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CO Concentration Reference Zones
Parking Garage Ventilation: Keeping CO and Exhaust Gases Down
Cars in a closed garage release carbon monoxide, nitrogen oxides and unburned fuel while they start, creep along ramps and idle at the barrier. Left alone, the gas collects in low spots and near the entrances, which is why codes treat garage ventilation as a safety system and not a comfort one. The maths is the same dilution idea as in the IAQ calculator, applied to a source that swings from nothing at 3 a.m. to a rush at 5 p.m.
What the Code Requires
Enclosed storage garages fall under Part 6 of the National Building Code (Article 6.2.2.3), and Part 9 extends it to residential garages for more than four vehicles. The clearest published wording is the BC Building Code, Article 6.3.1.4: a continuous supply of outdoor air of at least 3.9 L/s per square metre of floor area, or a system controlled by CO monitoring devices that holds CO to no more than 100 ppm. It also sets a 3 ppm nitrogen dioxide limit where most stored vehicles are diesel, and it lets the requirement drop by half where vehicles are parked by mechanical means. The 3.9 L/s per m² figure is about 0.77 CFM per square foot. Six air changes per hour is a handy rule of thumb that overshoots it for ordinary ceiling heights, so this calculator starts from the code rate. Other provinces adopt the NBC on their own schedules, so confirm the edition in force.
Worked Fixed-Rate Example
A 2,000 m² parkade with a 2.7 m ceiling holds 5,400 m³. At 3.9 L/s per m² it needs 7,800 L/s, which is 16,527 CFM and 5.2 air changes per hour. A flat 6 ACH design would pull 9,000 L/s instead, about 15% more. The fixed approach runs the fans at that rate whenever the garage is in service, whether or not a car has moved.
The CO-Controlled Alternative
Where the code allows it, CO sensors replace the flat rate, so fans slow down or cycle off when the garage is quiet and speed up as vehicles arrive. Peak airflow still has to be right, which is why the CO mode here finds it from the vehicle activity, and the fans then need a way to modulate. A variable frequency drive does it for the fan, and the variable speed savings calculator estimates what that modulation saves. The saving depends on how busy the garage is. A residential parkade that sits idle most of the day gains far more than a shopping centre that never empties.
Sizing From the CO Load
The mass balance is airflow equals generation divided by the allowed rise, with 1 ppm of CO equal to 1.145 mg/m³ at 25 °C. Eight vehicles at 3,000 mg/min each release 24,000 mg/min. To hold 25 ppm with 1 ppm outdoors, that takes 14,556 L/s (30,843 CFM), about 9.7 changes an hour in 5,400 m³. Allow the 100 ppm code limit and it falls to 3,529 L/s, or 2.4 changes an hour, which shows how strongly the target drives the answer. The 3,000 mg/min per vehicle is a conservative assumption, and a warmed modern car emits much less, so treat the result as a design check and use measured or local data where you have it. The calculation also assumes the air mixes well, and a long, low-ceilinged parkade rarely does, so a ramp or dead end can run well above the average. The carbon monoxide calculator covers CO levels more generally.
Sensors and Setpoints
Place sensors where cars idle and gas collects: near entrance and exit ramps, at the barrier and in any dead end. Staging is a design choice, and a common pattern starts the fans around 25 ppm, ramps between 25 and 100 ppm and runs full speed at 100 ppm, the BC limit. Minnesota's garage ventilation guidance is one published example of the lower start point. It requires the system to run at a CO level of 25 ppm or greater, and a nitrogen dioxide level of 3 ppm or greater, in enclosed garages with a minimum exhaust of 0.75 CFM per square foot, which is published by the state's Department of Labor and Industry. Sensors drift, so plan for calibration or replacement on a schedule. Once you have the peak airflow, the exhaust fan sizing calculator helps with the fans, and the pressurization calculator covers keeping garage air out of the building above.
Frequently Asked Questions
Enclosed storage garages are covered by Part 6 of the National Building Code (Article 6.2.2.3), and provincial codes follow it. British Columbia's Article 6.3.1.4, for example, requires a continuous supply of outdoor air of at least 3.9 L/s for each square metre of floor area, which is about 0.77 CFM per square foot. With a 2.7 m ceiling that works out to roughly 5.2 air changes per hour, so the common 6 ACH figure is a rule of thumb, not the code. Confirm the rate in the edition your province has adopted.
CO sensors spread through the garage report concentration to the fan controls, which slow or stop the fans when CO is low and speed them up as it rises. The BC code lets a system be controlled by CO monitoring devices as long as it holds CO to no more than 100 ppm, and many designers start fans well below that, near 25 ppm. How much energy this saves depends on how often the garage is busy, since a quiet garage runs at its minimum for most of the day.
Divide the CO the vehicles release by the concentration rise you will allow: airflow equals generation divided by the target minus the outdoor concentration, with 1 ppm of CO equal to 1.145 mg/m³ at 25 °C. Eight vehicles releasing 3,000 mg/min each and a 25 ppm target with 1 ppm outdoors need 14,556 L/s (30,843 CFM). Allowing the 100 ppm code limit instead needs only 3,529 L/s. The result assumes the air mixes well, which a long, low parkade seldom does, so sensor placement matters.
Where most of the stored vehicles are diesel-powered, the BC code requires the ventilation to hold nitrogen dioxide to no more than 3 ppm, and a CO sensor does not detect it. For gasoline vehicles, CO is the controlling gas. NO2 is not part of this calculator, so for a garage with diesel traffic, size the airflow with the code rate and add suitable monitoring.