Installing air handling units—such as FAHU (Fresh
Air Handling Units), MAHU (Make-Up Air Handling Units), or outdoor AHUs—in
residential buildings isn’t just about equipment placement. It’s about ensuring
healthy indoor air quality and meeting established ventilation standards like ASHRAE
62.1-2004.
This standard helps ensure that fresh air is
delivered in the right quantity and quality to maintain a comfortable and safe
indoor environment. Let’s break down how to approach ventilation design when
installing outdoor air handling units.
Why Ventilation
Design Matters
Good ventilation removes stale air, controls indoor
pollutants, and maintains oxygen levels. Poor design, on the other hand, can
lead to discomfort, odors, and even long-term health concerns.
Key Ventilation
Requirements by Space
🏠 Living Areas
Living spaces should ideally achieve 0.35 air
changes per hour (ACH).
- For smaller
rooms, designers should ensure a minimum airflow of:
- 15 CFM (cubic
feet per minute) per person, or
- 7.5 L/s per
person
If the space has higher occupancy or features like a
fireplace, additional fresh air must be supplied to maintain air quality.
🍳 Kitchens
Kitchens generate higher levels of pollutants
(including CO₂ and cooking fumes), so ventilation requirements are higher:
- 100 CFM for
intermittent exhaust systems
- 25 CFM when
natural ventilation (like openable windows) is available
If the outdoor unit cannot provide enough airflow,
installing a mechanical exhaust system is strongly recommended.
🚿 Bathrooms and
Toilets
These spaces require dedicated ventilation to remove
moisture and odors:
- 50 CFM for
intermittent ventilation
- 20 CFM if there is
an operable window
In some cases, airflow may be supported through adjacent spaces (like living rooms) via natural infiltration, but mechanical exhaust is preferred for reliability.
🚗 Garages
Garages are a major source of pollutants such as
carbon monoxide:
- 100 CFM per
car is typically required
In many residential setups, this requirement is
partially met through natural infiltration, but proper ventilation design
should still be verified.
Conclusion
Designing a ventilation system isn’t just about
installing an AHU or FAHU—it’s about ensuring each space gets the right amount
of fresh air based on its use and occupancy.
Meeting these ventilation requirements is not only
essential for comfort and health but also critical for code compliance and
permit approvals. A well-designed system will balance mechanical ventilation
and natural airflow to achieve optimal indoor air quality.
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