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3E) RTU Heating and Cooling Outdoor Air Temperature Lockouts
H Lock and C Lock
-
Parameter C Lock temperature disables the cooling stages based on the outdoor temperature.
-
Parameter H Lock temperature disables the heating stages based on the outdoor temperature.
RTU mode lockouts need to be properly set to keep heating or cooling equipment cycling to a minimum. It
is the responsibility of the installer to decide if priority of the system will be given to comfort or not. The
adjustments for both lockouts will be different based on specific regions load requirements.
-
A system located far north may require the RTU to deliver heating until a 75F outside air value is
attained due to the inertia of the building mass which will require heating during a cold night and
then will transition to a hot mid-season day.
-
A southern system application may require the RTU to always deliver cooling and never lock up the
cooling mode while imposing strong restrictions on the heating side of the system.
Heating and cooling RTU equipment cycling will only happen within the overlapping dead band value left
between the H Lock and C Lock parameter adjustments. The tighter the value between these two
parameters, the less cycling will be encountered.
Heat Lock = 75F
Cool Lock = 65F
Overlap = 10F
Outside Air Temperature
It is also possible to set the system to completely eliminate heating and cooling equipment cycling based
on outdoor air limitations if this type of operation is required. This of will have an impact on specific zone
performances.
3F) Critical Mid-Season Changeover
Heating and cooling RTU equipment cycling during mid-seasons is inevitable with a zoning VAV system if
any degree of comfort is to be maintained.
A properly setup system will be able to deliver comfort to conflicting zone demands during the mid-season
period by alternating heating and cooling at the RTU.
Normally, a lot of the unwanted heating and cooling switchovers can be eliminated by authorizing terminal
reheat or by limiting the RTU heating or cooling capacity throughput based on the outdoor temperature
( H
Lock and C Lock )
. However, limiting the RTU heating or cooling throughput based on outdoor
temperature will have an impact on control performance of certain zones when the required heating or
cooling capacity is not available due to the lockout conditions.
Typically, the number of RTU heating or cooling switchovers cycles during conflicting demand situations
will be around the same as the RTU CPH settings
(Default of 4 cycles per hour for both heating and
cooling).
This will translate into two cooling and two heating cycle periods per hour.
Outside Air Temperature
Heat Lock = 72F
Cool Lock = 72F
No Overlap
Heat Lock = 70F
Cool Lock = 75F
No Heat
No Cool
5F deadband
Outside Air Temperature
