SZVAV
Sequence of Operation
Single Zone VAV operates in DX and economizer cooling modes of operation when configured for System Type: VVZT (variable speed indoor fan type) or CVZT (multi speed indoor fan type). A valid space temperature and discharge air temperature sensor are required input. If the discharge air temperature input becomes invalid, the control automatically reverts to Single Loop Space Temperature Control. Symbio™ 700 operates in Single Loop Space Temperature Control in heating modes when staged gas heat and staged electric heat are configured.
Single Zone VAV is a control method of space temperature control that operates the supply fan at the lowest speed allowed for the cooling capacity required to satisfy the load in the space. As cooling capacity increases, the supply fan speed will increase to a defined minimum speed for the operating cooling capacity and modulate accordingly until it reaches 100 percent of its allowed range of operation. See Appendix A (In the application guide).
When there is no demand for heating or cooling the supply fan operates at the minimum speed setting while providing ventilation according to occupancy setpoints, see the following figure. Alternately, the supply fan can be setup to cycle off when there is no demand for heating or cooling via Supply Fan Configuration Command setting.
Figure 10. Supply fan operation - cooling only with staged gas or electric heat
Figure 11. Supply fan operation - heat pump heating and cooling
Single Zone VAV – Cooling
Cool
When there is a space cooling demand, DX cooling will initialize and increase to satisfy space temperature. The supply fan will continue to operate at a low, fixed speed at low cooling capacities until the cooling capacity requires additional airflow to keep compressor operation optimal. The supply fan speed will increase as the demand for cooling capacity increases. The minimum supply fan speed is calculated by DX cooling capacity. As the space requires additional cooling capacity, Space Temperature Control will calculate a lower discharge air temperature setpoint increasing DX cooling which in turn increases the supply fan speed. Cooling capacity and supply fan speed both can increase to 100 percent. See Appendix A (In the application guide).
If the unit discharge air temperature reduces to the Discharge Air Temperature Minimum Cool Limit setting or DX cooling reaches 100 percent capacity, cooling capacity increases will hold while the supply fan speed will continue to increase to 100 percent or modulate to manage space comfort cooling.
Cool – Economizer
If the unit is configured for a modulating outdoor air damper and conditions are suitable for economizer cooling, the supply fan will operate at minimum speed while the economizer damper modulates between the Outdoor Air Damper Minimum Position Setpoint and 100 percent to satisfy the discharge air temperature setpoint. If the economizer damper reaches 100 percent open and additional cooling capacity is required, the supply fan will increase toward 100 percent to provide additional cooling to the space.
Cool – Economizer + DX
Economizer Cooling + DX is a mode of operation when both economizer and DX cooling are active. If actively economizing, outdoor air damper is 100 percent and supply fan speed reaches 100 percent then DX cooling will be added if the unit is not satisfying space cooling requirements.
If DX cooling is active and economizer cooling enables, the control will transition to increase economizer damper above minimum position to 100 percent to satisfy space cooling while decreasing DX cooling. DX cooling will steadily be removed as long as economizer cooling is able to satisfy the cooling load. The supply fan operates to the lowest speed possible during the transition.
Occupied Cooling with Variable Speed Compressor
For normal cooling operation, the total cooling capacity will be modulated based on a PI algorithm in order to meet the calculated Active Discharge Air Cooling Setpoint, as on standard fixed speed compressor units. If enabled, economizer cooling will be utilized prior to energizing the DX capacity, per the user configured Economizer Mode.
At cooling start, if the economizer is enabled, it will be utilized as the first stage of cooling and will be modulated to maintain the calculated Discharge Air Cooling setpoint as on standard fixed-speed compressor units.
Once all economizing requirements have been met, compressor operation will be enabled if the economizer alone cannot meet the demand. Once compressor operation is started, the variable speed compressor will be energized, following all startup requirements, and will then be modulated to maintain the discharge air temperature to the calculated discharge air cooling setpoint.
Once the active cooling demand has been satisfied, the variable speed compressor will begin to modulate down to its minimum speed and then will be de-energized, while adhering to all shutdown requirements. Once the VSPD compressor has deenergized, the economizer (if enabled) will be allowed to close back to minimum position if there is no longer a demand.
Unoccupied Cooling with Variable Speed Compressor
For unoccupied DX and economizer cooling control, the unit will operate as a standard fixed-speed compressor unit. During all requests for cooling operation during Unoccupied mode, the unit will operate the variable speed compressor at 100 percent of the available capacity. When compressor operation is requested, the variable speed compressor will be started first and its speed will be increased up to 100 percent, based on the unit configuration. The unit will follow all startup requirements.
Supply Air Tempering – Space Temperature Control
If the Supply Air Tempering function is configured and the Discharge Air Temperature local sensor is valid, this function prevents excessively cold discharge air from being supplied to the space. Supply Air Tempering is an option for VVZTand multi-speed supply fan units when auxiliary heat is installed.
Supply Air Tempering is not supported when a Conventional TStat is configured.
The following requirements must be met to allow Supply Air Tempering when heat is installed:
- The unit is in any heating mode, including Heat, Emergency Heat, Morning Warmup, Max Heat but is not actively heating OR
- The unit is in any cooling mode except night purge, but not actively cooling and cooling capacity has been OFF for 5 minutes.
If the discharge air temperature drops to the Discharge Air Temperature Minimum Cool Limit - Active and the Space Temperature is less than the Active Space Temp Cooling Setpoint Status – 0.5°F, the Supply Air Tempering function will bring ON one stage of auxiliary heat. The supply fan operates at the defined minimum speed for the active stage of heating. Modulating gas heat supply air tempering is limited to the first stage, main manifold (burner 1). See Appendix A (In the application guide).
Supply Air Tempering terminates if the Discharge Air Temperature rises to 10°F above the Discharge Air Temperature Minimum Cool Limit - Active, or the Space Temperature rises to the Space Temp Cooling Setpoint Status. If the Space Heat Control function determines that one or more stages of Heat are required to meet the Space Temp Heating Setpoint Status, Supply Air Tempering will terminate and the unit will stage heating to meet the current space demand.
Unoccupied Cooling
Unoccupied mode is used when the building is unoccupied, and the space conditions are exceeding temperature limits. The unit is normally off in unoccupied mode, but unoccupied operation is allowed in any heat cool mode except OFF.
If a valid space temperature input rises above the Unoccupied Cooling Setpoint, unit operation starts unoccupied cooling to manage space temperature. The controller operates in Single Loop Space Temperature Control with ventilation disabled. DX or economizer cooling will steadily increase to 100 percent capacity. The supply fan will follow capacity control until it reaches 100 percent fan speed. Cooling continues until space temperature is 4°F less than the Unoccupied Cooling Setpoint, the unit will then cycle off.
Unoccupied Heating
When the unit is in unoccupied mode and the valid space temperature input falls below the Unoccupied Heating Setpoint, unit operation starts unoccupied heating to manage space temperature. The controller operates in Single Loop Space Temperature Control with ventilation disabled. The supply fan starts increases with heating capacity unit both reach 100 percent. Heating continues until space temperature is 4°F greater than the Unoccupied Heating Setpoint, the unit will then cycle off.
Application Guide Link
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.