sequence of
Operation operation
Occupied Sequence
All setpoints can be adjusted using the HI panel. Also, cooling/heating setpoints can be adjusted in the zone, if using one of the zone sensor options (BAYSENS020, BAYSENS021, BAYSENS108, BAYSENS110, BAYSENS019, or BAYSENS074). For a complete list of unit setpoint default values and ranges, see the
Occupied ZoneTemperature - Cooling The unit transitions from unoccupied to occupied when the occupied/unoccupied input on the RTM is open for more than five seconds after having been closed. This input can be received fromTracer Summit, the remote NSB zone sensor, the timed override function, or a field supplied contact. Dependent on unit options and the HI programming, the following sequence will occur:
•The unit will begin MWU and then switch to the occupied mode after the MWU setpoint is met.
•Purge will be enabled byTracer Summit. ThenTracer Summit will enable the occupied mode.
•The unit will switch from unoccupied to
occupied control immediately. Upon entering occupied mode,the IGV option will close while the supply fan remains on. The occupied/unoccupied relay will
Zone Temperature Control
(Unit Model Number Digit 9 = 4 or 5) A zone sensor located directly in the space sends input to the RTM while the CV unit is in occupied cooling mode. When the unit is in occupied cooling, the RTM controls the zone temperature within the cooling setpoint deadband by modulating the economizer option and/or staging mechanical cooling on and off as required.
Supply AirTemperature Control
(Unit Model Number Digit 9 = 1, 2, 3, or 6) When theVAV unit is in occupied cooling, the RTM controls the supply air temperature to the specified supply air cooling setpoint by modulating the economizer option and/or staging mechanical cooling on and off as required. The changeover relay contacts (field supplied) must be open on units with hydronic heat for cooling to operate.
Cooling
Upon entering occupied mode, the RTM receives an input from either the HI, RHI, Tracer Summit, or the GBAS to start the supply fan. The RTM supply fan contacts close and energize the supply fan contactor. OnVAV units with IGV, the fan delays until the IGV fully close. When the supply fan starts, the fan proving switch closes, signaling the RTM that airflow is established. Depending on unit options, either the IGV will begin to drive open, the VFD will ramp the fan, and/or the airside economizer dampers will open to the
When a cooling request is sent to the RTM from the zone sensor, the RTM evaluates the system operating condi- tions using the supply air and outdoor temperature input before sending the request to the MCM for mechanical cooling. If outdoor conditions (tempera- ture and humidity) are suitable or the EWT is within specified setpoints, the RTM will attempt to use “free cooling” without using any compressors. The RTM will use either the airside or waterside economizer option. When outdoor air conditions are not suitable, only mechani- cal cooling will function and outside air dampers will remain at their minimum position. If the unit does not have an economizer, mechanical cooling will operate to satisfy cooling requirements.
Units With Economizer
If the entering condenser water temperature (units with a WSE) or the outside air enthalpy (units with an ASE) is appropriate to use “free cooling,” the economizer will attempt to satisfy the cooling zone temperature setpoint.
Note: When using an ASE with economizer enabled, O/A temperature enable can be used instead of comparative enthalpy if the O/A temperature falls below the economizer setpoint.
Then compressors will stage on as necessary to maintain supply air tem- perature setpoint, which is
When both airside and waterside economizers are on a single unit, priority must be set at the HI. The economizer
with the highest priority attempts cooling first. Once it is operating at its maximum, and if additional cooling is necessary, the other economizer enables before mechanical cooling begins.
Cooling/Waterside Economizer Waterside economizing enables when the unit’s entering water temperature is below the unit’s entering mixed air temperature by 4°F plus the user adjustable economizer approach temperature. The approach temperature default is 4°F.
Waterside economizing disables when the unit’s entering water temperature is not below the unit’s entering mixed air temperature by at least the water economizer approach temperature (default value of 4°F). The economizer acts as the first stage of cooling. If the economizer is unable to maintain the zone (CV units) or supply air (VAV units) temperature setpoint, the compressor module will bring on compressors as required to meet the setpoint.
Cooling/Airside Economizer
On units with an airside economizer, a call for cooling will modulate the fresh air dampers open. The rate of economizer modulation is based on deviation of the zone temperature from setpoint; i.e., the further away from setpoint, the faster the fresh air damper will open. The first stage of cooling will start after the economizer reaches full open.
Note: The airside economizer will only function freely if ambient conditions are below the enthalpy control settings or below the return air enthalpy if unit has comparative enthalpy installed. If outside air is not suitable for “economizing,” the fresh air dampers drive to the minimum open position. A field adjustable, factory default setting at the HI panel or Tracer Summit can provide the input to establish the minimum damper position.
When outdoor air conditions are above the setpoint or comparative enthalpy control setting, only mechanical cooling will function and outside air dampers will remain at their minimum position.
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