Carbon Monoxide Calculator

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Carbon Monoxide in Canadian Buildings: Sources, Risks, and Ventilation

Carbon monoxide poisoning is a serious hazard in Canadian homes. Unlike most air quality issues that cause long-term harm through chronic exposure, CO can reach life-threatening concentrations in hours, or even minutes, from a single malfunctioning appliance. Understanding the buildup dynamics helps HVAC professionals size ventilation correctly and advise clients on where alarms need to go.

CO Buildup Physics

In a well-mixed space with a constant source and constant ventilation, CO concentration follows an exponential curve toward a steady-state value. The steady-state concentration equals the emission rate divided by the total ventilation flow rate, plus any background level. Without any ventilation, CO rises linearly until the source stops or the occupants leave. The time constant for the system is the room volume divided by the ventilation flow rate, identical to the time constant used in the dilution air calculator. A tightly sealed modern home with 0.05 ACH natural infiltration has a time constant of 20 hours, meaning CO from a moderate source can accumulate for many hours before approaching steady state. The calculator converts between emission rate and concentration at 1.165 mg/m3 per ppm (20 C).

Health Canada CO Guidelines

Health Canada's residential indoor air quality guideline for carbon monoxide sets a 24-hour limit of 10 ppm (11.5 mg/m3) and a 1-hour limit of 25 ppm (28.6 mg/m3). Both are averaged limits derived to keep blood carboxyhaemoglobin below 2.0%, so a brief excursion above 10 ppm is not by itself a 24-hour exceedance. The calculator compares the modelled concentration at the end of the run time against them, which is a conservative screen. Symptoms depend on both level and time: commonly cited exposure charts put a headache at around 200 ppm after 2 to 3 hours and life-threatening effects at 400 ppm after about 3 hours, and NIOSH sets the immediately dangerous to life or health level at 1200 ppm. Canadian CO alarms certified to CSA 6.19 are built to sound at 70 ppm sustained for 60 to 240 minutes, so they are not designed to trigger at the guideline levels of 10 to 25 ppm.

Furnace Heat Exchangers

A cracked heat exchanger is a serious CO source in Canadian homes because the furnace can distribute combustion gases through the supply air stream and the rest of the building. The calculator's furnace-leak preset is 500 mg/hr. In its example, a 173 m3 basement at 0.15 ACH settles near 17 ppm, above Health Canada's 24-hour limit but well below the 70 ppm alarm level, which is how a slow leak can go unnoticed until symptoms appear. Health Canada advises a professional inspection of fuel-burning appliances at least once a year. The heat exchanger crack test calculator covers crack testing of the exchanger itself, and the combustion air calculator covers makeup air sizing for combustion appliances.

Attached Garages

Attached garages are a significant CO source in Canadian winters because vehicles are often warmed up before driving. Health Canada advises keeping the door between the house and the garage closed and not idling vehicles in the garage, even with the garage door open. The calculator's vehicle preset is 2000 mg/hr, in line with the 2.4 to 5.4 g/hr that CPSC staff cite for idling mid-size cars of the late 1990s. In a sealed 72 m3 garage that preset reaches the 25 ppm level in about an hour and the 70 ppm alarm level in about 3 hours, and a cold engine emits more while its catalytic converter warms up. If there is any air pathway into the living space, such as around the entry door, through shared wall penetrations, or through ductwork, some of that CO follows it. Install CO alarms in hallways outside sleeping areas, as Health Canada advises.

Range Hoods and Kitchen Ventilation

Gas ranges produce CO from incomplete combustion, particularly at low flame settings and on older unserviced burners, and measured rates vary widely with burner condition. The calculator's range preset is 50 mg/hr. At that rate, about 1.2 L/s of total ventilation holds CO at Health Canada's 10 ppm 24-hour limit with a 0.2 ppm background, so the concern is a small, tightly closed kitchen with several burners running: four burners at the preset in a 48 m3 kitchen at 0.2 ACH settle near 18 ppm. The kitchen exhaust calculator sizes the exhaust airflow for the cooking side, and this calculator shows whether that airflow also covers CO. See the CO2 level calculator for kitchen CO2 accumulation from gas combustion alongside CO.

Portable Generators

Portable generators are a major CO hazard during power outages. The CPSC staff briefing on generator CO emissions reports that a typical engine powering a 5 kW generator emits a weighted average of about 1,500 g/hr of CO, roughly the output of 450 idling cars, and counts 659 generator-related CO deaths in the US from 2004 to 2012. The calculator's generator preset uses that 1,500 g/hr figure, and it passes the 70 ppm alarm level in well under 15 minutes in a closed garage. Health Canada says never to run a generator indoors, in a garage, or in a shed, to operate it at least 6 metres (20 feet) from buildings with the exhaust pointed away from doors and windows, and to keep CO alarms working while it runs. During extended winter outages the temptation to move a generator indoors for warmth creates a serious risk.

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