6.5.2.2 Hydronic System Controls

The heating of fluids in hydronic systems that have been previously mechanically cooled, and the cooling of fluids that have been previously mechanically heated, shall be limited in accordance with Sections 6.5.2.2.1 through 6.5.2.2.3

Hydronic systems that use a common return system for both hot water and chilled water shall not be used.

Systems that use a common distribution system to supply both heated and chilled water are acceptable provided all of the following are met:

1. 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.
2. 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.
3. 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:

1. 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).
2. 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.