Honeywell AUTOMATIC CONTROL SI Edition User Manual

ENGINEERING MANUAL OF AUTOMATIC CONTROL
INDOOR AIR QUALITY FUNDAMENTALS
165
Fig. 4. HVAC Flow Tracking.
FLOW TRACKING CONTROL SYSTEM
Figure 4 provides a simplified hardware schematic of a
variable volume supply and return fan system with flow stations
in the supply and return to control the return fan volume to
match the supply fan volume less any exhaust volume. A volume
differential is maintained between the supply and return to
pressurize the building. There may be only one such system in
a building or there may be many. The supply fan static pressure
setpoint, and thus the output volume, may be reset by the VAV
terminal box requirements that the fan supplies. Typically there
are 30 to 70 VAV terminal units per fan system. The sample
VAV terminal unit shown is a fan powered unit but the units
could be non-fan powered. Without an air flow station in the
outdoor air, this system might not deliver the minimum air flow
at all fan volumes, because the mixing box negative static
pressure and therefore the OA airflow varies with the supply
fan load. If mixed air temperature is used to calculate the
percentage of outdoor air, as the outdoor temperature
approaches 10°C the margin of error increases because of the
narrowing differential between outdoor air and mixed air.
OA SA
RA
EA
RETURN FAN
M10486
F
F
F
SUPPLY FAN
VAV TERMINAL BOX
CT
VSD
CT
VSD
RA FAN
STATUS
RA FAN
SPEED
RA
FAN
RA
TEMPER-
ATURE
RA FLOW
SIGNAL
OA
TEMPERATURE
MA
DAMPER
MA
TEMPER-
ATURE
HEATING
VALVE
COOLING
VALVE
SA
FAN
SA FAN
SPEED
SA FAN
STATUS
SA
TEMPER-
ATURE
SA FLOW
SIGNAL
SA
PRESSURE
AIR
FLOW
FLOW
DAMPER
REHEAT
HOT WATER
VALVE
ROOM
TEMPER-
ATURE

Honeywell AUTOMATIC CONTROL SI Edition Specifications

General

Brand	Honeywell
Model	AUTOMATIC CONTROL SI Edition
Category	Control Systems
Language	English