96
RT-SVX36G-EN
Unit Start Up
decreases, the effective (reset) setpoint value is adjusted downward toward the minimum outside
air CFM setpoint.
Low Charge Protection
For each refrigeration circuit, the entering and leaving evaporator temperatures are used to
calculate superheat. When the calculated superheat exceeds the Evaporator Temperature
Differential Setpoint minus 5°F but not the Evaporator Temperature Differential Setpoint, an
information only, auto-reset, High Superheat diagnostic is initiated. If the calculated superheat
exceeds the Evaporator Temperature Differential Setpoint, a manual reset, low refrigerant charge
diagnostic is initiated and all compressors on the circuit are locked out.
Frostat
™
Control
The compressor module utilizes an evaporator temperature sensor (3RT14 & 15), mounted on the
suction line of each circuit, to protect the evaporator from freezing. If the evaporator temperature
approaches the specified setpoint, adjustable between 25°F and 35°F, the compressor(s) will be
cycled “off”. The compressors will not be allowed to restart until the evaporator temperature has
risen 10°F above the specified cutout temperature and the compressor(s) have been off for a
minimum of three minutes.
Lead-Lag Operation
When Lead-Lag is enabled, each time the system cycles after having stages 1 and 2 “On”, “Stage
2” (K11 on SCM or K3 on MCM) and the corresponding condenser fan output “2A” will start first.
The compressor module cycles the compressors “On” and “Off” to keep the zone temperature
within the cooling setpoint deadband. The condenser fans are cycled “On” and “Off” to maintain
the saturated refrigerant temperature within the specified controlband.
Units Equipped with 100% Modulating Exhaust Dampers
The exhaust dampers are controlled through an Exhaust/Comparative Enthalpy Module (ECEM).
The ECE module receives input form a space transducer and modulates the exhaust dampers to
maintain the space pressure to within the specified setpoint controlband.
Units equipped with Hot Gas Reheat (Modulating Dehumidification)
When space humidity exceeds setpoint and the unit is loaded 50% or less, the modulating
dehumidification function activates the reheat mode, providing dehumidification of the space.
The reheat valve and cooling valve are modulated to control the discharge air temperature to the
reheat discharge air temperature setpoint. In reheat mode, the reheat valve is commanded to
control the discharge air (15 to 85%) to the reheat setpoint; the cooling valve mirrors the reheat
valve position (85 to 15%). The reheat coil pumpout valve is energized when the unit is in active
cooling, and de-energized during all other times - including reheat mode.
Figure 43. CO
2
Reset
