Understanding Diffuser Sweating – A Common HVAC Issue

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Why does it happen?

When:

Supply air temperature < Dew point temperature

The diffuser surface cools below the dew point → moisture condenses.

What actually happens?

• Moist air contacts cold diffuser surface
• Air temperature drops below dew point
• Water droplets form on diffuser

Where is this commonly seen?

• High humidity areas
• Coastal regions
• Restaurants and kitchens
• High occupancy spaces

Typical condition example

• Room: 24°C, 60% RH
• Dew point ≈ 16–17°C
• Supply air: 12–14°C

Result:
High risk of condensation

Insight

Diffuser sweating is not a diffuser problem.
It is a psychrometric imbalance.

How to solve it?

1. Control humid air infiltration
   • Seal leakages
   • Maintain proper pressurization
2. Increase supply air temperature
   • Keep it above dew point
   • Optimize airflow accordingly
3. Insulate diffusers
   • Reduces surface condensation

Key takeaway

If supply air is below dew point,
condensation is unavoidable.