Comprehensive Understanding of Fan - Coil Units

1. System Structure
   The FCU is composed of a fan, a coil, and a casing.
   The FC allows the indoor return air to directly enter the unit for cooling, dehumidification, or heating treatment.
   The water pipes connected to the FC are:
   (1) Supply water pipe
   (2) Return water pipe
   (3) Condensate water pipe
   Thought: What is the spatial position of the three water pipes?
   An automatic air - vent valve is installed at the highest point of the supply and return water pipes, and it is also required to be installed at local highest points.

1. Fresh air infiltrates naturally due to the negative pressure formed by the exhaust in indoor bathrooms, etc.
   Advantages: Low initial investment and operating costs.
   Disadvantages: Poor indoor hygiene conditions and uneven indoor temperature field.
   Application: Places with few people and low requirements.
2. Fresh air is introduced through a wall hole and directly enters the unit. This method can be used if the fan - coil unit is installed against the exterior wall.
   Although the fresh air volume can be guaranteed, it is troublesome to manage. The fresh air inlet will damage the building facade and increase pollution and noise.
   Application: Buildings with low requirements.
   For energy - saving purposes, a fan - coil unit with a total heat exchanger can be installed. There should be two air inlets for fresh air and exhaust on the exterior wall.
3. Fresh air is supplied by an independent fresh air system. This solution improves the flexibility of system regulation and operation, and the supply water temperature to the fan - coil unit can be appropriately increased, and the dew - condensation phenomenon of the water pipes can be improved. It is the most widely used method.

1. Explanation
   Since the chiller has been provided with optimized control by the intelligent controller of the equipment supplier, this system only considers monitoring and simple control of the chiller.
2. The following functions can be achieved
   (1) Linked start - up sequence: Cooling tower fan - Cooling tower electric butterfly valve - Condenser of the chiller - Electric butterfly valve - Cooling water pump - Evaporator of the chilled water - Electric butterfly valve - Chilled water pump - Chiller.
   (2) Linked shutdown sequence: Chiller (with a 5 - minute delay) - Chilled water pump - Electric butterfly valve of the condenser of the chiller - Cooling water pump - Electric butterfly valve of the evaporator of the chilled water - Cooling tower electric butterfly valve - Cooling tower fan.
   (3) Differential pressure bypass monitoring: Pressure sensors are installed on the main inlet and main return water pipes. By calculating the pressure difference between the supply and return water, comparing the pressure difference with the set value, and using the PI control method to adjust the electric two - way valve, the pressure difference is maintained within the set range. When the chiller system stops, the bypass valve is fully closed.
   (4) Unit optimization control: According to the total load of the cold source system (temperature difference between chilled water return and supply × total flow), the number of operating chillers is controlled. The number of units is matched with the load to achieve the optimal start - stop time control of the chiller. According to the change of the temperature of the supply/return water collection tank, the change of the system heat load is calculated through a specific algorithm, and the number of operating cold and heat sources is adjusted according to its change to achieve the purpose of optimized energy - saving.
   (5) Cooling tower optimization control: Detect the actual load on the load side and automatically turn on the number of cooling towers in groups to achieve the purpose of energy - saving.
   (6) Balanced operation control: Automatically accumulate the operating time of each chiller, and preferentially start the unit with relatively less operating time, so that the operating time of each unit is approximately equal.

   

   
   (7) Display: Display the operating status and fault status of the chiller, water pump, and cooling tower, as well as the status of the electric butterfly valve and water flow switch.
   (8) Record: The central station has a color dynamic graphic display, recording various statuses and alarms; recording start - stop times; accumulating operating time and its historical data and other parameters.
   (9) Timing central monitoring: For various temperatures, equipment operating status and alarms in the system, and the start - stop of various equipment, time - running programs such as holidays and working hours can be compiled. Equipment is reasonably operated in different time periods to save energy.
   II.

Fan - Coil Unit

Control Schematic Diagram of Fan - Coil Unit for Combined Cooling/Heating

III. Fan - Coil Unit with Fresh Air System

1. Fresh Air System

2. Working Principle of Fan - Coil Unit

• Simple control of fan - coil unit: Use a three - speed switch to directly and manually control the three - speed conversion and start - stop of the fan.
• Temperature control of fan - coil unit: Use a thermostat to compare and calculate the set temperature and the actual detected temperature, and automatically control the opening and closing of the electric two - / three - way valve, the three - speed conversion of the fan, or directly control the three - speed conversion and start - stop of the fan, so as to achieve constant temperature by controlling the water flow or air volume of the system.
  (2) Composition of Fan - Coil Unit
  The fan - coil unit is mainly composed of a fan, a heat - exchange coil, and a casing. According to the external static pressure of the fan - coil unit, it can be divided into standard type and high - static - pressure type. According to the number of rows of the heat - exchange coil, it can be divided into two - row and three - row types. The heat - exchange coil generally uses copper tubes with aluminum fins. The outer diameter of the copper tubes is 10 - 16mm, the fin thickness is about 0.15 - 0.2mm, and the spacing is 2.0 - 3.0mm. The fan generally uses a double - inlet forward - curved blade centrifugal fan.

