Fan coil units(FCUs) are widely used in central air conditioning systems in hotels, shopping malls, and other buildings. However, as the years go by, the number of complaints about the quality of fan coil projects increases. Designers often focus on the thermal performance of the units when selecting models, neglecting the condensation issue. In fact, once condensation occurs, the consequences can be severe. It can damage interior decorations, lead to dust accumulation, and promote mold growth, which negatively impacts both the environment and health. Therefore, designers must actively address the condensation problem to ensure the unit’s overall performance is fully optimized.

There are two causes of condensation problems: primary and secondary condensation.

Primary condensation

Fan coil units(FCUs) are generally equipped with condensate pans. The water condensed by the coil cooler should fall into the pan. From there, it should be drained through the pan’s drain port into the condensate pipe for centralized drainage. However, in practice, condensate drips for various reasons:

Small condensate pan

The condensate pan under the unit is too small to fully cover the unit. The water condensed by the cooler drips outside the pan. Additionally, if no condensate pan is placed under the manual or electric valves, condensate tends to drip at the valve handle and connections.

Poor insulation of cold water pipes and connecting air ducts

When cold water pipes and connecting air ducts lack proper insulation, condensation forms on the pipe wall. This happens when the insulation material doesn’t fully contact the pipe surface. The condensed water soaks the insulation cotton and worsens the condensation. If the insulation cotton is too thin, the surface temperature drops too low, causing condensation.

The slope of the condensate pipe is not enough

An unreasonable slope in the condensate pipe prevents the condensate from being discharged promptly and smoothly. As a result, the condensate accumulates in the water pan and eventually overflows.

The supports and hangers of air ducts and water pipes create cold bridges.

The supports and hangers directly or indirectly contact the air ducts and water pipes. If insulation is not properly applied, the supports and hangers’ surface temperature drops, causing condensation when they come into contact with the air, leading to dripping.

Condensation forms on the fcu fan coil unit casing

Poor insulation of the chilled water fan coil unit itself leads to condensation on the casing surface. The national standard specifies that the thermal resistance of the insulation layer should not be less than 0.68 m²K/W, so it is important to choose the right insulation material.

The connection between the condensate pan and the cooler lacks insulation, and the screws are not insulated

The cooler’s low surface temperature, combined with insufficient insulation at the condensate pan and cooler connection, creates a cold bridge, making condensation and dripping more likely. Additionally, some fan coil screws lack insulation, further contributing to condensation issues.

To prevent the primary condensation problem, take the following measures:

• Ensure the condensate pan is large enough to catch all the condensate from the unit.
• Make sure the insulation layer of the cold water pipe and air duct fits closely to the pipe to prevent air from entering.
• Guarantee the insulation layer’s thickness to keep the surface temperature of the insulation cotton from dropping too low and forming condensation.
• Ensure the condensate pipe has a proper slope to facilitate condensate drainage.
• Insulate the air duct, water pipe supports, hangers, and other areas where “cold bridges” are likely to form.
• Insulate the fan coil units(FCUs) internally to prevent condensation on the unit shells.
• Insulate the connection between the condensate pan and the unit.
• Install the unit away from frequently opened and closed doors, open windows, or areas with potential steam.
• Increase the inlet water temperature, reduce the water flow, or increase the air volume to prevent condensation.

Secondary condensation

According to the national standard GB/T19232-2019, the condensation test is usually carried out under the conditions of an ambient dry-bulb temperature of 27°C and a wet-bulb temperature of 24°C, with the fan coil running at a low speed for 4 hours. The test results show that there should be no condensation on the surface of the box of cassette fan coil units and exposed fan coil units, and there should be no water droplets on the air outlet.

Use of inferior insulation materials for the condensate pan

The material’s density, thickness, elasticity, and other performance indicators are inadequate. Even when pressed lightly with fingers, the insulation cotton does not recover easily. When condensate collects in the pan, the low temperature of the condensate, combined with the low thermal resistance of the insulation cotton, causes condensation.

Poor treatment of the edge of the condensate pan

The edge of the condensate pan is not tilted enough, and the insulation is insufficient, leading to condensation dripping out.

Improper assembly of the fan coil units(FCUs) with the condensate pan

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Condensate pipe insulation problem

During installation, workers often focus on insulating the cold water pipes but neglect the condensate pipes. When the temperature of the condensate water is low, poor insulation of the condensate pipe causes secondary condensation, resulting in engineering accidents.

To solve the problem of secondary condensation, take the following measures:

• Eliminate inferior materials. Ensure the insulation density and thickness meet the requirements, and the thermal resistance meets the standards.
• Tilt the edge of the condensate pan inward at a certain angle.
• Insulate the outer edge of the condensate pan to prevent condensation from hanging and dripping.
• Ensure the condensate pan is tilted towards the drain port.
• Insulate the condensate pipe with sufficient thickness, and make sure the insulation material fits closely to the pipe wall to prevent air from entering.