logic model to see if “free cooling” is available
3. If ambient temperature is equal to or less than setpoint (55 degrees F), the economizer will open and will begin to provide free cooling
4. Logic module monitors mixed air temperature and will modulate the return and outside air dampers to maintain 55 degrees F
5. 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

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

Dual Enthalpy

• More energy savings than dry bulb or single enthalpy

1. Thermostat or BAS sends cooling call (Y1) to RTU
2. RTU checks with economizer logic model to see if “free cooling” is available
3. If ambient enthalpy is less than return air enthalpy, the economizer will open and will begin to provide free cooling
4. Logic module monitors mixed air temperature and will modulate the return and outside air dampers to maintain 55 degrees F
5. If the mixed air temperature is above 55, then stage one compressor will be energized
6. If the thermostat calls for stage 2, the economizer closes to minimum position and ends “free cooling” mode
7. Barometric relief will provide building pressure relief as needed
8. *Make sure the unit is set up to allow compressor operation during economizer mode
9. *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?