Common mistakes in specifying dedicated outdoor air units (DOAS) in humid climates

Introducing outdoor air into buildings is not just a technical recommendation, it’s a requirement to ensure occupant health and well-being. Various international standards, including ASHRAE, WHO, and numerous national and local codes, mandate proper ventilation in all occupied areas: offices, hotel rooms, kitchens, event spaces, gyms, and any enclosed area where people spend time.

In hot and humid climates, such as many regions across the Caribbean and Latin America, proper specification of Dedicated Outdoor Air Systems (DOAS) is essential. However, several recurring mistakes still impact comfort, energy use, and equipment longevity. Below are the most common:

1. Using incorrect climate design conditions

A frequent mistake is selecting DOAS units based on total cooling or evaporative conditions, rather than the dehumidification conditions recommended by ASHRAE (see ASHRAE Handbook – Fundamentals, chapter on “Climatic Design Information”).

In areas like Cancún, Barranquilla, or Punta Mita, latent load (humidity) far exceeds the sensible load. If units are not designed to meet this challenge, they fail to control indoor relative humidity—leading to odors, mold, damage to finishes, and occupant complaints.

It’s also important to note that the dew point peak doesn’t necessarily occur during the hottest part of the day. Early mornings can have higher humidity levels, resulting in a higher dew point despite a lower dry-bulb temperature.

Recommendation: Always use the most critical humidity conditions for the project location (refer to dehumidification design condition tables) and, when applicable, cross-reference with sources like CIBSE, EN 16798, or the WELL Building Standard.

2. Missing or undersized reheat system

Many designs either omit or undersize the reheat system. DOAS units cool the air below the dew point (12–14 °C) to remove moisture, but this cold air must be reheated before entering the occupied space.

Without reheat, the dehumidified fresh air will overcool the space, causing visible condensation on grilles, diffusers, and glass windows. This results in thermal discomfort for guests, potential damage to materials and finishes, failures in sensitive electronic systems, and over time, will cause musty odors in the room.

Recommendation: Avoid using electric resistance heaters due to their high energy consumption, especially in humid climates. Instead, use recovered heat sources such as:

• Hot gas reheat or variable speed compressors in DX systems
• Wrap-around heat pipes in chilled water systems

Be cautious with “energy-saving” technologies that are not suited for tropical climates. For example:

• HRV systems do not remove moisture
• Enthalpy wheels should not be used as a reheat substitute in regions like the Caribbean
• Desiccant wheels are often too costly and complex for comfort applications

3. Not eliminating latent load from indoor units

A properly designed DOAS should handle 100% of the building’s latent load. However, many projects specify DOAS units that only handle part of the humidity, leaving the remainder to indoor units (fan coils, VRF, etc.).

This is especially problematic in DX systems, where indoor units are not optimized for dehumidification (SHR > 70). When forced to do so, they operate at very low evaporating temperatures, which reduces efficiency, increases energy use, and results in poor humidity control and frequent cycling.

In chilled water systems, it’s easier to separate sensible and latent loads by using dedicated coils and adjusting water temperature.

Recommendation: Make sure the DOAS supplies air with a dew point lower than the target dew point in the space (e.g., < 12.9 °C to maintain 24 °C at 50% RH). This allows indoor units to be designed for sensible load only—lowering size, cost, and energy consumption, especially in DX systems.

4. Chillers operating above desired dew point

Many chilled water plants in hotels and residential buildings are designed to operate with 6.7 °C supply and 12.2 °C return water. This is not sufficient for effective dehumidification when it is needed at terminal units or through the DOAS.

If the target dew point in the space is 12.9 °C (to maintain 24 °C at 50% RH), but the return water is above that, the coils will not reach a surface temperature low enough to condense water vapor. The result: high indoor humidity, discomfort, odors, surface condensation, and—in severe cases—mold growth.

Recommendation: Consider configuring colder chilled water temperatures for the DOAS, even if that means using a separate loop or mixing valve to protect other, more sensitive units.

5. Lack of coordination between architecture and HVAC systems

In hotels, residences, or resorts, it’s common to find that outdoor air ducts don’t reach every space because the architecture didn’t account for them. This leads to improvised solutions: exposed ducts, ceiling modifications, uninstalled equipment, or rooms without proper ventilation.

Recommendation: Integrate DOAS planning with architecture, interior design, and structure from early design stages.

Conclusion

Dedicated Outdoor Air Systems (DOAS) are essential for comfort, health, and energy efficiency in humid climates. Proper specification should address:

• Dehumidification design conditions
• Adequate reheat strategy
• Sensible-only load for indoor units
• Early coordination with architectural design

Doing this helps avoid costly operational issues and maintenance problems.

References

• ASHRAE Handbook – Fundamentals (Chapter: Climatic Design Information)
• ASHRAE Standard 62.1 – Ventilation for Acceptable Indoor Air Quality
• ASHRAE Standard 90.1 – Energy Requirements
• ASHRAE Technical Feature: Wrap-Around Heat Pipe in Humid Climates
• CIBSE Guide A – Environmental Design
• EN 16798-1:2019 – Indoor environmental input parameters for energy performance design
• LEED v4.1 BD+C – Indoor Environmental Quality Credits

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