Causes of vent dew and effective ways to fix it

Inlet dew is a common condition in air conditioning systems, when water vapor condenses at the louver surface, often appearing in slit-shaped louvers. Significantly affect the quality of the work, cause moldy plaster ceiling and cause discomfort to users. To know the causes and how to fix it, please follow the following article of the company P69 !

What is the phenomenon of vent dew?

Dew point occurs when the temperature difference between the low-temperature material surface and the higher-temperature space reaches the dew point, which is determined based on the parameters on the humid air graph.

What is the phenomenon of vent dew?
What is the phenomenon of vent dew?

In the field of air conditioning and ventilation, the problem of fogging and how to fix it is always the most shared and interested topic. Dew condensation not only causes unsightly but also affects the architecture of the building such as water seepage, rotting the decorative plaster ceiling, shorting electrical equipment and damaging the interior below.

Causes of vent dew

The typical environment of Vietnam is tropical climate: hot and humid, temperature fluctuations during the day can be very large. At the location of low-temperature air-conditioning vents, when exposed to humid air in the room, if the temperature is lower than the dew point temperature, the humid air will turn into dew and cause dew condensation at the vents. This phenomenon often occurs with the first operation of the ventilation system, or when the weather changes.

There are also the following basic reasons:

1. Due to indoor unit

  • Throttle valve is broken, or clogged
  • The indoor unit fan is weak, not enough air flow
  • Condensate pump weak, or damaged
  • Heat exchanger indoor unit, dust return filter mesh

2. Due to construction techniques

Insulation is not properly applied, the door box is not insulated, the thickness of the layer is not correct

The cold air duct is open, the flexible duct is torn or bent, causing the air flow to decrease

Too long hard and soft wind pipes create too much resistance, making the wind speed out at the door weak

The condensate line does not ensure the slope, the condensate does not drain properly, overflows the trough, returns to the coil and is blown down the vent.

In indoor units with high pressure head and fan capacity, more condensate traps should be arranged

3. By design

Calculation of the flow distribution on the pipeline, the outlet at the vent positions of the AHU, FCU is not accurate

The main air duct, branch pipe, flexible pipe to the air outlet are arranged too far, the indoor unit pressure head is not enough

The arrangement of the number of air vents is not suitable in terms of quantity and location. Avoid placing air vents in high temperature areas: near glass doors, avoid hot air circulation: atrium locations, main doors often come in and out…

The calculation and design of the ducts are not suitable: size, length, branching… leading to too high or too low wind speed.

Normally, we calculate the wind speed in the pipeline as follows:

  • Main pipeline wind speed is: 11m/s
  • The return pipe wind speed is: 6m/s
  • The wind speed at the louver, the wind mouth is: 2.5m/s
  • In flexible ductwork is: 3.5m/s

The selection of cooling capacity is lacking, the outlet temperature at the vent is usually from 10°C to 15°C because there is not enough cooling capacity for the room space, so the temperature difference at the surface of the louver and the surrounding air is very big

4. Improper arrangement of cold air outlet

When designing, it is necessary to arrange the air vent to avoid placing too close to the glass door or the glass wall that is directly exposed to the sun outside, which will cause a temperature difference that is easy to dew on the glass wall and the wind mouth.

5. Due to technical insuring of cold air duct insulation

– When constructing duct insulation, cold air outlet should be carefully calculated to avoid dew condensation. Technical characteristics of the insulation layer should be checked such as thickness, density,  and thermal conductivity coefficient of materials .

– It should be noted that outdoor and indoor installation spaces in air-conditioned spaces will have different insulation thicknesses.

6. Design without cooling load

– Lack of cooling load is one of the frequent causes of dew at the mouth of the wind.

– The air outlet temperature is usually from 10 °C to 14 °C, so after using the air conditioner, the room temperature does not decrease, there will be a phenomenon of dew at the mouth, to overcome it is necessary to determine the air-conditioning space, calculate suitable load, for special spaces, not based on experience, but must calculate the design by specialized software to calculate the design.

– Design of water trap at indoor units with large fan head is not technical, leading to condensate being sucked into the unit.

How to fix the phenomenon of vent dew.

How to fix the phenomenon of vent dew.
How to fix the phenomenon of vent dew.

It must be affirmed that it is impossible to completely solve the problem of dew. However, solutions can be used to limit the accumulation of dew at the mouth of the wind. That solution depends on the use of the project and investment costs.

As mentioned in the basic causes above, we have 3 main methods to deal with the root:

1. Increase the air outlet surface temperature

For some designs, there is a small resistance at the vent surface to warm the surface above the dew point of the air state in the conditioned space.

2. Isolate the surface causing dew

This is the treatment method commonly used by operational engineers. By increasing the wind velocity at the vent, we can drive away the surrounding moist air layers and limit their contact with the vent surface.

However, this method can only be used in a very small number of cases. Because the increased wind speed at the mouth will cause noise and when the ambient humidity is too high, this is not effective and can also cause additional energy consumption in the fan. ☞ Fix dew in the mouth of the wind.

3. Use dehumidification processes

The dehumidifier can be used directly or the dehumidifier (the dehumidifier can be used separately or in the same room as the air conditioner).

4. Design Solution

– For constructions, it is absolutely required not to drop water on objects inside. The system of ducts and louvers must be designed to go underground. The direction of blowing for diffusion as well as heat recovery is upward or horizontal. The fresh air duct system must undergo dehumidification treatment. The calculation of wind speed in the ductwork, at the supply and return air intakes must also be calculated and adjusted accordingly.

– With works with lower requirements. It is possible to arrange ventilation systems, air vents on the ceiling and downwards. With this system, it will help reduce investment costs to match the budget.

5. Operation Solution

For projects that have been put into operation but still have the above problems, solutions for system operation can be used to limit them.

– Check the wind speed at the air inlets, the wind speed is too small “Output air is too cold” ⇒ It will cause long-term heat accumulation at the location around the air outlet. Exacerbation of dew. Then increase the speed of the air conditioner fan. In addition, check the air ducts for blockages or distortions.

– Check the air valve to see if the opening has reached. If the valve opening is too small, it can also obstruct the air flow, affecting the wind speed.

– Check doors and windows. If it is not closed, it must be closed completely to isolate the environment inside and outside the building.

– Check the flow of fresh air, if the amount of fresh air is too large. It is possible to adjust the fan rotation speed or close the air flow control valves

Contact us today for a free consultation and answer at:

Contact information P69 – M&E Contractor

Address: No. 6/165C Xuan Thuy, Cau Giay District, Hanoi


Hotline: 02422121212 – 0965937799




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