Product Overview

BAC-1xx63/1xxx63 Series (6 Relays and 3 Analog Outputs) Section 54 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 (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. After 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 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 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 pro-
gramming to be so.
The method of controlling the valve is based on the fan conguration as
well as the valve conguration:
• For a modulating valve conguration, 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 heating proportional band. 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 cool-
ing setpoint plus cooling proportional band, modulating linearly
in between. Once the valve closes fully in either heating or cool-
ing mode, all water available ags are turned o.
• For a two-position valve conguration, with the fan mode set
to “Auto,” the valve will open 100% if the space temperature
falls below active heating setpoint minus heating proportional
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 setpoint
plus cooling proportional band and fully closes if the space temp
falls below the active cooling setpoint if cooling water is avail-
able.
Any time there is a fan failure alarm, the valve will be fully closed.