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// CODE SNIPPET

C403.3 Simple HVAC Systems and Equipment (Prescriptive)

DC Energy Conservation Code 2013 > 4 [CE] Commercial Energy Efficiency > C403 Building Mechanical Systems > 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.

C403.3.1 Economizers

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.
  1. Individual fan-cooling units with a supply capacity less than the minimum listed in Table C403.3.1(1).
  2. 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.
  3. Systems that serve residential spaces where the system capacity is less than five times the requirement listed in Table C403.3.1(1).
  4. Systems expected to operate less than 20 hours per week.
  5. Where the use of outdoor air for cooling will affect supermarket open refrigerated casework systems.
  6. Where the cooling efficiency meets or exceeds the efficiency requirements in Table C403.3.1(2).
TABLE C403.3.1(1)
ECONOMIZER REQUIREMENTS
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.
  1. 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.
TABLE C403.3.1(2)
EQUIPMENT EFFICIENCY PERFORMANCE EXCEPTION FOR ECONOMIZERS
CLIMATE
ZONES
COOLING EQUIPMENT PERFORMANCE
IMPROVEMENT (EER OR IPLV)
2B 10% Efficiency Improvement
3B 15% Efficiency Improvement
4B 20% Efficiency Improvement

C403.3.1.1 Air Economizers

Air economizers shall comply with Sections C403.3.1.1.1 through C403.3.1.1.4.

C403.3.1.1.1 Design Capacity

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.

C403.3.1.1.2 Control Signal

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).

C403.3.1.1.3 High-Limit Shutoff

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).

 

TABLE C403.3.1.1.3(1)
HIGH-LIMIT SHUTOFF CONTROL OPTIONS FOR AIR ECONOMIZERS
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
  1. Electronic enthalpy controllers are devices that use a combination of humidity and dry-bulb temperature in their switching algorithm.
TABLE C403.3.1.1.3(2)
HIGH-LIMIT SHUTOFF CONTROL SETTING FOR AIR ECONOMIZERS
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.
  1. 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.
  2. 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.

C403.3.1.1.4 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.

C403.3.2 Hydronic System Controls

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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