sequence of
Operation operation
Mechanical Cooling
If the zone temperature cannot be maintained within the setpoint deadband using the economizer option or if there is no economizer, the RTM sends a cooling request to the MCM. The compressor module checks the compressor protection circuit before closing stage one. After the first functional stage starts, the compressor module monitors the saturated refrigerant temperature and closes the condenser fan output contact when the saturated refrigerant temperature rises above the lower limit setpoint.
The compressor module closes the condenser fan output contact when the saturated refrigerant temperature rises above the lower limit setpoint.
Water-Cooled Units Only
The WSM modulates the condenser coil water valves to maintain condenser temperature, if applicable. Otherwise, it will check the entering condenser water temperature to ensure it is greater than 54°F or if not, it will lock out cooling.
Auto Changeover (Units with Heat Only) When the system mode is in auto, the mode will change to cooling or heating as necessary to satisfy the zone cooling andheating setpoints. The zone cooling and heating setpoints can be as close as 2°F (1.1°C).
Occupied ZoneTemperature - Heating Relies on input from a sensor directly in the space, while a system is in occupied heating mode or an unoccupied period, to stage electric heat on and off or modulate the hydronic heating valve as required to maintain the zone temperature within the heating setpoint deadband. The supply fan will operate when there is a request for heat.
Electric Heat
On units with electric heat, the zone temperature can be controlled to a heating setpoint during the occupied mode by cycling a single stage electric heater. An interface is provided for field supplied single stage electric heat. The zone temperature heating setpoint and deadband are user defined at the HI panel.
Hydronic Heat: HotWater or Steam
On units with hot water or steam heating, the zone temperature can be controlled to a heating setpoint during the occupied mode. The zone temperature heating setpoint and deadband are user defined at the HI panel or zone sensor.VAV occupied heating initiates by closing a
Supply Air Setpoint Reset (VAV Units Only)
Supply air reset can be used to adjust the supply air temperature setpoint on the basis of a zone temperature or outdoor air temperature. Supply air reset adjustment is available at the HI panel for supply air heating and supply air cooling control.
Reset based on outdoor air temperature Outdoor air cooling reset is sometimes used in applications where the outdoor temperature has a large effect on building load. When the outside air temperature is low and the building cooling load is low, the supply air setpoint can be raised, thereby preventing subcooling of critical zones. This reset can lower usage of mechanical cooling, thus savings in compressor kW, but an increase in supply fan kW may occur.
Outdoor air heating reset is the inverse of cooling, with the same principles applied.
For both outdoor air cooling reset and heating reset, there are three user defined parameters that are adjustable through the human interface panel.
•Beginning reset temperature
•Ending reset temperature
•Maximum amount of temperature reset
Reset based on zone temperature Zone reset is applied to the zone(s) in a building that tends to overcool or overheat. The supply air temperature setpoint is adjusted based on the temperature of the critical zone(s). This can have the effect of improving comfort and/or lowering energy usage. The user- defined parameters are the same as for outdoor air reset.
SupplyAirTempering (HotWater and Steam VAV Units Only)
When supply air temperature falls below the supply air temperature deadband low end, the heating valve modulates open to maintain the minimum supply air temperature setpoint.
Daytime Warmup (Units with Supply Air Temperature Control Only)
During occupied mode, if the zone temperature falls to a preset, user- defined zone low limit temperature setpoint, the unit is put into daytime warmup. The system changes over to CV heating, theVAV boxes drive full open. However, unit airflow modulation control operates to maintain duct static setpoint, and full heating capacity is provided until the daytime warmup setpoint is reached. The unit is then returned to normal occupied mode.
Supply Air Tempering
Supply air tempering is available on units without volume control and with hot water, steam, or electric heat or units with supply air temperature control with steam or electric heat. When the unit is in heat mode but not actively heating, if the supply air temperature drops to 10°F (5.5°C) below the occupied zone heating temperature setpoint, electric heat will stage on or the hydronic valve will modulate to maintain a minimum supply air temperature. The unit transitions out of heat mode if the supply air temperature rises to 10°F (5.5°C) above the occupied zone heating temperature setpoint.
Changeover
This mode only functions on units with supply air temperature control with hydronic heat. When the changeover binary input is closed the unit will control to a discharge air heating setpoint. This setpoint is entered from the HI, and can be a higher temperature than the supply air cooling setpoint. This function maintains duct static pressure.
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