Preheat Control Processes; Preheat Control With Face And Bypass Dampers - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

AIR HANDLING SYSTEM CONTROL APPLICATIONS

PREHEAT CONTROL PROCESSES

The preheat process heats the air in preparation for subsequent
conditioning. Preheat is sometimes necessary when high
percentages of low temperature OA must be handled by the
system. Considerations before a preheat component is installed
in an air conditioning system are:
1. Preheat coils are often exposed to subfreezing
temperatures. Provision to prevent freezing the coils must
be made.

PREHEAT CONTROL WITH FACE AND BYPASS DAMPERS

Functional Description
3
OPEN
OA
4
4
7
VALVE POSITION
SCHEDULE
OUTDOOR
AIR
TEMPERATURE
10
1.5
Item
No. Function
1-2 Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
3 OA damper opens on fan startup, closes upon
fan shutdown (in some cases the OA damper
may be part of a mixed air system).
4 OA temperature determines valve position.
5 Valve controls heating medium flow to coil.
11
CONTROL
PROGRAM
8
PERCENT
44
OPEN
TO COIL
N.O.
N.C.
N.C.
N.O.
NORMAL
5
66
2
VALVE
PERCENT
OPEN
CONTROL
PROGRAM
0
100
2. Accurate sizing of preheat coils is important. They should
be sized so it is possible to allow OA for cooling yet not
overheat the space.
In most of the following illustrations, preheat is shown in
the OA preceding the mixing of OA and RA. In complete HVAC
systems, the final preheat coil leaving air temperature setpoint
should be dictated by demands for heating or cooling.
RA
9
10
10
10
6
6 Control program coordinates fan interlock and
valve control.
7 Valve position determined by OA temperature.
8-10 Face-and-bypass dampers controlled to
maintain face-and-bypass leaving air
temperature setpoint.
11 Control program coordinates fan interlock and
face-and-bypass damper control.
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