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RT-SVD03L-EN

utilizing full speed indoor fan by energizing the
appropriate output(s) as described above. The two-
speed indoor fan unit will only utilize two speeds
during the modes described below.

Cooling Operation

For cooling operation, the unit will utilize low fan speed
during fan only and economizer only unit modes.
When the unit receives a call for cooling, if the
economizer is enabled, the unit will energize the fan at
low speed and begin to modulate the OA damper open
above minimum position up to 100% as necessary to
meet the mixed air setpoint. If the OA damper reaches
100% the supply fan will increase to high speed and if it
remains there for 5 minutes, the unit will begin to stage
up compressors and the supply fan will remain at high
speed for the remainder of the call for cooling.

For 2-step cool units, when the unit enters Cool 1 with
economizer disabled, the supply fan will energize at
low speed for the duration of Cool 1. When the unit
determines that the 2nd compressor is necessary to
meet cooling demands, the unit will energize the 2nd
compressor and the supply fan will switch to high
speed. For 3-step cool units the unit will utilize low fan
speed during requests for Cool 1 and Cool 2 during
cooling operation. High fan speed will be utilized only
for Cool 3.

The unit will stage down compressors for cooling in
reverse order that they were staged and the supply fan
will follow the unit mode transitions. Once it is
determined through normal cooling control that the
2nd compressor is no longer necessary to meet cooling
demand, the 2nd compressor will be staged back and
the supply fan will switch to low speed until the unit
stages all cooling off or receives a call for Cool 2 again.
Once the supply fan is no longer requested ON after a
cooling call is cleared, the supply fan will remain
energized at low speed for 60 seconds.

Economizer Minimum Position Handling for
Two-Speed Fan

Due to the low and high speeds of the indoor fan,
When the unit is in Fan Only, Cool 1, or Economizer
Only modes the control will require the setting of two
economizer minimum positions. The economizer
minimum position that will be utilized while the supply
fan is at low speed will be set with the RTEM DCV
minimum position pot (0-100%) and the minimum
position that will be utilized while the supply fan is at
high speed will be set with the RTEM Building Design
minimum position pot (0-50%). If the Building Design
minimum position is set to be higher than the DCV min
Position, the setpoints will be capped at the DCV min
position and the setpoint for the min position at low
and high fan speed will be the DCV min position.

Demand Controlled Ventilation Operation

DCV for units with two-speed supply fans will require
an additional module (RTVM) for the 2 additional
setpoint potentiometers necessary.

The new DCV scheme will require the user to select 4
OA damper minimum position setpoints in addition to
the Design and DCV CO2 setpoints:

•

Design Min Position @ Low Fan Speed Command
(RTVM R130)

•

Design Min Position @ High Fan Speed Command
(RTEM Design Min)

•

DCV Min Position @ Low Fan Speed Command
(RTVM R41)

•

DCV Min Position @ High Fan Speed Command
(RTEM DCV Min)

The speed at which the supply fan operates will dictate
which design and DCV minimum position setpoint is
used.

Space Pressure Control (Powered Exhaust/
Statitrac

™

)

For two-speed supply fan units, the user will select an
exhaust enable setpoint during the full fan speed
command. Once selected, the exhaust enable setpoint
for low fan speed operation will be calculated.

Failure and Override Modes

•

Supply Fan Proving - If there is a supply fan failure
condition all outputs associated with the supply fan
output control will be de-energized.

•

Ventilation Override Mode - For all VOM modes that
require supply fan operation (Purge and
Pressurize), the supply fan will operate at high
speed.

•

All failure modes that require the Supply Fan to
operate, the Supply Fan will energize at high speed.

Fan Speed Selection for Two-Speed Analog
Output Units

For units equipped with an analog speed controlled
motor for two-speed indoor fan control, the user will
have the ability to fine tune the high and low fan
speeds by adjusting the potentiometer provided on the
RTOM board (R136 DACOOL-FAN SPD) to get the
desired output voltage for high fan speed. The RTOM
will provide 50% and 100% of this user selected output
voltage for the low and high fan speeds respectively.
During service test mode when the RTOM indoor fan
speed output is set to 100% for Cool 2, Reheat, and all
Heating steps, the user can adjust the RTOM
potentiometer to provide the appropriate 0-10Vdc input
signal to the analog speed controlled motor for high
speed. For all other steps in which the RTOM indoor
fan speed output is providing 50%, the motor will
receive ½ of the adjusted voltage for low speed.