Airside Economizers

By Suyash Vidwans on Thursday 14 February 2019

Commercial buildings often require mechanical cooling even when the outdoor air temperatures are mild or cool. Internal core spaces of commercial buildings often contribute to these mechanical cooling loads because heat losses at the building envelope are unavailable to offset the internal heat gains e.g. computers, lighting & occupants. Solar gains can also be a contributing factor for higher cooling loads, particularly in west & south-facing spaces with significant amounts of glazing.

Airside economizers provide a perfect low-cost solution to this design requirement. Airside economizers are a duct and damper arrangement with an automatic control system that together allow a cooling system to supply outdoor air to reduce or eliminate the need for mechanical cooling during mild or cold weather. They serve a dual purpose of saving cooling energy and improving indoor air quality by supplying additional outdoor air. The effectiveness of an economizer depends on loads characteristics of the building, type of HVAC system and the local climate.

With obvious benefits of integrating this component in an HVAC system, ASHRAE Standard 90.1 which is one of the most widely referenced standards for building performance, has included airside economizers in both prescriptive and performance-based approaches. The Energy Cost Budget (ECB) method of ASHRAE Standard 90.1 only exempts climate zones 1a and 1b from having an airside economizer.  In these “Very Hot & Humid” (CZ 1a) and “Very Hot & Dry (CZ 1b)” climates, the higher enthalpy of outside air throughout the year eliminates the advantages of having an economizer. In mainland United States, a few southern counties of Florida and Texas are exempt from having an HVAC system with integrated economizer operation. All remaining climate zones need an airside economizer when system cooling capacity is more than 4.5 tons (~16 kW).

Figure 1: Modeling fixed Dry-Bulb Temperature and differential enthalpy economizer as high limit shutoff in ApacheHVAC

Airside economizers need to be implemented carefully since inaccurate control is counter-productive and can increase the cooling energy by adding excessive unconditioned outside air. California Energy Commission requires functional testing of economizer controls to ensure their reliability.

To avoid excess outside air during unfavorable weather conditions, economizers use high limit shutoff control. The four control strategies referenced by ASHRAE Standard 90.1 are:

  1. Fixed Dry Bulb Temperature
  2. Differential Dry Bulb Temperature
  3. Fixed Dry Bulb Temperature & Fixed Enthalpy
  4. Differential Enthalpy & Fixed Dry Bulb Temperature

Note: ASHRAE Standard 90.1-2013 eliminates the use of electronic enthalpy and Dew Point Temperature & Dry Bulb Temperatures control options which were available in the earlier versions.

Airside economizers with all four high limit shut off control settings can be easily modeled in ApacheHVAC application of IESVE Software. Figure 2 shows output of economizer operation with “differential enthalpy and fixed dry-bulb” (Option 4) high limit shutoff control. The integrated economizer brings in excess outdoor air (blue) when the following conditions are met:

  1. Outside air Dry Bulb Temperature (green) is below 75oF AND
  2. Outside air enthalpy (red) is lower than return air enthalpy (black)

When one or both conditions are not satisfied, the economizer maintains minimum ventilation outside airflow rate required by the system (E.g. ASHRAE 62.1).

Figure 2: Hourly data in IESVE Software demonstrates optimum economizer operation

All four economizer control strategies were modeled on a 15,000 ft2 (~1,400m2) office building in Chicago (CZ 5a). The results show that “differential enthalpy and fixed dry-bulb” (Option 4) control strategy is the optimal choice with 1,900 operational hours in economizer mode and 18% savings on cooling energy when compared a system without an economizer.

The ApacheHVAC prototype library of HVAC systems has prebuilt economizers on the airside network with different high limit shutoff controls. A designer gets an opportunity to analyze the economizer operation on a (sub)-hourly time step. This provides more granular results on the effectiveness of the economizer and optimizing the control strategy for maximum cooling energy savings.

Figure 3: Comparison of cooling energy savings and economizer operational hours

Visit the IES Content Store to get a free copy of the ApacheHVAC file with all four economizer control strategies.