BAC-1xx63/1xxx63 Series (6 Relays and 3 Analog Outputs) Section 43 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
modications.
When the fan runs, whether in “Auto” or “On” mode, the fan speed is
determined by the Fixed Speed seing. If the Fixed Speed seing 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 congured. The fan slows down in the reverse sequence.
This operation allows the FCU to aempt 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 congured.
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 conguration. 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 (the valve is fully closed), the
FlexStat will initiate a water evaluation mode. During the water evalu-
ation mode, all fan outputs are de-energized and the water valve opens
for three minutes. Aer three minutes, the valve closes and a water
temperature reading is immediately taken. If the water temp is below
63° F (17.2° C), a chilled water available ag is started. If the water temp
is above 90° F (32.2° C), a heating water available ag is started. 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 op-
posite 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 “op-
posite water available” mode and the unit remains calling for the same
temperature mode (heating or cooling), every een 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 evalua-
tion 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 conguration as
well as the valve conguration:
• For a modulating valve conguration, if the fan mode for a
particular occupancy mode is set to constant 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 temperature reaches active
heating setpoint minus deadband. The valve modulates closed as
the space temp rises towards the active heating setpoint. Like-
wise, the valve will generally begin opening as the space temper-
ature 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 ags
are turned o.
• For a two-position valve conguration, with the fan mode set to
“Auto,” the valve will open 100% if the space temperature 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 set-
point if cooling water is available.
Any time there is a fan failure alarm, the valve will be fully closed.
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