2 Pipe vs 4 Pipe Fan Coil
A 2-Pipe FCU system can be used to heat or cool a room that it is supplying air to. The following table describes the main components of a 2-Pipe FCU and their functions briefly:
| Component | Function |
| Fan Coil System | Receives Hot or Cold water from the Boiler or Chiller respectively into a heat transmitting coil and accordingly pumps out Hot or Cold air into the room using its Fan |
| Chiller | Takes in Hot Water only and Cools it down |
| Boiler | Takes in Cool Water only and Boils it up |
| Inlet and Outlet Pipes | To transport Cold or Hot water to the FCU from the Chiller and the Boiler as required |
| 3-Way Mixing Valves | Valves with 3 possible openings, lets say A, B and AB, such that only 2 of the 3 openings operate at a time |
| Pumps | To provide the necessary water pressure gradient for smooth transport of water through the entire system (both the Chiller and Boiler get one of these) |
| Thermostat | To measure the room temperature |
Let’s first consider the situation where the FCU system wants to heat up a room.
The thermostat is telling the FCU at this point that the temperature in the room is lower than ideal. Now, the 3-Way valves will be set so that the water is first sent into the Boiler, where it is heated up. This water is taken through the Pump and the 3-Way valves into the FCU system. As the hot water enters the FCU through the inlet pipe, it heats up the air in the FCU system. The FCU system, as a result, blows hot air into the room using it’s fan. Ideally speaking, the hot water was able to dump it’s excess heat into the air in the FCU. If this process continues long enough the room will naturally get heated up. Once the water exits the FCU through the outlet pipe, it is cooler than it was when it entered the FCU. This cold water is redirected to the Boiler to be heated again, using the motor actuators that control the direction of water flow inside the 3-Way valves.
Let’s say now that FCU system wants to cool down the room.
The thermostat is now telling the FCU to pump our cold air into the room. The 3-Way valves will now be set up such that the water flows into the chiller first. The cold water coming out of the chiller is sent to the FCU through the Pump and the Valves. This has the opposite effect on the air inside the FCU, i.e. , it gets cooled down by the incoming water. This time the water leaving the FCU through the outlet pipe, must get heated up in the FCU. This relatively hot water is redirected to the Chiller and in due time the room is colder. This sums up the working of a 2-Pipe Fan Coil System.
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very good article about fan coil units and 2 pipe vs 4 pipe systems