C403.4 Complex HVAC Systems and Equipment. (Prescriptive)
- 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.
Exception: Economizers on VAV systems that cause zone level heating to increase due to a reduction in supply air temperature.
- 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.
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.
- 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.
- 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.
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.
- 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.
- 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.
- 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.
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|