Fan coil units explained, with fault finding
A fan coil unit is mainly a fan, a coil and a casing. By external static pressure it is standard or high-static; by coil rows it is two-row or three-row. The coil is usually copper tube with aluminium fins, tube outer diameter 10 to 16 mm, fin thickness about 0.15 to 0.2 mm, pitch 2.0 to 3.0 mm. The fan is a double-inlet forward-curved centrifugal fan driven by a capacitor 4-pole single-phase motor with three speeds; the casing and drain pan are insulated.
The unit keeps circulating room air through the coil to cool or heat it, holding the required temperature and relative humidity. The chilled or hot water comes from a central source; fresh air processed by the fresh-air plant is delivered through dedicated ducts to each room for hygiene.
Compared with central systems, the fan-coil plus fresh-air system has no large ducts, only water pipes and small fresh-air ducts, so it is easy to lay out and install, takes little space and allows individual control. It is widely used where precision is modest, rooms are many and small, and separate control is wanted.
A fan coil is simple: think of it as a fan that blows air at the temperature you need. Structure. Fan: a low-noise induction motor drives it; input voltage sets the speed in high, medium and low, controlled by a switch, setting the cooling or heating capacity. The fan is the air mover and the disturbance that strengthens external convection at the coil, and with the motor it is the main noise source. Coil: a finned-tube air to water heat exchanger. Chilled or hot water flows inside; because its temperature is below the dew point, condensate forms on the tubes - a wet surface that exchanges both heat and mass, raising effectiveness. The coil carries most or all of the room load, usually 3 to 4 rows. Drain pan: sits under the coil to collect condensate and discharge it outside. Air filter: sits under the coil to catch dust from the return air.
Working principle and cooling cycle. The unit has a water circuit and an air circuit. The water circuit carries chilled or hot water from the central plant through the coil. The air circuit draws room air through the return, across the coil, where it exchanges heat with the water, cools and dehumidifies, then supplies it back. Central air conditioning is essentially heat transfer; in cooling, heat moves from the room to the refrigerant and out.
Fan-coil plus fresh-air: pros and cons. This system pairs a central fan coil with a fresh-air system; the fan coil is the terminal, the fresh-air system covers the ventilation load. It is a main form of water-system air conditioning and common in civilian buildings. Pros: 1. Flexible control - adjust each room temperature by use. 2. Small size, easy layout and install, even in retrofits. 3. Easy zoned control by orientation, use and time. Cons: 1. Units are scattered and many, so maintenance is heavy. 2. Indoor air quality is poorer; secondary filtration is hard and condensate leaks can occur. 3. Fresh air is hard to add because the fan static is low and throw is limited; it suits rooms with depth under 6 m.
Selection points. 1. Cooling check: nameplate capacity is at full run; real capacity is capacity times average run time. Higher nameplate is not better - oversizing lowers run rate and air changes, raises cost and hurts effect, so capacity is only one factor. 2. Airflow check: base it on air changes for room quality; lower supply difference and more changes mean better comfort. Nameplate airflow is measured with no water and zero pressure difference, so deduct 20 to 30% for real airflow. 3. Supply or return: form the air pattern; low external static cuts throw and leaves rooms uneven, so pick the model by mid-range airflow and speed against the room shape and depth. 4. Other: keep noise below 40 dB, adding a silencer with resistance under 10 Pa; watch insulation, condensate slope and joints; choose a vertical water system where it helps condensate drainage and floor height.
Control. Simple control uses a three-speed switch for the fan. Temperature control uses a thermostat that compares set and actual temperature and opens or closes a two- or three-way valve and sets the fan speed, holding the temperature by water flow or airflow.
Classification and parameters. By form: concealed horizontal, exposed horizontal, concealed vertical, exposed vertical, cassette. By thickness: slim, standard. By condensate pump: standard, deluxe. By static: 0 Pa, 12 Pa, 30 Pa, 50 Pa, 80 Pa. By rows: two-row, three-row. By circuit: two-pipe, four-pipe. Rows mean copper-tube rows in the coil: two rows = 2 by 8 = 16 tubes; three rows = 3 by 8 = 24. More tubes, better cooling. Two-pipe: chilled water in summer, hot in winter. Four-pipe: both at once, so one room cools while another heats. Parameter effects: each 1°C rise in supply temperature cuts cooling about 10% and dehumidification. At fixed supply, lower airflow cuts capacity and raises enthalpy difference with little change in fan power. A wider supply-return difference cuts flow and the coil coefficient, so capacity falls; at 7°C supply, raising the difference from 5°C to 7°C cuts capacity about 17%.
Common faults. Noise: 1. bearing damage - replace bearing; 2. rub against ceiling - adjust hanger nuts or the diffuser; 3. air in pipes - bleed at the coil, floor and terminal vents. Won start or slow: 1. broken thermostat - test with a tester and replace; 2. slow: feel the motor temperature, spin the blades by hand, replace the bearing if stiff, replace the start capacitor, or measure motor windings and replace the motor. Poor or no cooling: open the coil vent and check water temperature; check inlet-outlet difference - if small, check the two-way valve and Y-strainer; if normal, check room insulation and closed doors or windows; if large with low flow, check the filter and fan speed. Odour: clean the inlet filter and coil fins, clear the duct of dust and water, and check the drain pan. Leak: poor drainage - clear the pan and pipe and fix the slope; restore lost insulation; tighten the bleed valve; close the total valve before replacing a soft hose, valve or fitting. Burst pipe emergency: open the drain to relieve pressure; stop the host, expansion-tank make-up pump and circulation pump; close the zone total valve and power; if near a lift, stop it above the burst floor; move valuables and clear the water.