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C403.4 Complex HVAC Systems and Equipment. (Prescriptive)
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C403.4.1.1 Design Capacity
Water economizer systems shall be capable of cooling supply air by indirect evaporation and providing up to 100 percent of the expected system cooling load at outdoor air temperatures of 50°F dry bulb (10°C dry bulb)/45°F wet bulb (7.2°C wet bulb) and below.
Exception: Systems in which a water economizer is used and where dehumidification requirements cannot be met using outdoor air temperatures of 50°F dry bulb (10°C dry bulb)/45°F wet bulb (7.2°C wet bulb) shall satisfy 100 percent of the expected system cooling load at 45°F dry bulb (7.2°C dry bulb)/40°F wet bulb (4.5°C wet bulb).
C403.4.1.2 Maximum Pressure Drop
Precooling coils and water-to-water heat exchangers used as part of a water economizer system shall either have a water-side pressure drop of less than 15 feet (4572 mm) of water or a secondary loop shall be created so that the coil or heat exchanger pressure drop is not seen by the circulating pumps when the system is in the normal cooling (noneconomizer) mode.
C403.4.1.3 Integrated Economizer Control
Economizer systems shall be integrated with the mechanical cooling system and be capable of providing partial cooling even where additional mechanical cooling is required to meet the remainder of the cooling load.
- Direct expansion systems that include controls that reduce the quantity of outdoor air required to prevent coil frosting at the lowest step of compressor unloading, provided this lowest step is no greater than 25 percent of the total system capacity.
- Individual direct expansion units that have a rated cooling capacity less than 54,000 Btu/h (15 827 W) and use nonintegrated economizer controls that preclude simultaneous operation of the economizer and mechanical cooling.
C403.4.1.4 Economizer Heating System Impact
HVAC system design and economizer controls shall be such that economizer operation does not increase the building heating energy use during normal operation.
Exception: Economizers on VAV systems that cause zone level heating to increase due to a reduction in supply air temperature.
C403.4.2 Variable Air Volume (VAV) Fan Control
Individual VAV fans with motors of 7.5 horsepower (5.6 kW) or greater shall be:
- Driven by a mechanical or electrical variable speed drive;
- Driven by a vane-axial fan with variable-pitch blades; or
- The fan shall have controls or devices that will result in fan motor demand of no more than 30 percent of their design wattage at 50 percent of design airflow when static pressure set point equals one-third of the total design static pressure, based on manufacturer's certified fan data.
C403.4.2.1 Static Pressure Sensor Location
Static pressure sensors used to control VAV fans shall be placed in a position such that the controller setpoint is no greater than one-third the total design fan static pressure, except for systems with zone reset control complying with Section C403.4.2.2. For sensors installed down-stream of major duct splits, at least one sensor shall be located on each major branch to ensure that static pressure can be maintained in each branch.
C403.4.2.2 Set Points for Direct Digital Control
C403.4.3 Hydronic Systems Controls
The heating of fluids that have been previously mechanically cooled and the cooling of fluids that have been previously mechanically heated shall be limited in accordance with Sections C403.4.3.1 through C403.4.3.3. Hydronic heating systems comprised of multiple-packaged boilers and designed to deliver conditioned water or steam into a common distribution system shall include automatic controls capable of sequencing operation of the boilers. Hydronic heating systems comprised of a single boiler and greater than 500,000 Btu/h (146 550 W) input design capacity shall include either a multistaged or modulating burner.
C403.4.3.1 Three-Pipe System
Hydronic systems that use a common return system for both hot water and chilled water are prohibited.
C403.4.3.2 Two-Pipe Changeover System
Systems that use a common distribution system to supply both heated and chilled water shall be designed to allow a dead band between changeover from one mode to the other of at least 15°F (8.3°C) outside air temperatures; be designed to and provided with controls that will allow operation in one mode for at least 4 hours before changing over to the other mode; and be provided with controls that allow heating and cooling supply temperatures at the changeover point to be no more than 30°F (16.7°C) apart.
C403.4.3.3 Hydronic (Water Loop) Heat Pump Systems
C403.4.3.3.1 Temperature Dead Band
Hydronic heat pumps connected to a common heat pump water loop with central devices for heat rejection and heat addition shall have controls that are capable of providing a heat pump water supply temperature dead band of at least 20°F (11.1°C) between initiation of heat rejection and heat addition by the central devices.
Exception: Where a system loop temperature optimization controller is installed and can determine the most efficient operating temperature based on realtime conditions of demand and capacity, dead bands of less than 20°F (11°C) shall be permitted.
C403.4.3.3.2 Heat Rejection
C403.4.3.3.2.1 Climate Zones 3 and 4
For Climate Zones 3 and 4:
- If a closed-circuit cooling tower is used directly in the heat pump loop, either an automatic valve shall be installed to bypass all but a minimal flow of water around the tower, or lower leakage positive closure dampers shall be provided.
- If an open-circuit tower is used directly in the heat pump loop, an automatic valve shall be installed to bypass all heat pump water flow around the tower.
- If an open- or closed-circuit cooling tower is used in conjunction with a separate heat exchanger to isolate the cooling tower from the heat pump loop, then heat loss shall be controlled by shutting down the circulation pump on the cooling tower loop.
C403.4.3.3.2.2 Climate Zones 5 Through 8
For Climate Zones 5 through 8, if an open- or closed-circuit cooling tower is used, then a separate heat exchanger shall be provided to isolate the cooling tower from the heat pump loop, and heat loss shall be controlled by shutting down the circulation pump on the cooling tower loop and providing an automatic valve to stop the flow of fluid.
