Hot Water Plant Considerations - Honeywell AUTOMATIC CONTROL Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS

HOT WATER PLANT CONSIDERATIONS

See Chiller, Boiler, and Distribution System Control
Applications section for additional information on hot water
system control.
Because unit ventilators have large OA dampers in close
proximity to the water coils, the hot water pump should run
anytime the OA temperature is less than freezing, independent
of hot water temperature.
Unit ventilator and fan coil unit hot water temperatures should
be reset based upon the OA temperature to match the hot water
temperature to the load. This allows much smoother control,
and keeps adequate water velocities in the hot water coils if
modulating valves are used. Dual schedules are recommended
with significantly warmer water provided during unoccupied
ENGINEERING MANULA OF AUTOMATIC CONTROL
periods to aid in the convection and warm-up efforts. A triple
water temperature schedule may be considered where the
convection mode has a higher water temperature than warm-
up or occupied modes. The setpoint shift between modes should
be ramped over a 20 to 30 minute duration.
The hot water pump should also run during unoccupied
periods on a drop in room temperature to about 60F. In this
mode the coils will act as a radiator and add heat to the building
perimeter without running the unit ventilator fans.
Unit Ventilators and fan coil units and their associated heating
and cooling plants should be started by a global optimum start
program with the OA damper closed until occupancy time.
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