Combustion Air Calculator

How to Use This Calculator

Combustion Air Requirements Under CSA B149.1

Every gas-fired appliance needs air for combustion, and appliances with a draft hood also draw dilution air that mixes with the flue gases. Getting that air to the appliance reliably, especially in tighter modern building envelopes, is one of the most safety-critical tasks in residential and light commercial HVAC work in Canada.

How CSA B149.1 Sizes Combustion Air

Clauses 8.2 to 8.7 of CSA B149.1 cover passive combustion air supply openings and ducts, as SkilledTradesBC's fuel gas systems training text explains. The code splits installations at 120 kW (400,000 Btu/h): up to and including 120 kW is low volume and sized from Tables 8.1 to 8.4, and above 120 kW is high volume and sized from clause 8.4. Two things drive the tables: whether the building is tight, and whether the appliance has a draft control device such as a draft hood, draft diverter, or barometric regulator. Appliances with draft control need combustion and dilution air, and appliances without it need less. Direct vent appliances take their air through their own pipe and fall outside these clauses. This calculator assumes tight construction, which the training text advises when in doubt, so it follows Tables 8.1 and 8.2 for low volume installations and does not model the loose-building Tables 8.3 and 8.4.

Low Volume and High Volume Sizing

For high volume installations, clause 8.4 gives formulas: 310 mm2 of free area per kW up to 293 kW plus 155 mm2 per kW above that for appliances with draft control, and 70 mm2 per kW for appliances without it. A separate ventilation opening, at least 10% of the supply area and never under 6,500 mm2, sits as high as practical and at least 300 mm from the supply opening. The supply opening connects to outdoors and sits 150 to 450 mm above the floor. Low volume installations use Tables 8.1 and 8.2, which this calculator approximates at 320 and 155 mm2 per kW. Those rates reproduce the table points in the training text, about 11 in2 at 75 MBH and 22 in2 at 150 MBH for draft control appliances in a tight building, though the code rounds up to its table steps. When both appliance types share a room, the low volume result is the larger of the draft control table and the all-appliance table. A rectangular supply duct never goes below 9 in2 of free area, and a single non-direct-vent water heater of 50 MBH (14.65 kW) or less with no other room-air appliances needs no outdoor supply. In this calculator's default example, an 88 kW furnace and a 15 kW water heater, both with draft hoods, total 103 kW, which is low volume and needs about 330 cm2 of free area, or roughly 26 x 26 cm of louver at 50% free area.

Data Plate Ratings and Units

Data plates usually give input in Btu/h or MBH (thousand Btu/h), while this calculator works in kW. Divide Btu/h by 3,412 to get kW, so the 120 kW low volume limit is 400 MBH. The BTU to kWh converter handles the other conversions. Use the input rating on the plate rather than the output rating, since the code sizes air supply from the fuel input.

Direct Vent and Sealed Combustion Appliances

Direct vent appliances, including two-pipe condensing furnaces and boilers, draw combustion air through a dedicated pipe and don't count toward room-air sizing. A single-pipe condensing furnace still takes its air from the room, so it belongs with the fan-assisted appliances in this calculator. When a high-efficiency direct vent furnace replaces an older one, the room may no longer need the old supply opening for the furnace, but an atmospheric water heater or fireplace in the same room still does, and that's where the calculation matters.

Depressurization and Backdrafting Risk

Backdrafting, the reversal of flue gas flow down the chimney into the living space, is caused by negative pressure in the building relative to outdoors. Every exhaust fan, clothes dryer, and combustion appliance competes for the same air, and the worst case is all exhaust loads running while the furnace fires. Modern tight homes can develop significant negative pressure from a single large kitchen exhaust fan. This calculator compares the exhaust left over after infiltration with the combustion air the appliances need. In the default example, a range hood, bath fan, and dryer total 135 L/s, and 0.15 ACH across 480 m3 replaces about 20 L/s, so about 115 L/s is left against roughly 83 L/s of combustion air, which scores as very high. It is a screening indicator, not a pressure test. When exhaust exceeds what infiltration can replace, a dedicated makeup air system is the fix, and the makeup air calculator sizes that supply. Our carbon monoxide calculator models CO accumulation if backdrafting does occur.

Combustion Safety After Air Sealing

Health Canada's guidance on preventing carbon monoxide exposure recommends a professional inspection of fuel-burning appliances at least once a year, and CO alarms in the hallways outside sleeping areas. Tightening the envelope or adding an exhaust fan removes some of the accidental air supply the appliances relied on, so a licensed gas technician should verify combustion safety after that work. Openings that lead outdoors need corrosion-resistant screens, and the training text notes that fixed louver openings cannot be smaller than 6 mm, that automatic dampers are only permitted when interlocked with the burner, and that manually adjustable dampers are not permitted.

Frequently Asked Questions

Related Air Quality Tools