  

  The motor is a capacitor - type 4 - pole single - phase motor with three - speed. The casing and the condensate water pan are insulated.
  (3) Working Principle of Fan - Coil Unit Air - Conditioning System
  With the help of the fan - coil unit, the indoor air is continuously circulated, passing through the coil to be cooled or heated, so as to maintain the required room temperature and a certain relative humidity. The chilled water or hot water used in the coil is supplied by the centralized cold source and heat source. At the same time, the fresh air processed centrally by the fresh air air - conditioning machine room is sent into each air - conditioned room through special fresh air ducts to meet the hygiene requirements of the air - conditioned rooms.
  Compared with the centralized system, the fan - coil unit air - conditioning system has no large air ducts, only water pipes and smaller fresh air ducts. It has the advantages of convenient layout and installation, small occupation of building space, and good individual adjustment. It is widely used in comfort air - conditioning systems where the requirements for temperature and humidity accuracy are not high, there are many rooms, the rooms are small, and individual control is required.
  The working principle of the fan - coil unit is not as complex as that of the central air - conditioning system. In fact, we can vividly regard the fan - coil unit as an electric fan, except that the air blown out by this fan is at the temperature we need. There are many fan - coil units on the market. To save costs, many companies will choose domestic fan - coil units and import central air - conditioning hosts, which does not affect the operation and use effect of the entire central air - conditioning system.

• Advantage 1: Flexible control, with the superiority of individual control. It can flexibly adjust the temperature of each room, and the start - stop of the fan - coil unit can be determined according to the usage status of the room.
• Advantage 2: The fan - coil unit is small in size, occupies little space, and is convenient for layout and installation. It is even suitable for the renovation of existing buildings.
• Advantage 3: It is easy to achieve system zoning control. The cooling and heating loads can be divided into several regional systems according to the room orientation, use purpose, use time, etc., to implement zoning control.

• Disadvantage 1: Due to the decentralized installation of the units and the large number of units, the maintenance and management workload is large.
• Disadvantage 2: The indoor air quality is relatively poor. It is difficult to carry out secondary filtration, and there is a high risk of condensate water seepage on the ceiling.
• Disadvantage 3: It is difficult to solve the fresh air volume problem with the fan - coil unit itself. Because the static pressure of the unit fan is small, the air distribution is limited, and it is suitable for rooms with a depth of less than 6 meters.

4. Explanation
   As the terminal device of the central air - conditioning system, the fan - coil unit is widely used in many public places. Its main characteristics are as follows:
   (1) Self - contained unit with flexible adjustment
   The fan - coil unit has three - speed adjustment, and the water - way system can adjust the temperature regulating valve of cold and hot water automatically according to the user - set room temperature. Thus, each room can adjust the room temperature independently to meet the needs of different air - conditioning users. When a room is unoccupied, it can be manually shut down or automatically shut down at a fixed time. This also enables developers to avoid excessive initial investment, which is convenient for their rolling development. They can turn on different rooms according to the occupancy of customers, thereby reducing the overall operating cost of the system.
   The entire system is relatively easy to control in zones. It can be divided into several zones according to the orientation, floor, use, and usage time of the rooms, and zonal control can be carried out according to the different usage needs of customers, avoiding the unreasonable aspect of centralized control that the large - air - duct system must adopt.
   (2) Small size, flexible layout, convenient installation, and less space occupation
   The fan - coil unit is small in size, flexible in layout, convenient for installation, and occupies less building space, which is convenient for coordinating with interior decoration construction. However, when choosing a better fan - coil unit for practical projects according to the different needs of owners and in combination with design drawings, the following points should be fully considered:

