Product Overview

BAC-1xx36/1xxx36 Series (3 Relays and 6 Analog Outputs) Section 64 Application Guide S
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
modications.
When the fan runs, whether in “Auto” or “On” mode, the fan speed is
determined by the Fixed Speed seing. If the Fixed Speed seing 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 congured. The fan slows down in the reverse sequence.
This operation allows the FCU to aempt 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 congured.
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 conguration. 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 ag
is started and water evaluation mode ends. If the water temp is above
90° F (32.2° C), a heating water available ag is started and water evalu-
ation mode ends. These ags 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 ag 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 fteen
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 pro-
gramming to be so.
The method of controlling the valve is based on the fan conguration:
• 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 tem-
perature reaches active heating setpoint minus heating propor-
tional band. The valve modulates closed as the space temp rises
towards the active heating 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 cooling proportion-
al band, modulating linearly in between. Once the valve closes
fully in either heating or cooling mode, all water available ags
are turned o.
• With the fan mode set to “Auto,” the valve operates in a “two-
position” fashion. The valve will open 100% if the space temper-
ature falls below active heating setpoint minus heating propor-
tional band 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 set-
point plus cooling proportional band 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.