Viconics VWZS Application Guide - Manual (Page 23)

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3D) RTU Heating and Cooling Supply Air Temperature Lockouts
One problematic aspect of any VAV zoning system is high demand for (heating or cooling) when most of
the zone VAV dampers are closed. This leads to most of the supply air being re-circulated through the
pressure by-pass and can lead to extremely hot or cold supply temperature.
- To prevent high supply temperatures (specifically with gas heating RTU), adjust discharge air
temperature high limit to required value.
Discharge air temperature high limit default value is: 80°F
Range is: 70°F to 150°F (21°C to 65°C) (increments: 0.5° or 5°)
- To prevent low supply temperatures (specifically to prevent freezing of RTU DX coils when a high
by-pass ratio is in effect ), adjust discharge air temperature low limit to required value.
Discharge air temperature low limit default value is: 55°F
Range is: 35 to 65°F (2.0°C to 19.0°C) (increments: 0.5° or 5°)
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.
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