Making a home more airtight for energy efficiency can inadvertently reduce the combustion air available to an existing atmospheric gas appliance.
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Quick answer
An atmospheric gas boiler or water heater relies on drawing combustion air from the surrounding room or a dedicated opening, and when a home is air-sealed, re-insulated, or has new high-performance windows installed, the reduced natural air leakage that previously supplied that combustion air can leave the appliance without enough air to burn cleanly. This can increase carbon monoxide production and raise backdraft risk. Any home undergoing significant air-sealing or insulation work with atmospheric gas appliances present should have combustion air adequacy checked as part of that project.
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Older, leakier homes naturally leak enough outdoor air through gaps around windows, doors, and the building envelope to supply combustion air to atmospheric gas appliances without anyone designing for it specifically. This incidental leakage was never engineered as a combustion air source, but it worked well enough that many homes operated for decades without a documented combustion air problem, simply because the house itself was never airtight to begin with.
Energy-efficiency upgrades change this equation directly. New windows, added insulation, air sealing around penetrations, and weatherstripping all reduce the amount of outdoor air that leaks into the home. That's the intended benefit for comfort and energy costs, but it also removes the incidental air supply an atmospheric appliance may have depended on.
An atmospheric boiler or water heater starved of adequate combustion air can burn incompletely, producing more carbon monoxide than it would with proper airflow, and can also become more prone to backdraft, since the appliance's own combustion process can create enough localized negative pressure to disrupt its own draft in a very tight space. This is a gradual, easy-to-miss problem rather than a sudden failure, which is part of why it's often discovered only after a renovation, not before.
| Assessment Step | What It Determines |
|---|---|
| Calculate the mechanical room's volume | Whether the space alone provides enough air by code |
| Check for existing air openings to adjacent spaces or outdoors | Whether supplemental air is already available |
| Worst-case depressurization test | Whether draft holds up under realistic operating conditions |
| Dedicated combustion air duct if needed | Provides a direct outdoor air supply to the appliance |
Where a mechanical room doesn't have enough volume or existing openings to meet requirements on its own, a dedicated combustion air duct routed to outdoors is a common fix, sized appropriately for the appliance's input rating. This is a standard, well-understood solution, but it needs to be identified and installed correctly, not skipped because the appliance seemed to be working fine before the renovation.
Anyone planning significant air-sealing, insulation, or window upgrades in a Metro Vancouver home with an atmospheric gas boiler or water heater should raise combustion air adequacy with their contractor at the planning stage. It's a straightforward thing to address as part of a broader renovation and considerably more disruptive to retrofit afterward once finishes are already in place.
Safety note
If you smell gas, hear hissing near a gas line, or your carbon monoxide alarm sounds, leave the home immediately with everyone inside, do not operate switches or light anything, and call FortisBC at 1-800-663-9911 from outside the house. Gas and combustion work must be carried out by a Technical Safety BC licensed gas fitter.
Yes, if the boiler is atmospheric and was relying on incidental air leakage through old windows for combustion air. Sealing that leakage without providing another air source can create a real problem.
A technician can calculate this based on your mechanical room's volume, existing openings, and the appliance's input rating, and can test actual performance under worst-case conditions.
It can, especially if it encloses what was previously a more open space around the boiler or water heater. This should be checked as part of any basement finishing project.
A dedicated combustion air duct routed to outdoors, sized for the appliance, is a common and effective solution when the existing space doesn't provide enough air on its own.
Before, ideally, since it's easier and less disruptive to plan for combustion air during the renovation than to retrofit a solution afterward.
Yes, since it draws combustion air directly from outdoors through a dedicated intake rather than from the room, making it far less sensitive to how airtight the rest of the home is.
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