Causes and Prevention of "Primary Condensation" and "Secondary Condensation" in Fan - Coil Units

2025-01-10

Fan - coil units are not only widely used in central air - conditioning systems in hotels, shopping malls, etc., but also their usage quantity is increasing year by year.

Causes and Prevention of

However, in actual use, the number of engineering quality complaints about fan - coil units is increasing year by year. When selecting fan - coil units, designers often only consider performance indicators such as air volume, power, cooling/heating capacity, and do not pay much attention to the condensation problem of the units. After the unit is installed and used, the consequences and impacts caused by the condensation problem are relatively serious.

Causes and Prevention of

The causes of the condensation problem are divided into two types: primary condensation and secondary condensation.

Primary Condensation
Generally, fan - coil units are equipped with condensate pans. The water condensed by the coil surface cooler should all fall into the pan, and then be collected from the drainage outlet of the pan into the condensate pipe and discharged centrally. But in practice, the condensate water drips outside for various reasons:

Small condensate pan: The area of the condensate pan under the unit cannot completely cover the unit, and the water condensed by the surface cooler will drip outside the pan. In addition, if there is no condensate pan under the manual or electric valves on the connection between the fan - coil unit and the cold water pipe, condensation water is likely to drip from the valve handle and the valve connection.
Poor insulation of cold water pipes and connecting air ducts: When the insulation of cold water pipes and connecting air ducts is not well done, and the insulation material is not closely attached to the pipe surface, once it comes into contact with the outside air, condensation will occur on the pipe wall, wetting the insulation cotton and further deteriorating the condensation. When a certain amount of water accumulates, the condensed water drips outside, causing serious consequences. If the thickness of the insulation cotton of cold water pipes and connecting air ducts is insufficient, the surface temperature of the insulation cotton will be too low, resulting in condensation.
Inadequate slope of the condensate pipe: The slope of the condensate pipe is unreasonable, and the condensate water cannot be discharged timely and smoothly.
As a result, the condensate water accumulates more and more in the pan and finally overflows.
Cold bridges formed by air duct and water pipe supports and hangers: The supports and hangers of air ducts and water pipes are in direct or indirect contact with the air ducts and water pipes. If the insulation is not properly treated, the surface temperature of the supports and hangers will be relatively low. When in contact with the air, condensation water will drip.
Condensation on the outer shell
- Condensation on the outer shell of the fan - coil unit: Due to poor insulation of the unit itself, condensation occurs on the surface of the casing.
- No heat - insulation treatment for the connection between the condensate pan and the surface cooler, and no insulation for the screws: Since the surface temperature of the surface cooler is relatively low, if the connection between the condensate pan and the surface cooler is not properly treated, a cold bridge will be formed, and it is prone to the accident of condensed water dripping outside. Some screws of the fan - coil unit are not externally insulated, which also causes the condensation problem.
- Condensation on the fan - coil unit screws

For the primary condensation problem, the following measures can be taken to prevent condensation:

The area of the condensate pan should be large enough so that all the condensate water of the unit can drip into the pan.
The insulation layer of the cold water pipe and the connecting air duct should be closely attached to the pipe to prevent air from entering.
The thickness of the insulation layer should be ensured to prevent the surface temperature of the insulation cotton from being too low and causing condensation.
The condensate pipe should have a certain slope to facilitate the discharge of condensate water.
Insulation should also be added to places where “cold bridges” are likely to be formed, such as air duct and water pipe supports and hangers.
The fan - coil unit should be internally insulated to prevent condensation on the outer shell of the unit.
A heat - insulating gasket should be added at the connection between the condensate pan and the unit.

Secondary Condensation
The condensation test conditions are an ambient dry - bulb temperature of 27°C, a wet - bulb temperature of 24°C, and the fan - coil unit operates at low speed for 4 hours. There should be no condensed water on the outer surface of the cassette - type and exposed - mounted unit casings, and no condensed water should drip from the air outlets; for concealed - mounted unit casings, no condensed water should drip outside.

Condensation on the outside of the condensate pan
- Use of inferior insulation materials for the condensate pan: The performance indicators such as the density, thickness, and elasticity of the material are not ideal. Even if gently pressed with a finger, the insulation cotton is not easy to recover. When there is condensate water in the condensate pan, due to the low temperature of the condensate water, the surface temperature of the insulation cotton with low thermal resistance is also low after heat conduction, resulting in condensation.
- Poor treatment of the edge of the condensate pan: The slope of the edge of the condensate pan is insufficient, and the insulation is poor, causing condensed water to drip outside.
- Condensation on the outside of the condensate pan
- Improper assembly of the fan - coil unit and the condensate pan: If the condensate pan is installed horizontally, it is not conducive to the smooth discharge of condensate water. If a large amount of condensate water remains in the pan, on the one hand, it may accumulate more and more and overflow; on the other hand, if the insulation of the condensate pan is poor, it is also likely to cause secondary condensation on the insulation surface and drip outside, resulting in serious consequences.
- Insulation problem of the condensate pipe: In the installation and construction of the fan - coil unit, attention is often paid to the insulation of the cold water pipe, while the insulation of the condensate pipe is ignored. When the temperature of the condensate water is low, if the insulation of the condensate pipe is poor, secondary condensation will also occur, causing engineering accidents.

For the secondary condensation problem, the following measures can be taken to prevent condensation:

Eliminate inferior materials. The insulation density and thickness should meet the requirements, and the thermal resistance should meet the standards.
The edge of the condensate pan should have a certain slope, inclining towards the inside of the pan.
The outer edge should also be insulated to prevent condensed water from hanging and dripping outside.
The condensate pan should have a certain slope towards the drainage outlet.
The insulation of the condensate pipe should have a certain thickness, and the insulation material should be closely attached to the pipe wall to prevent air from entering.