The following describes the supported features and modes for the CV systems. The system consists of several components that include the following:
- RTU Controller
- System Manager
- RTU, heat pump, or split system equipment
Verasys supports the TEC3000 Thermostats or the SMART Equipment Controls to control CV units. The TEC3000 Thermostat is the only supported thermostat for the Verasys system. The TEC3000 Thermostat communicates to third-party RTUs.
The following sections provide a description of system behavior in the various operating modes.
Cooling
When the unit is in occupied mode, the supply fan runs based on the parameters selected. It either cycles with a call for cooling or it continuously runs. When the zone temperature is below the occupied cooling setpoint, all cooling is off. When in unoccupied mode, the economizer is closed unless there is a call for free cooling, and free cooling is available.
If the zone temperature rises above the occupied cooling setpoint and if the outside air conditions are suitable, the mixed air dampers modulate to maintain the setpoint. If the outside air conditions are not suitable and the zone temperature rises above the occupied cooling setpoint, the first stage (Y1) of cooling is energized. A further rise in zone temperature initiates additional cooling stages (Y2 to Y4), if equipped. Cooling stages operate with minimum OFF and ON times. As the zone temperature falls below the occupied cooling setpoint, stages of cooling de-energize in reverse order. If you have proportional cooling, the cooling valve changes proportionally to the difference between the zone temperature and cooling setpoint.
When the unit is in unoccupied mode, cooling turns on when the zone temperature rises above the unoccupied cooling setpoint. It remains on until the zone temperature is below the unoccupied cooling setpoint for 30 seconds.
The fan does not run unless a call for cooling occurs. The economizer remains closed unless a call for free cooling occurs and free cooling is available.
Heating
When zone temperature is above the occupied heating setpoint, all heating is off. As zone temperature drops below the occupied heating setpoint, the first stage (W1) of heating is energized. A further drop in zone temperature initiates additional heating stages (W2 and W3), if equipped. Heating stages operate with minimum off and on times. As the zone temperature rises above the occupied heating setpoint, stages of heating de-energize in reverse order. If you have proportional heating, the heating valve changes proportionally to the difference between the zone temperature and heating setpoint.
When the unit is in unoccupied mode, heating turns on when the zone temperature drops below the unoccupied heating setpoint. Heating remains on until the zone temperature is above the unoccupied heating setpoint for 30 seconds.
The fan does not run unless a call for heating occurs.
Off
Shut down the system in the following two ways:
Instant Shutdown: The application performs an instant shutdown on heating, cooling, and the fan when the input is set to Shutdown. Minimum on times are ignored in this mode.
Control Mode: The application performs a shutdown on heating, cooling, and the fan when the Control mode input is Off. Minimum on times are observed in this mode. Table 1 provides minimum on and off times for each stage of staged heating and cooling.
| RTU Stage | Minimum On Time | Minimum Off Time |
|---|---|---|
| Heating | 2 min | 2 min |
| Cooling | 3 min | 5 min |
RTU
All heating and cooling is off. The supply fan is off.
Fan only
When the system is in fan only, the supply fan runs continuously regardless of occupancy. Cooling and heating is off.
CO2 DCV operation
RTU
When DCV Enabled is set to True and a CO2 sensor is reliable, the control modulates the economizer position to remain within ±100 ppm of the CO2 setpoint. The DCV control of the economizer position does not exceed the user-set maximum value (Demand Ventilation Maximum Economizer Position).
Dehumidification operation
RTU
The Verasys system supports dehumidification operation on RTUs equipped with a factory installed HGR coil and SMART Equipment or control board.
Dehumidification is initiated when the humidity rises 5% above the dehumidification setpoint and continues until the humidity is 5% below the dehumidification setpoint. The request for dehumidification starts the fan, hot gas relay, and appropriate cooling stages based on the user-set mode.
The dehumidification sequence supports two modes: normal and alternate. In both modes, requests for dehumidification with no cooling turns on the first compressor (C1) and the HGR coil. When one stage of cooling (Y1) is requested, the mode is checked. If the mode is set to normal, the HGR coil is turned off or remains off. The request for cooling is then satisfied by the economizer, if present, or the first compressor. If the mode is alternate, the first compressor, HGR coil, and the second compressor (C2) are turned on. For both normal and alternate modes, a call for two stages of cooling (Y2) turns off the HGR and turns on the first and second compressors.
See Table 2 for a summary of the dehumidification sequence in both modes.
| Request | Normal Mode | Alternate Mode | ||||
|---|---|---|---|---|---|---|
| HGR | C1 | C2 | HGR | C1 | C2 | |
| Dehumidification | On | On | Off | On | On | Off |
| One Stage of Cooling (Y1) | Off | On | Off | On | On | On |
| Two Stages of Cooling (Y2) | Off | On | On | Off | On | On |
Power exhaust control
The Verasys system supports three types of power exhaust: constant volume EF, modulating exhaust damper, or VFD exhaust fan.
Economizer Damper Position for Exhaust Fan to Turn On: The constant volume exhaust fan turns on when the supply fan is running and the economizer is open more than the setpoint.
Economizer Damper Position for Exhaust Fan to Turn Off: The constant volume exhaust fan turns off when the supply fan is off or when the economizer is open less than the setpoint or 10% open, whichever value is greater.
The exhaust damper modulates to maintain the static pressure setpoint for the building. When the exhaust damper opens more than the Exhaust Damper Position for Fan to Turn On setpoint, the power exhaust fan turns on. When the exhaust damper is open less than the Exhaust Damper Position for Fan to Turn Off setpoint, the power exhaust fan is turned off or 10% open, whichever is greater.
The VFD exhaust fan modulates to maintain the building static pressure setpoint. The power exhaust fan turns on when the supply fan is on. The exception is when the building static pressure is 0.02 in. (0.508 mm) W.C. or more below the setpoint and the VFD exhaust fan is at 0%. The power exhaust fan turns on again when the building static pressure is 0.02 in. (0.508 mm) W.C. or more above the setpoint.
Load shed
The Verasys system supports a centralized demand shed for user-specified zones. A predefined BI on an IOM is used to trigger the load shed. All zones with load shed enabled shift their setpoints by a user-specified amount. You can also set an adjustable rate limit which controls how quickly the setpoint changes from its current setpoint to its shed setpoint in either direction. The setpoint shift occurs during occupied and standby modes.
CV system strategies
Shutdown
The System Manager can shut down the system.
Control mode
Control mode allows the user to place the system into the following operating states: Heat, Cool, Fan Only, Off, or Auto.
- In Heat state, the RTU adjusts the zone temperature to meet the zone heating setpoint.
- In Cool state, the RTU adjusts the zone temperature to meet the zone cooling setpoint.
- In Fan Only state, the supply fan runs, and heating and cooling is off.
- In Off state, the system shuts down heating and cooling and turns off the supply fan. Off mode is not an instant shutdown so minimum times are observed.
- In an Auto state, the system operates normally based on the zone temperature.