Outdoor air intake placement and design

The outdoor air intake is where a building's ventilation begins, and every contaminant, snowflake or raindrop that enters it ends up in the air handler or the occupied space. Intakes are often placed late in design, wherever there is room left on the roof or wall, and the consequences show up for decades: odour complaints, wet filters, frozen coils and snow-packed plenums. Careful placement and detailing cost little at design and save a great deal in operation.

This guide covers intake location, separation distances, weather protection, louvre selection and other details that matter in Canadian climates.

Location principles

Keep away from contaminant sources

Intakes should be located away from anything that could contaminate the air. ASHRAE Standard 62.1 includes a table of minimum separation distances from intakes to common sources. Values in recent editions include roughly:

These are abbreviated, approximate values; consult the edition of 62.1 referenced by your project and any provincial or municipal requirements. CSA B149.1 has separate termination clearances for gas appliance vents, and NFPA 96 sets distances for kitchen exhaust. The distances are minimums: wind patterns around buildings can carry exhaust well beyond them. See odour control and exhaust re-entrainment.

Consider prevailing wind and building aerodynamics

Where possible, put intakes upwind of exhausts relative to prevailing winds, on a different face of the building, or at a different elevation. Avoid placing roof intakes inside recirculation zones near the downwind side of penthouses and screens, where exhaust from nearby stacks can collect.

Avoid ground-level hazards

Intakes at grade are exposed to vehicle exhaust, landscaping chemicals, dust, debris, and deliberate tampering. Where intakes must be low, raise them as high as practical, avoid locations near loading docks, drop-off zones and parking, and consider security measures.

Plan for snow

In much of Canada, snow is the dominant design issue. Intakes should be located above expected snow accumulation, including drifting against walls, parapets and rooftop equipment. The bottom of the louvre should be well above the roof or grade surface; roof snow loads in the NBC give an indication of potential depth, and drifts near parapets and level changes can be far deeper than general accumulation. Avoid locating intakes on the leeward side of tall elements where drifts form.

Weather protection

Rain

Wind-driven rain enters intakes at louvres with high face velocity. Louvre airflow, pressure drop and water penetration performance are tested under AMCA Standard 500-L, including a wind-driven rain test, and licensed ratings are published under the AMCA 511 certified ratings program. A common design guideline is to keep the velocity through the louvre's free area at or below the beginning point of water penetration published for that model, often in the range of 2.5 to 4 m/s (500 to 800 fpm) for standard drainable louvres. Wind-driven rain louvres perform better at higher velocities.

Snow

Fine, dry snow is drawn through most louvres. Strategies include:

• Lower intake face velocities, which reduce snow entrainment
• Snow hoods or intake hoods that turn the air upward or downward before entry
• Larger plenums behind louvres that allow snow to drop out before reaching filters
• Drained plenum floors so melting snow does not reach the air handler
• Heated or sloped plenum bottoms in severe locations

Wet filters from snow intrusion can freeze, collapse or bypass. Air handler outdoor air sections should be designed to drain.

Ice and hoarfrost

Moist exhaust air drawn back into intakes can freeze on louvres and screens in cold weather, restricting airflow. Separating exhaust from intakes helps, as does avoiding fine bird screen where icing is likely, within code and pest control requirements.

Louvre and plenum sizing

1. Determine the maximum outdoor airflow, usually at full economizer, not just minimum ventilation.
2. Select a louvre type based on exposure: drainable, wind-driven rain or acoustic.
3. Size the louvre so free-area velocity stays below the model's water penetration point and below any snow velocity limit set by the designer.
4. Account for bird or insect screens, which add pressure drop and reduce effective area.
5. Provide a plenum or transition that distributes air evenly across downstream dampers and filters.
6. Include access doors to inspect and clean the plenum, and drainage at its low point.

Pressure drop through the louvre and plenum should be included in fan static calculations.

Freeze protection and mixing

Outdoor air entering an air handler at -30 °C (-22 °F) or colder can freeze coils and trip freezestats if it is not mixed properly or preheated. Intake design affects this: uneven airflow across the outdoor air damper, or outdoor air entering on one side of the mixing box, leads to stratification. Preheat coils, blenders and correct damper arrangements address it. See mixing boxes, stratification and freeze protection.

Airflow measurement and controls

Measuring outdoor airflow is essential for confirming ventilation rates. Airflow measuring stations installed in the outdoor air path, or integrated louvre-damper measurement assemblies, need straight approach lengths or flow conditioning to read accurately. Low velocity at minimum outdoor air conditions often falls below sensor accuracy limits, so a separate minimum outdoor air damper and measurement section is common. Measured airflow should be verified by the balancing contractor during air balancing.

Security and safety

Intakes accessible from grade or adjacent roofs can be used to introduce harmful substances deliberately. For high-risk buildings, relocate intakes to secure elevated locations, restrict roof access and monitor intake areas. Intakes should also be located so that maintenance staff can reach louvres, screens and plenums safely.

Key takeaways

• Locate intakes away from exhausts, vents, loading docks, traffic and cooling towers, using ASHRAE 62.1 separation distances as minimums.
• Consider wind patterns, rooftop recirculation zones and snow drifting, not just plan distances.
• Size louvres to stay below water penetration velocity and use hoods and drained plenums to control snow.
• Size for maximum economizer airflow and include louvre pressure drop in fan calculations.
• Provide outdoor airflow measurement, freeze protection, access and security appropriate to the building.