VAV w/ Reheat (No Fan)

4 Minute Read

| How it Works

75F VAV profiles operate by ASHRAE GPC36. To read a full description please read GPC36 in its entirety. Below you will find a highly summarized description of how this VAV application will operate. The description below does not cover all details of GPC36 but is meant to be used as a quick and dirty field-level understanding of the VAV box operation. 

Cooling Mode

If the system is supplying cool air and the zone requires cooling, the damper actuator will modulate toward its maximum cooling position until the airflow setpoint is achieved. If the system is supplying cool air but the zone requires heating, the damper will be maintaining its minimum heating airflow setpoint and will stage on the reheat to raise the DAT. As the DAT rises past the DAT setpoint then the damper actuator will begin to modulate open to its max heating airflow setpoint to mix the air back down to its DAT setpoint. 

Heating Mode

If the system is supplying warm air and the zone needs cooling then the damper actuator will modulate down to its minimum heating airflow setpoint. If the system is supplying warm air and the zone needs heating then the damper actuator will modulate open to its maximum damper position. 

Recirculate - If the zone is satisfied the damper actuator will modulate near closed to maintain its minimum airflow setpoint. 

| Wiring

Electric Reheat

User 18gauge thermostat wire for power and actuator connections.

I/O
VAV Reheat - No Fan - Staged Electric
Inputs/ Outputs
#	DI	AI	DO	AO	Point	Comments
AI1		X				0-10Vdc
AI2		X				0-10Vdc
TH1		X			Discharge Air Temp	10k Type II or DI
TH2		X			Entering Air Temp	10k Type II or DI
AO1				X	Damper Actuator	0-10Vdc
AO2				X		0-10Vdc
R1			X		Reheat Stage 1	Dry contact
R2			X		Reheat Stage 2	Dry contact
Other Ports
Damper 1	8 Pin RJ45					Primary Smart Damper
Damper 2	8 Pin RJ45					Secondary Smart Damper
RTS	3 Pin Molex Sensor Bus				Temperature/ Humidity/ Duct Pressure	3 pin cable connector (No local interface option)
RS485	4 Pin Molex Sensor Bus				Temperature/ Humidity Local control	4 pin cable connector (Local Interface option)

 

Modulating Reheat

I/O
VAV Reheat - No Fan - Modulating Reheat
Inputs/ Outputs
#	DI	AI	DO	AO	Point	Comments
AI1		X				0-10Vdc
AI2		X				0-10Vdc
TH1		X			Discharge Air Temp	10k Type II or DI
TH2		X			Entering Air Temp	10k Type II or DI
AO1				X	Damper Actuator	0-10Vdc
AO2				X	Modulating Reheat	0-10Vdc
R1			X			Dry contact
R2			X			Dry contact
Other Ports
Damper 1	8 Pin RJ45					Primary Smart Damper
Damper 2	8 Pin RJ45					Secondary Smart Damper
RTS	3 Pin Molex Sensor Bus				Temperature/ Humidity/ Duct Pressure	3 pin cable connector (No local interface option)
RS485	4 Pin Molex Sensor Bus				Temperature/ Humidity Local control	4 pin cable connector (Local Interface option)

 

| Configuration

Once you've paired the SmartNode as a VAV profile, the sequences for VAV applications in GPC36 will be followed. What you will need to do is select a few specific configuration parameters that define the physical characteristics of your box. 

 

Damper Type - Select the type of damper actuator. The most common is a 2-10Vdc actuator but other types exist. For example, if you are using Smart Dampers, you will select Smart Damper as the damper type. 

Size - Tells the system the size of the duct. This helps the system accurately calculate the CFM. 

Shape - Also helps the system calculate the CFM. 

Reheat Type - Tells the system which output to use as the reheat. Staged and modulating options both exist. 

Zone priority - Zone priority is a concept of 75F which equates to the importance multiplier in guideline 36. For more information on its significance and influence refer to the importance multiplier section in the article Damper Position & Zone Priority Driven Trim and Respond Requests.

Damper Min/ Max Positions - Sets the hard limit for damper actuator minimum and maximum positions. 

Enable Occupancy Control - Only used if an occupancy sensor exists and is not a standard option. If you have the option and you enable this feature the occupancy sensor will set the zone to occupied during unoccupied hours if occupancy is detected. 

Enable IAQ Control - Only used if an IAQ sensor exists and is not a standard option. This will enable the trim and respond logic from this controller for IAQ. 

Enable CO2 Control - Only used if a CO2 sensor exists and is not a standard option. This will enable the trim and respond logic from this controller for DCV. 

Enable CFM- To enable operation based on the Airflow in cubic feet per minute calculation. For more information refer to True CFM (Cubic Feet per Minute)

| Operation

The VAV Series Fan terminal profile, with Reheat capability, provides two major operations to provide the necessary conditioning required for the zone, they are as follows:

• Damper position operation
• Reheat Operation

Damper Position Operation

The damper position operation satisfies the cooling load requirement for the zone. The operation is usually influenced by the current and desired temperature of the zone, but the user can also configure for a CO2, IAQ, and CFM-based damper operation as well. 

For more information on the Damper position calculation refer to VAV Terminal Profile- Damper Position Calculation & Operation

Reheat Operation

The reheat operation satisfies the heating load requirement for the zone. when the system is in cooling operation mode. 

The Reheat type can be staged, or modulating based on the selection made during the configuration.

The modulating reheat is enabled and ramped from 0% to 100%, based on reheatPos or reheatLoopOuput, which is the reheat load calculated as a loop output based on the discharge air temperature. 

 The staged reheat is also enabled based on reheatPos or reheatLoopOuput, which is the reheat load calculated as a loop output based on the discharge air temperature. The table below explains the operation of each stage in detail:

Reheat Stages	ON	OFF
Reheat Stage 1	(reheatLoopOutput) > 0% + relayActivationHysteresis.	(reheatLoopOutput) = 0%
Reheat Stage 2	(ReheatLoopOutput) > 50% + relayActivationHysteresis.	(reheatLoopOutput) <= 50%

| Failsafe Mode

When the SmartNode loses the connection with the CCU, the system enters the failsafe mode where the required controls are still made available from the SmartNode itself.
The Occupancy, IAQ and VOC controls will not be available in Fail Safe, only CFM and all its tuners and configs will work as intended in Fail Safe.

| Equip Graphics for VAV w/Reheat (No Fan)

Equipment graphics is available for all the terminal profiles. It provides a graphical representation of system-level equipment and enables monitoring of key point parameters associated with terminal profiles, along with the alert rings and alert severity legends displayed.

The highlighted list of points is predefined for visualization alongside the graphic for the respective profile.