Sequence of Operation
44 RT-SVX25Q-EN
the normal 2-compressor dehumidification mode
cycle will be used.
5. The purge cycle will ignore the Low Ambient
Dehu
midif
ication lockout feature.
6. A purge cycle takes priority over normal cooling or
dehumi
dification requests, but will discontinue for all
high priority lockouts and alarms.
Two-Speed Indoor Fan (Title 24)
Control
For units equipped with two-speed indoor fan control,
standard unit operation for cooling, heating, and
dehumidification will continue and all unit functions will
operate normally, except for the control of the indoor fan
speed. All units configured with any type of heating or
reheat/dehumidification will heat and reheat/dehumidify
the supply air and space as normal utilizing full speed
indoor fan by energizing the appropriate outputs. The two-
speed indoor fan unit will only utilize two speeds during
the cooling operation modes described below.
Cooling Operation
For cooling operation, the unit will operate at Low Fan
Speed during Fan Only, Economizer Only, and the first
stage of compressor unit modes by Default. When the unit
receives a call for cooling, if the economizer is enabled, the
unit will energize the associated Low Speed output and
begin to modulate the OA Damper open above minimum
position up to 100% as necessary. By default, the supply
fan will remain at low speed for the duration of
Economizer-Only cooling. If the OA damper reaches 100%
and remains there for a few minutes, the unit will begin to
stage up compressors. If the supply fan is on when the first
stage compressor is energized, it will remain energized or
energize at low speed for the duration of Cool 1. When
there is a call for Cool 2, the second compressor is
energized and the Supply Fan will switch to High Speed.
Note: For 3-st
ep co
ol units the unit will utilize Low Fan
Speed during requests for Cool 1 and Cool 2 during
cooling operation. High Fan Speed will be used
only for Cool 3.
The unit will stage down compre
ssors for coo
ling 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 second
compressor is no longer necessary to meet cooling
demand, the second 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 as on traditional
units.
For units under thermostat control
,
if the economizer is
enabled and is being utilized as the first stage of cooling
(Y1 active), if the unit receives a Y2 request, the unit will
energize the first compressor and will switch the supply
fan to high speed until the Y2 request is cleared.
Economizer Minimum Position Handling
for 2-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 and the
indoor fan is at low speed, the unit will not be bringing in
as much fresh air as normal. By setting a higher minimum
damper position this problem could be solved, but that
would mean more fresh air than may be desired when the
supply fan is at high speed. To overcome these situations,
the control will require the setting of two economizer
minimum positions to be used when the fan is at low and
high speed to compensate for the lower amount of fresh
air at lower fan speeds and the potential for more fresh air
than desired at higher fan speeds. These minimum
positions will be obtained from the Bldg Design Min
Position and DCV Min Position pots on the RTEM. The
economizer minimum position that will be utilized while
the supply fan is at low speed will be set with the DCV Min
Position pot with a range of 0-100% and the minimum
position that will be utilized while the supply fan is at high
speed will be set with the Building Design Min Position pot
with a range of 0-50%. If the Building Design Min 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
Demand Controlled Ventilation for units with 2-Speed
Supply Fans will require an additional module (RTVM) for
the 2 additional setpoint potentiometers necessary in
order to overcome the non-linear characteristics of the
airflow through the outside air damper along with the
multiple supply fan speeds.
The new Demand Controlled Ventilation scheme will
r
equire the user to select
4 OA Damper Minimum Position
setpoints in addition to the Design and DCV CO2 Setpoints:
1. Design Min Position @ Low Fan Speed Command
(RTVM R130)
2. Design Min Position @ High Fan Speed Command
(RTE
M Design Min)
3. DCV Min Position @ Low Fan Speed Command (RTVM
R41)
4. DCV Min Position @ High
Fan Speed Command (RTEM
DCV Min)
The speed at which the Supply Fan oper
ates will dictate
which Design and DCV Minimum Position setpoint is used
in the calculation for the OA Damper Minimum Position
Target for Demand Controlled Ventilation. All other
functionality for Demand Controlled Ventilation will
operate as on traditional units.
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