Wash. Admin. Code § 51-11C-40353 - Section C403.5.3-Air economizers

C403.5.3 Air economizers. Air economizers shall comply with Sections C403.5.3.1 through C403.5.3.5.

C403.5.3.1 Design capacity. Air economizer systems shall be configured to modulate outdoor air and return air dampers to provide up to 100 percent of the design supply air quantity as outdoor air for cooling.

C403.5.3.2 Control signal. Economizer controls and dampers shall be configured to sequence the dampers with the mechanical cooling equipment and shall not be controlled by only mixed air temperature. Air economizers on systems with cooling capacity greater than 65,000 Btu/h shall be configured to provide partial cooling even when additional mechanical cooling is required to meet the remainder of the cooling load.

EXCEPTION :

The use of mixed air temperature limit control shall be permitted for systems that are both controlled from space temperature (such as single zone systems) and having cooling capacity less than 65,000 Btu/h.

C403.5.3.3 High-limit shutoff. Air economizers shall be configured to automatically reduce outdoor air intake to the design minimum outdoor air quantity when outdoor air intake will no longer reduce cooling energy usage. High-limit shutoff control types shall be chosen from Table C403.5.3.3. High-limit shutoff control settings for these control types shall be those specified in Table C403.5.3.3.

Table C403.5.3.3 High-Limit Shutoff Control Setting for Air Economizers^b

Device Type	Required High Limit (Economizer off when):		Required High Limit For Cycling Fansc (Economizer off when):
Equation	Description	Equation	Description	Description
Fixed dry-bulb	TOA75°F	Outdoor air temperature exceeds 75°F	TOA70°F	Outdoor air temperature exceeds 70°F
Differential dry-bulb	Toa [GREATER THAN] Tra	Outdoor air temperature exceeds return air temperature	TOA [GREATER THAN] (TRA- 5)	Outdoor air temperature exceeds return air temperature - 5
Fixed enthalpy with fixed dry-bulb temperatures	hOA [GREATER THAN] 28 Btu/lba or TOA [GREATER THAN] 75°F	Outdoor air enthalpy exceeds 28 Btu/lb of dry aira or outdoor air temperature exceeds 75°F	hOA [GREATER THAN] 26 Btu/lba or TOA [GREATER THAN] 70°F	Outdoor air enthalpy exceeds 26 Btu/lb of dry aird or outdoor air temperature exceeds 70°F
Differential enthalpy with fixed dry-bulb temperature	hOA[GREATER THAN] hRA or TOA [GREATER THAN] 75°F	Outdoor air enthalpy exceeds return air enthalpy or outdoor air temperature exceeds 75°F	hOA [GREATER THAN] (hRA - 2) or TOA [GREATER THAN] 70°F	Outdoor air enthalpy exceeds return air enthalpy or outdoor air temperature exceeds 70°F

For SI: °C = (°F - 32) × 5/9, 1 Btu/lb = 2.33 kJ/kg.

^aAt altitudes substantially different than sea level, the fixed enthalpy limit shall be set to the enthalpy value at 75°F and 50 percent relative humidity. As an example, at approximately 6,000 feet elevation the fixed enthalpy limit is approximately 30.7 Btu/lb.

^bDevices with selectable setpoints shall be capable of being set to within 2°F and 2 Btu/lb of the setpoint listed.

^cWhere fans cycle on only to provide heating and cooling, limits are adjusted lower to compensate for fan energy use in economizer mode.

^dFor cycling fans at altitudes substantially different than sea level, the fixed enthalpy limit shall be set to the enthalpy value at 70°F and 50 percent relative humidity.

C403.5.3.4 Relief of excess outdoor air. Systems shall be capable of relieving excess outdoor air during air economizer operation to prevent over-pressurizing the building. The relief air outlet shall be located to avoid recirculation into the building.

C403.5.3.5 Economizer dampers. Return, exhaust/relief and outdoor air dampers used in economizers shall comply with Section C403.7.8.

Notes

Wash. Admin. Code § 51-11C-40353

Adopted by WSR 19-24-040, Filed 11/26/2019, effective 7/1/2020 Amended by WSR 20-21-080, Filed 10/19/2020, effective 2/1/2021