6.5.2 Simultaneous Heating and Cooling Limitation
Zone thermostatic controls shall prevent
Where reheating is permitted by other parts of this standard, zones that have both supply and return/exhaust air openings greater than 6 ft above floor shall not supply heating air more than 20°F above the space temperature set point.
- The system is designed to allow a dead band between changeover from one mode to the other of at least 15°F outdoor air temperature.
- The system is designed to operate and is provided with controls that will allow operation in one mode for at least four hours before changing over to the other mode.
- Reset controls are provided that allow heating and cooling supply temperatures at the changeover point to be no more than 30°F apart.
Hydronic heat pumps connected to a common heat pump water loop with central devices for heat rejection (e.g., cooling tower) and heat addition (e.g., boiler) shall have the following:
- Controls that are capable of and configured to provide a heat pump water supply temperature dead band of at least 20°F between initiation of heat rejection and heat addition by the central devices (e.g., tower and boiler).
- For Climate Zones 3 through 8, if a closed-circuit cooling tower (fluid cooler) is used, either an automatic valve shall be installed to bypass all but a minimal flow of water around the tower (for freeze protection) or low-leakage positive closure dampers shall be provided. If an open-circuit cooling 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-circuit cooling tower is used in conjunction with a separate heat exchanger to isolate the tower from the heat pump loop then heat loss shall be controlled by shutting down the circulation pump on the cooling tower loop.
Where humidity controls are provided, such controls shall prevent reheating, mixing of hot and cold airstreams, or other means of simultaneous heating and cooling of the same air-stream.
Humidifiers with preheating jackets mounted in the airstream shall be provided with an automatic valve to shut off preheat when humidification is not required.
Humidification system dispersion-tube hot surfaces in the airstreams of ducts or air-handling units shall be insulated with a product with an insulating value of at least R-0.5.
Exception to 22.214.171.124.2
Systems where mechanical cooling, including economizer operation, does not occur simultaneously with humidification.
Preheat coils shall have controls that stop their heat output whenever mechanical cooling, including economizer operation, is occurring.
Units that provide ventilation air to multiple zones and operate in conjunction with zone heating and cooling systems shall not use heating or heat recovery to warm supply air above 60°F when representative building loads or outdoor air temperature indicate that the majority of zones require cooling.