Key Takeaways
- Free Cooling Concept: Economizers use outdoor air instead of mechanical cooling when conditions are favorable, significantly reducing energy consumption while still requiring power for fans and controls
- Core Components: System includes outdoor/return air dampers, sensible or enthalpy sensors, damper actuators, control boards, and relief systems to manage building pressure
- Sensor Selection: Choose between sensible temperature sensors (dry bulb only) or enthalpy sensors (total heat content) based on your climate and humidity conditions
- Fresh Air Benefits: Beyond energy savings, economizers provide ventilation and can integrate with CO2 sensors for demand-controlled ventilation
- Control Integration: Economizer controls work in sequence with rooftop unit controls, determining whether to use free cooling or mechanical cooling based on outdoor conditions
What Is an Economizer in HVAC?
Simply put, it is a mechanical device that is designed to reduce the consumption of energy, whether it be fuel, electricity, or other resources.
According to Wikipedia, the first economizer was patented by Edward Green in 1845. It was used to increase the efficiency of stationary steam boilers.
In this article, the discussion will revolve around air-side economizers.
You will typically see them as an accessory built into rooftop units used for the purpose of “free cooling.”
Free cooling is a funny term because it’s not actually “free” – the fan motor and economizer controls must be powered in order to operate, which consumes energy.
The term merely demonstrates the fact that less power consumption is taking place due to the fact we are utilizing outdoor air to cool a space rather than the use of a compressor or compressors.
Economizers also offer the added feature of providing fresh air to the building and its occupants. When properly configured with BMS control fundamentals, a carbon dioxide sensor can be integrated into the setup.
As CO2 levels increase within the building, the outdoor air dampers are commanded to open, filling the space with fresh air. As CO2 levels drop off, the dampers return to their minimum position.
The Guts of an Economizer
The economizer setup employs several parts in order to operate correctly. Understanding each component is crucial for proper troubleshooting and maintenance.
Outdoor Air Dampers
A set of outdoor air dampers that are directly linked to the return air dampers are used to control air flow. They move together as one – as the outdoor air dampers begin to open, the return air dampers begin to close and vice versa.
Outdoor Air Sensor
This sensor is responsible for determining if the outdoor air is acceptable for free cooling. In most cases, there will be an option between a sensible temperature sensor or an enthalpy sensor.
#### Sensible Temperature Sensor
Measures dry bulb temperature of the air. This simpler option works well in dry climates where humidity isn’t a major factor.
#### Enthalpy Sensor
Measures heat content within the air measured in BTU/lb. This sensor takes both dry bulb temperature and wet bulb temperature into account for total heat content. In humid climates, this provides more accurate control since it considers moisture content.
Indoor Air Sensor
This sensor reads sensible temperatures and is responsible for maintaining mixed or discharge air temperature.
The damper assembly will modulate according to feedback from this sensor to maintain a pre-determined mixed or discharge air set point.
On newer economizer controls, like the Honeywell Jade, for example, you are able to set the mixed or discharge air temperature as desired.
Damper Actuator
The damper actuator receives a signal from the economizer control board and moves to the assigned position to maintain the mixed air or discharge air set point. These actuators typically operate on 24VAC or 0-10VDC signals.
Barometric Relief Damper or Power Exhaust System
When using free cooling you must remember that you are introducing fresh air – this added air into the space can cause positive pressure issues within a building.
To alleviate this problem, economizers in most cases will have a built-in barometric relief damper or power exhaust system. The barometric relief opens automatically based on building pressure, while power exhaust systems actively remove excess air.
Control Board
The control board is the heart and soul of the operation.
The control board receives sensor input signals, internally calculates the next step, and relays the output signals to the damper actuator and power exhaust motor if utilized. Modern control boards can integrate with BMS network architecture for centralized monitoring and control.
Order of Operations
To keep it simple, the following example will be based on a single-stage cooling rooftop unit complete with an economizer package. Understanding this sequence is essential for troubleshooting economizer issues.
Step 1: Call for Cooling
On a call for cooling from the thermostat or BAS (building automation system), the Y1 terminal will be powered.
Step 2: Signal Routing
In most cases, the signal will first move through the rooftop control board and over to the economizer control.
Step 3: Decision Point
At that point, the economizer control will then decide whether to proceed with free cooling or mechanical cooling based upon the outdoor air conditions – either using sensible temperature of the air or the heat content of the air measured in enthalpy.
Step 4A: Mechanical Cooling Mode
If the outdoor air is not suitable for free cooling, the control signal will be then relayed back to the main control board of the rooftop and initiate mechanical cooling (compressor operation).
Step 4B: Free Cooling Mode
If the outdoor air is suitable for free cooling, the outdoor air dampers will modulate from their minimum position to maintain the mixed air or discharge air set point until the space temperature is reached. The damper minimum position is set up during commissioning to maintain constant fresh air to the building and occupants.
Step 5: Cycle Complete
Once the thermostat or BAS has been satisfied, the call for cooling will cease and dampers return to minimum position.
Most air-side economizers in general work as explained above. It is best to contact the manufacturer of the equipment you are working on for technical advice or when issues pertaining to that system arise.
Maximizing Your HVAC Business Efficiency
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Common Economizer Issues and Maintenance
Regular maintenance of economizer systems is crucial for optimal performance. Just like following proper commercial HVAC maintenance scheduling, economizers require periodic attention to prevent common issues:
- Stuck or binding dampers due to dirt accumulation or mechanical wear
- Failed sensors providing incorrect readings leading to improper operation
- Actuator failures preventing proper damper modulation
- Control board malfunctions causing erratic operation
- Improper minimum position settings affecting ventilation requirements
📺 Check out this video of an onsite breakdown of a Honeywell Jade Economizer
Finally!
Understanding economizer operation is essential for any HVAC technician working on commercial rooftop units. These systems provide significant energy savings when properly maintained and configured. Remember that while the concept of “free cooling” sounds simple, the integration of sensors, dampers, and controls requires careful attention to detail for optimal performance.
Check out the link to my YouTube channel for more tips, tricks, and troubleshooting videos, and check out The HVAC Know It All podcast here or on your favorite podcast app.
Happy HVACing!
