Hot Water Plant Considerations - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS

HOT WATER PLANT CONSIDERATIONS

See Chiller , Boiler, and Distr ibution System Contr ol
Applica tions section f or ad ditional inf orma tion on hot w ater
system control.
Because unit v entila tor s have lar ge OA damper s in close
pr oximity to the w ater coils, the hot w ater pump should r un
anytime the O A temper atur e is less than fr eezing, independent
of hot water temper atur e.
Unit ventila tor and f an coil unit hot w ater temper atur es should
be r eset based upon the O A temper atur e to ma tch the hot w ater
temper atur e to the load . This allows much smoother contr ol,
and k eeps adequa te water v elocities in the hot w ater coils if
modula ting valves ar e used. Dual schedules ar e recommended
with signif icantl y war mer w ater pr ovided dur ing unoccupied
per iods to aid in the con vection and w ar m-up ef forts. A tr iple
water temper atur e schedule ma y be consider ed wher e the
convection mode has a higher w ater temper atur e than w ar m-
up or occupied modes. The setpoint shift betw een modes should
be r amped o ver a 20 to 30 min ute dur ation.
The hot w ater pump should also r un dur ing unoccupied
periods on a drop in room temperature to about 16
mode the coils will act as a r adia tor and ad d hea t to the b uilding
per imeter without r unning the unit v entila tor f ans.
Unit Ventila tor s and f an coil units and their associa ted hea ting
and cooling plants should be started by a global optimum start
pr ogram with the O A damper c losed until occupanc y time.
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C. In this