Product Overview

BAC-1xx63/1xxx63 Series (6 Relays and 3 Analog Outputs) Section 55 Application Guide S
FCU (Fan Coil Unit), 4-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.
Valve Control
The method of controlling the heating and cooling valves is based on the
fan conguration as well as the valve conguration:
• If the valves are congured for modulating action and the fan
mode for a particular occupancy mode is set to constant or
“On,” the valves function in a modulating fashion. Actual re-
sponse may vary slightly due to PI action of the control loops,
but generally, the heating valve will start to open as the space
temperature drops below the active heating setpoint and modu-
late to fully open when space temperature reaches active heating
setpoint minus heating proportional band. Likewise, the cooling
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
proportional band, modulating linearly in between.
• With the fan mode set to “Auto” or the valves are congured
for two-position operation, the heating 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. The cooling 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 at any time there is a fan failure alarm, the cooling and heating
valves are both forced closed.
Dehumidification
If the dehumidication sequence is enabled on models BAC-
10163C/11163C, the dehumidication mode is started and stopped as
described in the Common Features section above. Upon activation of the
dehumidication mode, the cooling valve is opened 100%. The heating
valve then reheats the space as called for in a modied heating sequence
to maintain the active cooling setpoint. Upon termination of the dehu-
midication mode, the valves return to their normal sequence.