Garage ventilation and CO/NO2 detection

Enclosed parking garages are found under almost every new condominium, office tower and hospital in Canadian cities. Vehicles release carbon monoxide (CO), nitrogen dioxide (NO2), particulate and fuel vapours, and in an enclosed space those contaminants can accumulate quickly, particularly in cold weather when engines idle longer and run rich during warm-up. A well-designed garage ventilation system keeps the air safe while running fans only as hard as needed.

This guide covers code requirements, system configurations, gas detection, and the testing and maintenance that keep a parkade system reliable.

Code requirements

In Canada, ventilation of enclosed storage garages is governed by Part 6 of the National Building Code of Canada and the provincial codes based on it. In general terms, the NBC requires a mechanical ventilation system that either:

• Supplies outdoor air continuously during operating hours at a prescribed rate per square metre of floor area (3.9 L/s per m2, roughly 0.77 cfm/ft2, in recent editions), or
• Is controlled by CO monitoring so that CO concentration is limited to not more than 100 ppm

Most designers choose the second option because continuous ventilation at the prescribed rate uses a great deal of fan and heating energy. The code also requires garages to be kept at negative pressure relative to adjacent occupied spaces, and requires vestibules or other separation between the garage and the rest of the building in many cases. Exhaust discharge locations must be kept away from air intakes and openings.

Code editions differ in their details, and provinces and municipalities may add requirements, for example NO2 detection where diesel vehicles are expected, or specific alarm requirements. Always confirm the adopted edition and any local amendments with the authority having jurisdiction. Occupational exposure limits for parking attendants and maintenance staff are set by provincial occupational health and safety regulations.

Why NO2 matters

Gasoline vehicles with modern catalytic converters emit much less CO than older vehicles, so CO levels in many garages are now low. Diesel vehicles, including delivery trucks, emergency vehicles and some passenger vehicles, emit relatively more NO2 and less CO. A CO-only system can therefore under-ventilate a garage with significant diesel traffic. Loading docks, bus garages, ambulance bays and municipal works yards usually warrant combined CO and NO2 detection. Many designers now include NO2 sensors in mixed-use parkades as a matter of course.

System configurations

Ducted supply and exhaust

Supply air enters at one side or end of the garage, and exhaust is drawn through ductwork with grilles distributed near the floor and ceiling. Ducted systems are predictable but take up headroom, which is often limited.

Jet fan systems

Ceiling-mounted jet fans or induction fans push air horizontally across the garage toward central exhaust points, replacing most of the ductwork. They save headroom and can be zoned, but their design usually requires computational fluid dynamics (CFD) modelling to confirm there are no dead zones. Jet fans may also be part of a smoke control strategy, which brings additional design and testing requirements.

Natural and hybrid ventilation

Open parking structures with sufficient wall openings may rely on natural ventilation and are classified differently under the NBC. Partially open structures sometimes use fans only for interior zones.

Make-up air and heating

Exhaust air must be replaced. Make-up air typically enters through the ramp opening, supply fans or dedicated make-up air units. In Canadian winters, untempered air near entrances can freeze sprinkler and domestic water piping, so many garages include heaters near ramps and in pipe-sensitive areas, and garage heating is often kept to a minimum temperature only. See make-up air units for related equipment.

Gas detection and fan control

A detection-controlled system runs fans at minimum or off during clean conditions and ramps up as gas levels rise.

A typical control approach:

1. Sensors report CO and NO2 concentrations to a dedicated gas detection controller or the BAS.
2. Each zone's fans stage or modulate based on the highest sensor reading in that zone.
3. A low setpoint starts or increases ventilation, and a higher setpoint drives fans to full speed.
4. An alarm setpoint activates visual and audible alarms and notifies building operations.
5. A minimum run schedule, such as a fixed number of minutes per hour, provides baseline air change even when sensors read clean.
6. Sensor fault or loss of communication drives fans to full speed (fail-safe).

Setpoints vary by jurisdiction and designer, but staging well below the code limits is common practice, with CO staging points typically in the tens of ppm and the alarm point at or below the code maximum. Setpoints should be written into the sequence of operation and confirmed with the AHJ where required.

Sensor selection and placement

Electrochemical sensors have finite lives and drift over time. A system with expired sensors may either run fans continuously, wasting energy, or fail to respond to real contamination. For general guidance, see sensor selection, location and calibration.

Energy considerations

Garage exhaust fans are often among the largest fan loads in a residential tower or office building. Detection-based control with variable-speed fans can dramatically reduce fan run hours compared with continuous operation, along with the cost of heating make-up air. Fan affinity laws mean that running at half speed uses roughly one-eighth of full-speed power. See reducing fan energy.

Common energy problems found during retro-commissioning include fans locked on in hand mode, failed sensors defaulting to full speed, and timers that override detection.

Testing and commissioning

Commissioning a garage system should include:

• Verification of exhaust and supply airflows against design by measurement
• Confirmation of garage pressure relative to adjacent stairs, elevator lobbies and occupied spaces
• Calibration verification of every sensor using certified calibration gas
• Functional testing of each staging setpoint, alarm and fail-safe response
• Testing of interlocks with smoke control, fire alarm and emergency power where applicable
• Verification of heaters and freeze protection near ramps

Our control system verification work frequently includes gas detection systems.

Frequently asked questions

Can garage fans be turned off overnight?

Only if the code option chosen and the control sequence allow it. Detection-based systems usually run at minimum or on a schedule and respond to gas levels at all hours, since vehicles may move at any time.

Do electric vehicles change garage ventilation?

They reduce exhaust emissions, but most garages will see mixed fleets for many years. EV charging also raises fire and battery safety questions that are handled under fire and building codes rather than ventilation rules.

Key takeaways

• The NBC allows either continuous ventilation at a prescribed rate or CO-controlled ventilation limited to 100 ppm; most designs use detection.
• Add NO2 detection where diesel vehicles operate, and confirm local requirements with the AHJ.
• Jet fan systems save headroom but need careful design, often with CFD.
• Electrochemical sensors need regular calibration and replacement, and failures should drive fans to full speed.
• Variable-speed, detection-controlled fans save significant fan and heating energy.