C403.4.3.3.3 Two Position Valve
Each hydronic heat pump on the hydronic system having a total pump system power exceeding 10 horsepower (hp) (7.5 kW) shall have a two-position valve.
C403.4.3.4 Part Load Controls
Hydronic systems greater than or equal to 300,000 Btu/h (87 930 W) in design output capacity supplying heated or chilled water to comfort conditioning systems shall include controls that have the capability to:
- Automatically reset the supply-water temperatures using zone-return water temperature, building-return water temperature, or outside air temperature as an indicator of building heating or cooling demand. The temperature shall be capable of being reset by at least 25 percent of the design supply-to-return water temperature difference; or
- Reduce system pump flow by at least 50 percent of design flow rate utilizing adjustable speed drive(s) on pump(s), or multiple-staged pumps where at least one-half of the total pump horsepower is capable of being automatically turned off or control valves designed to modulate or step down, and close, as a function of load, or other approved means.
C403.4.3.5 Pump Isolation
Chilled water plants including more than one chiller shall have the capability to reduce flow automatically through the chiller plant when a chiller is shut down. Chillers piped in series for the purpose of increased temperature differential shall be considered as one chiller.
Boiler plants including more than one boiler shall have the capability to reduce flow automatically through the boiler plant when a boiler is shut down.
C403.4.4 Heat Rejection Equipment Fan Speed Control
Each fan powered by a motor of 7.5 hp (5.6 kW) or larger shall have the capability to operate that fan at two-thirds of full speed or less, and shall have controls that automatically change the fan speed to control the leaving fluid temperature or condensing temperature/pressure of the heat rejection device.
C403.4.5 Requirements for Complex Mechanical Systems Serving Multiple Zones
Sections C403.4.5.1 through C403.4.5.4 shall apply to complex mechanical systems serving multiple zones. Supply air systems serving multiple zones shall be VAV systems which, during periods of occupancy, are designed and capable of being controlled to reduce primary air supply to each zone to one of the following before reheating, recooling or mixing takes place:
- Thirty percent of the maximum supply air to each zone.
- Three hundred cfm (142 L/s) or less where the maximum flow rate is less than 10 percent of the total fan system supply airflow rate.
- The minimum ventilation requirements of Chapter 4 of the International Mechanical Code.
Exception: The following define where individual zones or where entire air distribution systems are exempted from the requirement for VAV control:
- Zones where special pressurization relationships or cross-contamination requirements are such that VAV systems are impractical.
- Zones or supply air systems where at least 75 percent of the energy for reheating or for providing warm air in mixing systems is provided from a site-recovered or site-solar energy source.
- Zones where special humidity levels are required to satisfy process needs.
- Zones with a peak supply air quantity of 300 cfm (142 L/s) or less and where the flow rate is less than 10 percent of the total fan system supply airflow rate.
- Zones where the volume of air to be reheated, recooled or mixed is no greater than the volume of outside air required to meet the minimum ventilation requirements of Chapter 4 of the International Mechanical Code.
- Zones or supply air systems with thermostatic and humidistatic controls capable of operating in sequence the supply of heating and cooling energy to the zones and which are capable of preventing reheating, recooling, mixing or simultaneous supply of air that has been previously cooled, either mechanically or through the use of economizer systems, and air that has been previously mechanically heated.
C403.4.5.1 Single Duct Variable Air Volume (VAV) Systems, Terminal Devices
C403.4.5.2 Dual Duct and Mixing VAV Systems, Terminal Devices
C403.4.5.3 Single Fan Dual Duct and Mixing VAV Systems, Economizers
C403.4.5.4 Supply-Air Temperature Reset Controls
Multiple zone HVAC systems shall include controls that automatically reset the supply-air temperature in response to representative building loads, or to outdoor air temperature. The controls shall be capable of resetting the supply air temperature at least 25 percent of the difference between the design supply-air temperature and the design room air temperature.
- Systems that prevent reheating, recooling or mixing of heated and cooled supply air.
- Seventy five percent of the energy for reheating is from site-recovered or site solar energy sources.
- Zones with peak supply air quantities of 300 cfm (142 L/s) or less.
C403.4.6 Heat Recovery for Service Water Heating
Condenser heat recovery shall be installed for heating or reheating of service hot water provided the facility operates 24 hours a day, the total installed heat capacity of water-cooled systems exceeds 6,000,000 Btu/hr (1 758 600 W) of heat rejection, and the design service water heating load exceeds 1,000,000 Btu/h (293 100 W).
The required heat recovery system shall have the capacity to provide the smaller of:
- Sixty percent of the peak heat rejection load at design conditions; or
- The preheating required to raise the peak service hot water draw to 85°F (29°C).
- Facilities that employ condenser heat recovery for space heating or reheat purposes with a heat recovery design exceeding 30 percent of the peak water-cooled condenser load at design conditions.
- Facilities that provide 60 percent of their service water heating from site solar or site recovered energy or from other sources.
C403.4.7 Hot Gas Bypass Limitation
Cooling systems shall not use hot gas bypass or other evaporator pressure control systems unless the system is designed with multiple steps of unloading or continuous capacity modulation. The capacity of the hot gas bypass shall be limited as indicated in Table C403.4.7.
Exception: Unitary packaged systems with cooling capacities not greater than 90,000 Btu/h (26 379 W).
MAXIMUM HOT GAS BYPASS CAPACITY
MAXIMUM HOT GAS BYPASS CAPACITY
|RATED CAPACITY||MAXIMUM HOT GAS BYPASS CAPACITY
(% of total capacity)
|≤ 240,000 Btu/h||50|
|> 240,000 Btu/h||25|
For SI: 1 British thermal unit per hour = 0.2931 W.
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