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C403.3 Simple HVAC Systems and Equipment (Prescriptive)
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This section applies to buildings served by unitary or packaged HVAC equipment listed in Tables C403.2.3(1) through C403.2.3(8), each serving one zone and controlled by a single thermostat in the zone served. It also applies to two-pipe heating systems serving one or more zones, where no cooling system is installed.
Each cooling system that has a fan shall include either an air or water economizer meeting the requirements of Sections C403.3.1.1 through C403.3.1.1.4.
Exception: Economizers are not required for the systems listed below.
- Individual fan-cooling units with a supply capacity less than the minimum listed in Table C403.3.1(1).
- Where more than 25 percent of the air designed to be supplied by the system is to spaces that are designed to be humidified above 35°F (1.7 °C) dew-point temperature to satisfy process needs.
- Systems that serve residential spaces where the system capacity is less than five times the requirement listed in Table C403.3.1(1).
- Systems expected to operate less than 20 hours per week.
- Where the use of outdoor air for cooling will affect supermarket open refrigerated casework systems.
- Where the cooling efficiency meets or exceeds the efficiency requirements in Table C403.3.1(2).
CLIMATE ZONES | ECONOMIZER REQUIREMENT |
1A, 1B | No requirement |
2A, 2B, 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B, 5C, 6A, 6B, 7, 8 | Economizers on all cooling systems ≥33,000 Btu/ha |
For SI:1 British thermal unit per hour = 0.2931 W.
- The total capacity of all systems without economizers shall not exceed 300,000 Btu/h per building, or 20 percent of its air economizer capacity, whichever is greater.
CLIMATE ZONES |
COOLING EQUIPMENT PERFORMANCE IMPROVEMENT (EER OR IPLV) |
2B | 10% Efficiency Improvement |
3B | 15% Efficiency Improvement |
4B | 20% Efficiency Improvement |
Air economizers shall comply with Sections C403.3.1.1.1 through C403.3.1.1.4.
Air economizer systems shall be capable of modulating outdoor air and return air dampers to provide up to 100 percent of the design supply air quantity as outdoor air for cooling.
Economizer dampers shall be capable of being sequenced with the mechanical cooling equipment and shall not be controlled by only mixed air temperature.
Exception: The use of mixed air temperature limit control shall be permitted for systems controlled from space temperature (such as single-zone systems).
Air economizers shall be capable of automatically reducing 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 for specific climates shall be chosen from Table C403.3.1.1.3(1). High-limit shutoff control settings for these control types shall be those specified in Table C403.3.1.1.3(2).
CLIMATE ZONES | ALLOWED CONTROL TYPES | PROHIBITED CONTROL TYPES |
1B, 2B, 3B, 3C, 4B, 4C, 5B, 5C, 6B, 7, 8 | Fixed dry bulb | Fixed enthalpy |
Differential dry bulb | ||
Electronic enthalpya | ||
Differential enthalpy | ||
Dew-point and dry-bulb temperatures | ||
1A, 2A, 3A, 4A | Fixed dry bulb | Differential dry bulb |
Fixed enthalpy | ||
Electronic enthalpya | ||
Differential enthalpy | ||
Dew-point and dry-bulb temperatures | ||
All other climates | Fixed dry bulb | — |
Differential dry bulb | ||
Fixed enthalpy | ||
Electronic enthalpya | ||
Differential enthalpy | ||
Dew-point and dry-bulb temperatures |
- Electronic enthalpy controllers are devices that use a combination of humidity and dry-bulb temperature in their switching algorithm.
DEVICE TYPE | CLIMATE ZONE | REQUIRED HIGH LIMIT (ECONOMIZER OFF WHEN): |
|
EQUATION | DESCRIPTION | ||
Fixed dry bulb | 1B, 2B, 3B, 3C, 4B, 4C, 5B, 5C, 6B, 7, 8 | TOA > 75°F | Outdoor air temperature exceeds 75°F |
5A, 6A, 7A | TOA > 70°F | Outdoor air temperature exceeds 70°F | |
All other zones | TOA > 65°F | Outdoor air temperature exceeds 65°F | |
Differential dry bulb | 1B, 2B, 3B, 3C, 4B, 4C, 5A, 5B, 5C, 6A, 6B, 7, 8 | TOA > TRA | Outdoor air temperature exceeds return air temperature |
Fixed enthalpy | All | hOA > 28 Btu/lba | Outdoor air enthalpy exceeds 28 Btu/lb of dry aira |
Electronic Enthalpy | All | (TOA, RHOA) > A | Outdoor air temperature/RH exceeds the "A" setpoint curveb |
Differential enthalpy | All | hOA > hRA | Outdoor air enthalpy exceeds return air enthalpy |
Dew-point and dry bulb temperatures | All | DPOA> 55°F or TOA> 75°F | Outdoor air dry bulb exceeds 75°F or outside dew point exceeds 55°F (65 gr/lb) |
For SI: °C = (°F - 32) × 5/9, 1 Btu/lb = 2.33 kJ/kg.
- 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.
- Setpoint "A" corresponds to a curve on the psychometric chart that goes through a point at approximately 75°F and 40-percent relative humidity and is nearly parallel to dry-bulb lines at low humidity levels and nearly parallel to enthalpy lines at high humidity levels.
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.
Hydronic systems of at least 300,000 Btu/h (87 930 W) design output capacity supplying heated and chilled water to comfort conditioning systems shall include controls that meet the requirements of Section C403.4.3.
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