Control From Space With Supply Temperature Reset - Honeywell AUTOMATIC CONTROL Engineering Manual

AIR HANDLING SYSTEM CONTROL APPLICATIONS

CONTROL FROM SPACE WITH SUPPLY TEMPERATURE RESET

Functional Description
NORMAL
MA
3
00
N.O.
Item
No. Function
1-2 Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
3 Hot water valve modulates heating medium.
4,5 SA temperature controlled to maintain
setpoint.
6,7 SA temperature setpoint determined by space
heating load.
8,9 Space temperature is compared to space
temperature setpoint to determine SA
temperature setpoint.
10 Control program coordinates temperature
control and fan interlock.
FEATURES
1. Air is supplied at a temperature necessary to make up the
sensible heat loss in the space.
2. The SA temperature will not fall below a desired
minimum or rise above a desired maximum (unless
heating coil entering air rises).
CONDITIONS FOR SUCCESSFUL OPERATION
A multiple inline coil arrangement if a high temperature rise
is required.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
ENGINEERING CMANUAL OF AUTOMATION CONTROL
2
ON
4
1
10
CONTROL
PROGRAM
5
CURRENT SUPPLY AIR
TEMPERATURE SETPOINT
72
8
9 72
SPACE
TEMPERATURE
SUPPLY AIR
TEMPERATURE
SA
RESET SCHEDULE
SUPPLY AIR
TEMPERATURE
SETPOINT
95
100
7
95
55
6
1. Anytime the supply fan runs, the hot water valve shall be
modulated by an SA PID control loop to maintain the SA
temperature setpoint. The SA temperature setpoint shall
be reset from no lower than 55°F to no greater than 100°F
as the space temperature PID demand for heating varies
from 0% to 100%.
2. The N.O. hot water valve shall close upon fan shutdown.
PSYCHROMETRIC ASPECTS
The supply condition of the air depends on the condition of
the entering air and the temperature rise needed to satisfy the
space heating requirements.
In the following chart it is assumed that:
1. The space temperature control loop is set at 75°F.
2. MA condition is 50°F DB and 48°F WB.
3. A space heating load exists which is large enough to
require 95°F DB SA to meet the design condition heat
loss.
MA
50°F DB, 48°F WB
The following results are obtained:
Item
No. Explanation
1 The heating of the MA to the supply temperature
occurs along a line of constant moisture content.
2 Space air picks up moisture from the
occupants and contents of the space.
224
SPACE
HEATING
DEMAND
100
0
M10459
SPACE
75°F
2
SA 95°F DB
1
C2548-1