1. Cooling capacity verification
   Generally, products are selected according to the calculated cooling load. However, it should be noted that different fresh - air supply methods will lead to different cooling loads of the fan - coil unit. When fresh air is directly sent to the room through the exterior wall without heat - moisture treatment, the cooling capacity of the fan - coil unit = indoor cooling load + fresh - air cooling load; when an independent fresh - air system is set up, the cooling capacity of the fan - coil unit = indoor cooling load. The products on the market are generally marked with nominal cooling capacity, while the actual cooling capacity during operation should be the cooling capacity × the average operating time per unit time. That is, changing the operating time or air volume will affect the cooling capacity input by the unit. Therefore, it is not that the higher the nominal cooling capacity, the better. If the unit is selected only based on high cooling capacity, there will be excessive cooling capacity, resulting in a too - low startup rate, a decrease in the number of air changes, an increase in the room - temperature gradient, an increase in the system capacity and equipment investment, an increase in air - conditioning energy consumption, and a reduction in the air - conditioning effect. So, the cooling capacity is only one of the necessary conditions for selecting equipment, and other factors should also be considered.
2. Air - volume verification
   The number of air changes is mainly verified according to the room - quality requirements. The smaller the supply - air temperature difference and the more the number of air changes, the better the air quality and the more comfortable it is. The reason why some air - conditioned rooms have odors and feel stuffy is that the air - volume verification is not properly handled. Since the nominal air volume of the fan - coil unit is measured under the working condition of no water flow and zero air - inlet and outlet pressure difference, there are some unrealistic factors. Therefore, when actually determining the air volume, the air - volume value in this ideal state should be deducted. Through empirical calculation, this additional air volume should account for 20 - 30% of the nominal air volume.
3. Supply and return air methods
   The supply and return air methods form the so - called air distribution. Whether it is reasonable or not directly affects the uniformity and stability of the temperature field and velocity field in the air - conditioned room, that is, the quality of the air - conditioning effect. A reasonable air distribution requires a certain supply - air velocity to avoid short - circuit of the air flow and ensure a certain jet length. The wind speed depends on factors such as the external static pressure of the unit, the air supply volume, and the air supply outlet. If the external static pressure of the unit is too low, the air volume will decrease, the range will be reduced, and the room will have uneven cold and heat. There is a large difference between the designed air distribution and the actual operating state in the curve diagram. Therefore, according to the actual building layout, room structure, depth, height, etc., the fan - coil unit model should be selected with reference to the medium - grade air - volume and wind - speed indicators. At present, the external static pressure values of fan - coil units from various manufacturers on the market are not uniformly calibrated, and there are large differences. In addition, some construction sites use horizontal concealed - mounted units, with short external air ducts, filters, and large resistance values of the air - inlet and return grilles. Therefore, in actual ordering, the external static pressure value is determined to be 30Pa, and for some rooms, units with an external static pressure value of 50Pa are even selected, which is greater than the conventional resistance value of about 20Pa. Thus, a good uniform field is ensured in actual operation, and the expected air - conditioning effect is achieved.

1. The fan has noise.
2. The fan cannot start or runs very slowly.
3. The air - conditioning effect is poor or there is no air - conditioning effect.
4. The air - conditioner has an odor.
5. The air - conditioner leaks water.

1. Temperature switch damage
   Use a test pen to measure whether there is electricity at the input end of the temperature control switch and the fan output terminal. If the temperature switch is judged to be damaged, the temperature switch can be replaced or repaired.
2. Slow running speed
   a. Feel whether the surface temperature of the fan is normal.
   b. After stopping the machine, manually rotate the fan blade to feel whether the rotation is flexible. If there is resistance, replace the fan bearing.
   c. If the manual rotation is normal, replace the starting capacitor.
   d. Measure the resistance of the motor coil. If it is abnormal, replace the motor.

1. Open the coil exhaust valve and check whether the system circulating water temperature is normal.
2. Check the inlet and outlet temperatures:
   a. If the temperature difference is very small, open the coil exhaust valve and check whether the water temperature is normal. If the water temperature is normal, then check whether the two - way valve is open. If the two - way valve is not open, continue to check whether the power supply to the two - way valve is normal and check whether the Y - type filter is blocked.
   b. If the inlet - outlet temperature difference is normal, check whether the room insulation is normal, such as whether the doors and windows are closed. If the doors and windows are not closed, after air convection, the air - conditioner will have no effect. Good communication and explanation should be made with the customer.
   c. If the inlet - outlet temperature difference is large and the air volume at the air outlet is small, check whether there is dust on the air - inlet filter and check whether the fan speed is normal.

1. Check and clean the air - inlet filter.
2. Check whether there is dust on the coil fins. If there is dust, an appropriate cleaning plan should be implemented.
3. Check whether there are foreign objects, dust, or water accumulation in the air duct, and clean it to remove the odor.
4. Check whether there are foreign objects in the water - collecting pan.

1. Poor drainage
   a. Check whether the water - saving pan is blocked.
   b. Check whether the drain pipe is blocked.
   c. Check whether the slope of the drainage pipe is reasonable.
2. Insulation layer falling off
   Restore the insulation layer.
3. Exhaust valve leaking water
   Tighten the exhaust valve.
4. Hoses, valves, and pipe fittings leaking water
   Close the main valve and then replace them.

1. Open the drain valve to drain and relieve pressure.
2. Turn off the air - conditioning main unit, the expansion tank make - up water valve, the booster pump, and the circulation pump.
3. Close the main valve of the burst - pipe area.
4. Cut off the power supply of the burst - pipe area.
5. When it endangers the elevator, turn off the elevator power supply and try to stop the elevator at a floor higher than the burst - pipe floor.
6. Move important materials in time and clean up the water accumulation.