2 Pipe Fan Coil Logic

A Smart Stat can be used to control a 2 Pipe Fan coil using the wiring schematic in the 2 Pipe Fan Coil Installation article and pairing as a 2 Pipe Fan Coil. The unit will operate as follows under standard conditions (standard conditions are often adjustable):

Heating or Cooling? The central plant will dictate whether hot or cold water is being supplied to the FCU. The Smart Stat will recognize whether central plant is heating or cooling based on supply water temp (SWT). The plant will be said to be heating if SWT > 85 and will be said to be cooling when SWT < 65 default. 

Cooling: When there is a call for cooling from the space and the SWT < 65 then the fan and valve will energize until the desired temperature is achieved. When there is a call for cooling in the space and the SWT > 65 then the valve will close and the fan will be set to user defined setting or will be off if set to auto. 

Heating: When there is a call for heat from the space and the SWT > 85 then the fan and valve will energize until desired temperature is achieved. When there is a call for heating in the space and the SWT < 85 then the valve will close and fan will be set to user defined setting or will be off if set to auto. 

Fan Staging: You have the ability to control 3 stages of fan. There are 5 modes for fan control; Off, Auto, Fan Low, Fan Medium, and Fan High. If you choose Low, Medium, or High the fan will run at that level all the time during occupied hours. Auto fan will run Fan low whenever the space temp is 1 degree off from desired, Fan medium will run when space temp is 2 degrees from desired, and Fan high and all speeds will run when space temp is more than 3 degrees from setpoint. 

Water Sampling: If there is a call for heating or cooling when the SWT is below the heating threshold and above the cooling threshold (SWT said to be neither heating or cooling) then cycle the valve for 2 minutes every 5 minutes. This is what is called sampling. If the valve has not opened in the last hour on a call, the valve will open for 2 minutes of flow to ensure that the SWT is being accurately measured and the water does not stagnate.