As the name suggests, a 2-pipe fan coil unit comprises 2 pipes, allowing water supply with a fan that runs air over it. The air is further conditioned by the running water. The thermostat only has control over whether to turn on the fan and at what speed. The building's central plant determines whether the pipes are getting hot or cold water. As such, turning on the fan when the building is providing hot water and the current temp is above the desired temp is counterproductive.
A coil fed by 1 pair of supply pipes provides heating or cooling. The correct mode is based on sensing supply water temperature.
| How it Works
The 75F SmartStat 2 Pipe Fan Coil unit is a unique profile, that provides both heating and cooling conditioning.
Based on the conditioning and fan modes the system is in, and the temperature of the supply water from the chiller, the heating, and cooling operations are triggered, and the corresponding actions and conditioning are achieved.
The reheat capability is enabled with the auxiliary heating stage, which is only triggered during the Auto and Heat Only mode (where additional heat is required).
- 3 Stage Fan Speed Control
- Single Stage auxiliary heating equipment control
- Single-stage water valve control
- 2 Thermistor inputs connecting to Airflow Temperature Sensor & Supply Water Temperature
- Occupancy Sensing Control
Additionally, it also supports the Auto Force Occupied and Auto-Away occupancy-based controls.
| Application of Deadband and Hysteresis
The diagrams below show the application of the Deadband and Hysteresis Band and their role:
| Wiring Diagram
- Select the SmartStat option from the device screen.
- Select 2 Pipe FCU from the module type.
The pairing options are displayed on the screen which provides the steps to successfully pair the device.
- Click Pair to start the pairing.
- Select the displayed Bluetooth address.
- Enter the PIN displayed on the device and press Pair.
You can see the configuration screen for the 2 PIPE FCU with the following options available for configuration.
Note: The water valve control option and the supply water temperature are enabled by default, since they are the major actors in influencing the algorithm.
| Configuration Parameters
Once the SmartStat is paired as a 2 Pipe Fan Coil profile, you will need to enable the required configuration parameters as below, which define the physical characteristics of the device.
|Fan Medium Speed
|Activated when space temp is 2 degrees less or 2 degrees more from the heating or cooling desired setpoint respectively
|Fan High Speed
|Activated when space temp is 3 degrees less or 3 degrees more from the heating or cooling desired setpoint respectively
|Fan Low Speed
|Activated when space temp is 1 degree less or 1 degree more from the heating or cooling desired setpoint respectively
|Aux Heating Stage 1
|Activated when the central plant is in cooling, but this particular zone is below the setpoint.
|Controls the water valve to keep the zone close to the setpoint.
|Airflow Temperature Sensor
|Only select if using an airflow sensor. These are helpful for troubleshooting.
|Supply Water Sensor
|Required for gauging the supply water temperature.
|Enable Occupancy Setting
|Select if you would like to use the Occupancy Setting for setpoint control.
- Enable the required parameters as below.
- Click Set to confirm the parameters configured.
| Post Configuration
The zones paired with 2PIPE FCU profile is displayed in the CCU as below.
Below are widgets configured in the heatmap pages in the portals.
The user is also provided with profile reconfiguration options from the Internal portal as shown below, with all the features and capabilities that were available in CCU as explained in the above sections.
| Fan Modes
2 Pipe FCU profile supports the following fan and conditioning modes.
- Fan Low Current Occupied Period [From hardware changed]
- Fan Low Occupied Period
- Fan Low All Times
- Fan Medium Current Occupied Period [From hardware changed]
- Fan Medium Occupied Period
- Fan Medium All Times
- Fan High Current Occupied Period [From hardware changed]
- Fan High Occupied Period
- Fan High All Times
Note: The SmartStat hardware supports only the following fan modes at its end.
And when a change is made, it would translate to the following in the CCU.
- Auto------ Auto
- Low------Fan Low Current Occupied Period
- Medium-------Fan Medium Current Occupied Period
- High-------Fan High Current Occupied Period
| Conditioning Modes
- Cool Only
- Heat Only
| Thermistor Inputs for 2 Pipe FCU Profile
The Th2 or the supply water temperature sensing for chilled/hot temperature is used to determine the mode the system needs to operate.
A predefined set of values are set for the cooling and heating threshold using the relevant tuners for the 2 Pipe FCU profile, and the sensed Th2 value is compared with the threshold values set earlier. Based on whether the sensed Th2 value is less than or greater than the threshold values, the modes of operation are triggered.
The Th2 input (Supply Water Temperature) plays a crucial role in determining the functionality of the profile for further initiating several modes for the 2 Pipe FCU profile.
| Modes of Operation Based on Th2 Input and User Intents
Let us consider the multiple working modes of operation available under the 2 Pipe FCU Profile.
|All relays associated with multiple inputs will be non-functional and set OFF
|Heat Only Operation
|Cool Only Operation