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. |
Device Type | Required High Limit (Economizer off when): | Required High Limit For Cycling Fansc (Economizer off when): | ||
Equation | Description | Equation | Description | |
Fixed dry-bulb | TOA ˃ 75°F | Outdoor air temperature exceeds 75°F | TOA ˃ 70°F | Outdoor air temperature exceeds 70°F |
Differential dry-bulb | TOA ˃ TRA | Outdoor air temperature exceeds return air temperature | TOA ˃ (TRA- 5) | Outdoor air temperature exceeds return air temperature - 5 |
Fixed enthalpy with fixed dry-bulb temperatures | hOA ˃ 28 Btu/lba or TOA ˃ 75°F | Outdoor air enthalpy exceeds 28 Btu/lb of dry aira or outdoor air temperature exceeds 75°F | hOA ˃ 26 Btu/lba or TOA ˃ 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 ˃ hRA or TOA ˃ 75°F | Outdoor air enthalpy exceeds return air enthalpy or outdoor air temperature exceeds 75°F | hOA ˃ (hRA - 2) or TOA ˃ 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. |
a | At 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. |
b | Devices with selectable setpoints shall be capable of being set to within 2°F and 2 Btu/lb of the setpoint listed. |
c | Where fans cycle on only to provide heating and cooling, limits are adjusted lower to compensate for fan energy use in economizer mode. |
d | For 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. |
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. |
Device Type | Required High Limit (Economizer off when): | Required High Limit For Cycling Fansc (Economizer off when): | ||
Equation | Description | Equation | Description | |
Fixed dry-bulb | TOA ˃ 75°F | Outdoor air temperature exceeds 75°F | TOA ˃ 70°F | Outdoor air temperature exceeds 70°F |
Differential dry-bulb | TOA ˃ TRA | Outdoor air temperature exceeds return air temperature | TOA ˃ (TRA- 5) | Outdoor air temperature exceeds return air temperature - 5 |
Fixed enthalpy with fixed dry-bulb temperatures | hOA ˃ 28 Btu/lba or TOA ˃ 75°F | Outdoor air enthalpy exceeds 28 Btu/lb of dry aira or outdoor air temperature exceeds 75°F | hOA ˃ 26 Btu/lba or TOA ˃ 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 ˃ hRA or TOA ˃ 75°F | Outdoor air enthalpy exceeds return air enthalpy or outdoor air temperature exceeds 75°F | hOA ˃ (hRA - 2) or TOA ˃ 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. |
a | At 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. |
b | Devices with selectable setpoints shall be capable of being set to within 2°F and 2 Btu/lb of the setpoint listed. |
c | Where fans cycle on only to provide heating and cooling, limits are adjusted lower to compensate for fan energy use in economizer mode. |
d | For 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. |