BAC-1xx36/1xxx36 Series (3 Relays and 6 Analog Outputs) Section
53
Application Guide L
FCU (Fan Coil Unit), 2-Pipe
Fan Control
The fan follows the base sequence for fan operation as listed in the
Flex-
Stat (All Models) Common Features
section above with the following
modifications.
When the fan runs, whether in “Auto” or “On” mode, the fan speed is
determined by the Fixed Speed setting. If the Fixed Speed setting is set
to a particular speed (“1,” “2,” or “3”), then any time the fan is called for,
it will run at that speed. This allows fan speed to be set by a balancer. If
the Fixed Speed is set to “Auto” and the Fan Mode is set to “On,”then
the speed the fan runs is determined by the heating or cooling PI loops.
The fan is started at low speed upon call for heating or cooling and re-
mains there until the active heating or cooling PI loop reaches 60%. Once
above 60%, the fan is incrementally sped up based on the upper 40% of
the heating or cooling PI loop, whichever is active based on the number
of fan stages configured. The fan slows down in the reverse sequence.
This operation allows the FCU to attempt to maintain space temperature
setpoint using the valve at low fan speed. If space temp setpoint cannot
be maintained at low speed, increasing the fan speed will allow more
heat transfer to/from the coil. If the Fixed Speed is set to “Auto” and the
Fan Mode is set to “Auto,” then when the fan is called for, it runs on the
highest speed for which the unit is configured.
Water Valve Operation and H/C Water Available Determination
The application is capable of determining whether heating or cooling
water is available even in a stand-alone configuration. If the FlexStat has
a call for heating or cooling greater than 10% and the unit does not know
what type of water is available (both valves are fully closed), the Flex-
Stat will initiate a
water evaluation mode
. During the water evaluation
mode, all fan outputs are de-energized and the water valve opens for
three minutes. During the three minutes, the water temperature is read.
If the water temp is below 63° F (17.2° C), a chilled water available flag
is started and water evaluation mode ends. If the water temp is above
90° F (32.2° C), a heating water available flag is started and water evalu-
ation mode ends. These flags will stay on until the valve goes fully shut
for ten minutes (or as set in
min eval timer
) or the water drops 2.0° (1.1°
C) out of range. If the opposite type of water is available from what type
of water is needed (e.g., cooling water is available and there is a call for
heating), another flag indicating opposite water available is started. If
the unit goes into “opposite water available” mode and the unit remains
calling for the same temperature mode (heating or cooling), every fifteen
minutes (or as set in
opposite water timer
) the unit will reinitiate the
water evaluation mode. If neither heating nor cooling water is available,
the water evaluation mode is run every ten minutes (or as set in
min
eval timer
).
NOTE: If adjusted,
min eval timer
should always be set at least 30
seconds lower than
opposite water timer
and is limited in
programming to be so.
The method of controlling the valve is based on the fan configuration:
• If the fan mode for a particular occupancy mode is set to con-
stant or “On,” the valve functions in a modulating fashion.
Actual response may vary slightly due to PI action of the control
loops, but generally, if heating water is available, the valve will
start to open as the space temperature drops below the active
heating setpoint and modulate to fully open when space temper-
ature reaches active heating setpoint minus deadband. The valve
modulates closed as the space temp rises towards the active heat-
ing setpoint. Likewise, the valve will generally begin opening
as the space temperature rises above the active cooling setpoint
and be fully open by the time the space temp reaches the active
cooling setpoint plus deadband, modulating linearly in between.
Once the valve closes fully in either heating or cooling mode, all
water available flags are turned off.
• With the fan mode set to “Auto,” the valve operates in a “two-
position” fashion.
The valve will open 100% if the space tem-
perature falls below active heating setpoint minus deadband and
fully close when space temp rises above active heating setpoint if
heating water is available. The valve similarly opens 100% if the
space temp rises above the active cooling setpoint plus deadband
and fully closes if the space temp falls below the active cooling
setpoint if cooling water is available.
Any time there is a fan failure alarm, the valve will be fully closed.
