Three different methods can be used to determine if outdoor air contains more cooling capacity than the
return air and are described below. The different methods are suited for different applications and
environments.
- Comparative Enthalpy — Outdoor Air Enthalpy is compared with Return Air Enthalpy. This method is best suited for high humidity climates and applications in which humidity can affect the cooling capacity of the outdoor air or return air.
- Reference Enthalpy — Outdoor Air Enthalpy is compared with a reference enthalpy point. This method is best suited for high humidity climates in which humidity can affect the cooling capacity of the outdoor air, but not necessarily the return air.
- Reference Dry Bulb — Outdoor Air Temperature is compared with a user-set reference temperature. This method is best suited for low humidity climates and applications in which humidity does not strongly affect the cooling capacity of the outdoor air or return air.
Dry bulb temperature and relative humidity data are used to determine enthalpy. Economizer-based cooling is enabled only when outdoor air is determined to have more cooling capacity than the return air. The method used is according to the available data. When temperature and humidity data are available for the outdoor air and return air, the Comparative Enthalpy method is used. One of the other methods is used if data is invalid or unavailable. Ultimately, when there is insufficient data to use any of the three methods, economizer-based cooling is disabled.
The following Inputs Are Used
Mixed Air Sensor (MAS)
- MAS measures the dry bulb temperature of the air leaving the evaporator coil while economizing. Return air, outdoor air, and cooling caused by any compressor cooling make up the mixed air input. The MAS is plugged into the economizer actuator module (ECA).
Outdoor Air Sensor (OAS)
-
The OAS measures the ambient air surrounding the unit. It is located in the compressor section on the lower or upper left side for Precedent™ units depending on model, or the right side beneath the control panel on Voyager™ Light Commercial, or the economizer end of the unit for Voyager™ Commercial units. Ventilation holes in the access panel of the unit allow air movement across the sensor. The OAS connects to the RTRM module.
Figure 78. Outdoor air sensor (Precedent)
Outdoor Humidity Sensor (OHS)
- The OHS measures the relative humidity of the outside air. It is located inside the economizer hood. The OHS is plugged into the ECA.
Return Air Temperature Sensor (RAT)
- The RAT measures the return air temperature. It is located on the return air damper of the economizer. The RAT is plugged into the ECA.
Return Humidity Sensor (RHS)
- The RHS measures the relative humidity of the return air. It is located on the return air damper of the economizer. The RHS is plugged into the ECA.
Economizer Damper Enthalpy Layout
| Table 92. choice of enthalpy method | |
|---|---|
| The method use to determine economizer effectiveness | Required Data |
| Comparative Enthalpy | MAT, OAT, OAH, RAT, RAH |
| Reference Enthalpy | MAT, OAT, OAH |
| Reference Dry Bulb Enthalpy | MAT, OAT |
| The unit will not economize | MAS or OAT data is invalid or unavailable |
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WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.