Pacific Northwest National Laboratory
Bulding Systems Program

Terms

Freeze protection or supply temperature control Freeze protection is achieved by limiting the temperature to which the mixed air can drop. A temperature controller limits how much cool outside air is introduced. It is implemented either by sensing the mixed-air or supply-air temperature. This limit is required to prevent too much outside air from being supplied during cold weather, which could lead to coil freeze-ups or uncomfortably cold drafts in the conditioned spaces. Temperature control also generally includes an interlock to shut off the mechanical cooling system when the economizer is enabled.  
Minimum acceptable outside-air ventilation rate The volumetric flow-rate of outside air required to provide healthy, comfortable conditions for occupants can be determined from building codes, ASHRAE standards, or standard practice. It is usually expressed in terms of volumetric flow-rate (cfm) per occupant or per unit floor area.  
Packaged Units Packaged heating, ventilating, and air-conditioning (HVAC) units are generally found in small- to medium-sized commercial applications. The term "packaged unit" indicates that a single enclosure contains the heating, cooling, and air-handling components needed to condition the space. Packaged HVAC units are mounted beside the building at ground level or placed on a roof for horizontal or downflow installation. Multiple packaged units are often installed on roof tops to provide enough space conditioning capacity without requiring larger chillers and boilers.  
     
     
     

Equipment and Strategies

Air-handling Units The Outdoor Air Economizer (OAE) diagnostic module supports constant-air-volume and variable-air-volume systems with no volume compensation (outside-air intake is a constant fraction of supply air).
  • Constant-air-volume system - A constant-air-volume system is one that provides a fixed volume of air when operated.
  • Variable-air-volume (VAV) system with no volume compensation - A typical VAV system delivers a mixture of outdoor and recirculated return air to multiple spaces via local terminal units that control flow rate. While the amount of supply air delivered to each space varies with the thermal load, the amount of outdoor air remains constant.
  
Economizers Integrated economizer operation where simultaneous operation of the economizer and mechanical cooling is permitted
  • Packaged units with multiple stages or central units - Packaged heating, ventilating, and air-conditioning (HVAC) units are generally found in small- to medium-sized commercial applications. The term "packaged unit" indicates that a single enclosure contains the heating, cooling, and air-handling components needed to condition the space. Packaged HVAC units are mounted beside the building at ground level or placed on a roof for horizontal or downflow installation. Multiple packaged units are often installed on roof tops to provide enough space conditioning capacity without requiring larger chillers and boilers.
  • Differential dry-bulb temperature-based economizer control
  • Differential enthalpy-based economizer control
  • High-limit dry-bulb temperature-based economizer control
  • High-limit enthalpy-based economizer control
  
Economizer Control Most commercial buildings generally have a cooling requirement even during mild and cold weather conditions, because of the internal loads. A cooling system with an economizer can use cool outside air to satisfy all or part of the cooling demand. This reduces the cooling energy required by the system. A properly designed economizer will have no impact on the heating energy used by the building.

Economizers use controllable dampers to increase the amount of outside-air intake into the building when the outside air is cool and the building requires cooling. In addition to the controllable outside-air dampers, there are several other key components in an economizer system: return-air dampers, exhaust-air dampers, economizer controller, temperature controller, and minimum position limiter.

  • Damper System - An economizer sized to provide up to 100% outside air requires that the outside-air and return-air dampers be sized to provide the required volume. In addition, the relief- or exhaust-air dampers are required to prevent the building from being over-pressurized when large amounts of outside air are introduced.
  • Economizer Controller - A controller (high-limit or differential) is used to open/close the outside-air (return-air and exhaust) damper system. This controller arbitrates when to use outside air for cooling and how much to use. It is required to provide the correct amount of outside air, while preventing the economizer from inadvertently increasing heating or cooling loads by introducing more outside air than required.
  • Temperature Controller - A temperature controller limits the temperature of mixed air (return plus outside). It is implemented by sensing and controlling how cold the mixed-air or supply-air temperature gets. It overrides the economizer controller, limiting the amount of cold outside air, to prevent coil freezing and uncomfortably cold drafts in conditioned spaces during cold weather.
  • Minimum Position Limiter - When outside-air conditions are not favorable for economizing, the outside-air damper system is positioned to provide the minimum outside-air intake required to meet the fresh-air ventilation requirements for occupants.
  
Economizer Control Strategies The control strategies that are supported by the OAE include: differential dry-bulb temperature-based, differential enthalpy-based, high-limit dry-bulb temperature-based, and high-limit enthalpy-based.

With differential control strategies, the outside-air condition is compared with the return-air condition. As long as the outside-air condition is more favorable (for example, with dry-bulb temperature control, the outside-air dry-bulb temperature is less than the return-air temperature), outside air is used to meet all or part of the cooling demand. If the outside air alone cannot satisfy the cooling demand, mechanical cooling is used to provide the remainder of the cooling load.

With high-limit control strategies, the outside-air condition is compared to a single setpoint or fixed setpoint (usually referred to as a high limit). If the outside-air condition is below the setpoint, then outside air is used to meet all or part of the cooling demand; the remainder of the cooling load is provided by mechanical cooling.