17
Staged Air Volume (SAV) Units (FTYP = LEN VFD)
The SAV Option is a new method of controlling the supply fan in a
typical constant volume rooftop unit. This control method employs
a Variable Frequency Drive (VFD) to operate the supply fan at
different speeds in order to achieve energy savings through reduced
fan power. This method is specifically not concerned with
controlling static pressure in the supply duct, but rather with setting
different fan speeds for different operating conditions, such as
ventilation mode or part-load mechanical cooling. This option
should not be confused with a linearly variable VFD concept. The
different speeds are programmed per condition; there is no
linear-modulation of fan speed.
The SAV function is NOT
a Variable Air Volume (VAV) function.
The fan adapts its speed to one of seven based on mode and current
state to satisfy a demand. The seven speeds consist of off (0%) and
six configurable values. The six configurable fan speeds are:
Maximum Speed (FS.MX), Ventilation (FS.VN), IAQ Override
(FS.AQ), Free Cool Lo (FS.E1), Mech Cooling (FS.CL), and
Reheat2 (FS.RH).
The VFD is powered direct from the distribution block or circuit
breaker (CB) and is always on with power applied unless the CB is
tripped. When the thermostat or space sensor control conditions
require the fan on, the VFD will then ramp to desired speed. Fan
speed is always calculated by evaluating the current applicable
conditions. Each fan speed “mode” is evaluated independently, and
the highest fan speed is used. For example, if an IAQ Override
event occurs during Ventilation mode, the fan speed is set to the
higher of the two (FE.VN or FS.AQ). Refer to the speed
configurations below for when the fan will run at them. When first
turning on the fan from 0%, it will run at max speed (FS.MX) for 1
minute.
Supply Fan Maximum Speed (FS.MX)
Max speed is the highest fan speed allowed. This is typically
100% when pulleys are set to deliver design CFM to the space per
job requirement. Most safety conditions for the unit will override
the fan speed to this to help protect the unit. When in heat mode
this is the only speed available. In free cooling after the damper
has been at max for 5 minutes, the fan will be set to max before
mechanical cooling can be used and locked at max while damper is
100% and mechanical cooling is on. When running both cooling
stages the fan will be set to max.
Fan Speed - Ventilation (FS.VN)
This configuration defines the fan speed used in Ventilation
(fan-only) mode. Ventilation mode is when the supply fan is
running, but there is no demand for heating or cooling. In
thermostat mode, this is with just a G call. In space sensor control,
this is when the unit is Occupied mode and the indoor fan is
configured to always run while occupied (Configuration I.FAN
OC.FN = YES). If the indoor fan is configured for intermittent
fan (OC.FN = No), the fan will be off instead of this speed during
ventilation. The economizer damper will adjust its position based
on how far away this speed is from max speed.
IMPORTANT: It is important that the ventilation rate is checked
after setting this speed to verify that the unit can properly ventilate
the space per requirements. Adjusting this configuration or the
economizer minimum setting curve should be performed to meet
job requirements.
Fan Speed - IAQ Override (FS.AQ)
This configuration defines the fan speed used when an IAQ
Override is active. This pertains only to the Override function of
IAQ (Configuration AIR.Q IA.CF = 2) (Override IAQ) or
(Configuration AIR.Q II.CF = 3 or 4) (Override N/O or
N/C), not the DCV or Minimum Position functions. When in an
IAQ override condition the fan will run at this speed unless it is
configured for less than the current commanded speed. When the
IAQ override condition is cleared the speed will go to the next
commanded speed.
Fan Speed - Free Cool Lo (FS.E1)
This configuration defines the initial fan speed used when in Free
Cooling. Refer to the Economizer Controls Operation section for
details on free cooling. The fan will stay at this configured speed
whenever the damper is being used for free cooling. If the damper
is at 100% for 5 minutes the fan will ramp to max speed. It is
locked there until the actual damper position falls below 75% at
which time it will ramp back down to this configured speed.
Fan Speed - Mech Cooling (FS.CL)
This configuration defines the intermediate fan speed used when
one stage mechanical cooling is being requested. Fan speed is
based on how many cooling stages the unit has, how many cooling
stages are being requested, and how many reheat stages are being
requested. If the unit only has one circuit of cooling, then the fan
speed will be FS.MX whenever that stage is requested. If the unit
has more than one circuit of cooling but only one is being
requested, the fan speed will be set to this FS.CL. Any time more
than one stage is requested the fan speed will be set to FS.MX. On
Humidi-MiZer
t
equipped units fan speed is more complex. Refer
to the FS.RH and Table 5 for details.
Fan Speed - Reheat2 (FS.RH)
This configuration defines the fan speed used when Hot Gas
Reheat (reheat2) is being requested. Fan speed is based on how
many cooling stages the unit has, how many cooling stages are
being requested, and how many reheat stages are being
requested. If only reheat stages are being requested, then the fan
speed will be set to FS.RH. If the unit only has one circuit of
cooling, then the fan speed will be FS.MX when
dehumidification and cooling is being requested. If the unit has
more than one circuit of cooling and only one cooling stage is
being requested, the fan speed will be set to FS.CL. Any time
more than one cooling stage is requested the fan speed will be
set to FS.MX. If only one cooling stage is requested and reheat
is requested, then the fan speed will be set to the greater of the
two configurations (FS.CL or FS.RH). (See Table 5.)
Table 5 – Cooling Fan Speed Determination
Number
of
Circuits
Cooling Stages
Required
Reheat Stages
Required
HVAC
Mode
Fan Speed
1
1 0 Cooling FS.MX
1 >0 Reheat1 FS.MX
0 >0 Reheat2 FS.RH
>1
1 0 Cooling FS.CL
>1 0 Cooling FS.MX
>1 >0 Reheat1 FS. MX
1 >0
Reheat1/
Reheat 2
FS.CL or
FS.RH
0 >0 Reheat2 FS.RH
OAU Fan Boost Enable (OAFB)
This configuration (Configuration OAU OAFB) is available
when the unit is equipped with a factory installed Outside Air Unit
(OAU). If set to yes then the indoor fan can be used to help the
OAU if there is a low CFM alarm. When the alarm is active for 10
minutes the fan will ramp to the next highest configured speed and
wait 10 minutes. If the alarm is still active it will ramp to the next
highest configured speed for 10 minutes, and so on until
commanding FS.MX. If the alarm clears on its way up to max
speed or at max speed, the current speed will be locked in until a
standard speed driver commands something different.
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