Sealed combustion means a gas appliance draws its combustion air directly from outside through a dedicated intake pipe, keeping the burning process completely isolated from the air inside your home.
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Sealed combustion, also called direct vent, describes a gas appliance design where the combustion chamber is fully sealed from the surrounding room air. Instead of pulling in air from inside the mechanical room to support burning, the appliance draws combustion air through a dedicated intake pipe connected directly to the outdoors, and exhaust leaves through a separate sealed pipe. The burning process never interacts with indoor air at all.
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An atmospheric, or open combustion, gas appliance pulls the air it needs for burning from the room it's installed in, the mechanical room's air is drawn in around the appliance and used for combustion, then exhaust rises out through a vertical flue. This design has worked for decades, but it depends on the mechanical room having adequate air supply, and it means the space around the appliance is part of the combustion air pathway.
A sealed combustion appliance removes that dependency entirely. Combustion air comes through its own dedicated intake pipe from outside, and exhaust leaves through its own separate pipe, also to outside. The appliance's combustion chamber is essentially isolated in its own sealed loop with the outdoors, unaffected by what's happening in the room around it.
Because a sealed combustion appliance doesn't depend on room air for burning, it's less affected by things like a mechanical room being tightly sealed for other reasons, or exhaust fans elsewhere in the house creating negative pressure that could otherwise pull exhaust gases back into the appliance's flue, a phenomenon called backdrafting. This makes sealed combustion a particularly good fit for modern, tightly sealed, energy-efficient homes where atmospheric combustion appliances can sometimes struggle to get adequate air.
It also means the appliance's combustion process isn't drawing in dust, humidity, or other contaminants from a typical mechanical room or basement, since its intake air comes straight from outside instead.
As Metro Vancouver homes get more energy-efficient upgrades, better insulation, tighter windows and doors, air sealing work, the mechanical room can end up with less incidental air leakage than it used to have, which is exactly the scenario where an older atmospheric gas appliance can run into combustion air supply problems. Sealed combustion equipment sidesteps this concern entirely, which is one reason it's often recommended when replacing an older water heater or boiler in a home that's had other efficiency upgrades done.
It's also a natural fit anywhere a condensing appliance is being installed, since most high-efficiency condensing boilers and water heaters use sealed combustion by design, paired with polypropylene intake and exhaust venting.
Safety note
Any work on the gas piping, burner, venting, or pressure components of a boiler or gas water heater must be done by a Technical Safety BC licensed gas fitter. CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas, hear hissing near a gas line, or suspect a carbon monoxide problem, leave the home immediately and call FortisBC at 1-800-663-9911 from outside before doing anything else.
Sealed combustion removes dependency on room air supply and reduces backdrafting risk from household exhaust fans, which many technicians consider an advantage, particularly in tightly sealed modern homes. Both types can be safe when properly installed and maintained by a licensed gas fitter.
Most condensing, high-efficiency gas water heaters and boilers use sealed combustion as part of their design, though it's worth confirming the specifics of a particular model with your technician.
It offers more placement flexibility than atmospheric venting since it doesn't rely on room air supply, but it still needs proper access to an outside wall or roof for the intake and exhaust pipes, and must meet code clearance requirements.
An atmospheric combustion appliance relies on air being drawn into the room to support combustion, which can create a noticeable draft, especially in a tightly sealed space. This is one of the things sealed combustion design avoids.
It can, since it requires running two dedicated pipes to outside rather than a single flue, and the appliances themselves are often higher-efficiency models. A licensed technician can give you a ballpark based on your specific home.
Generally this means replacing the appliance itself with a sealed combustion model, since the combustion chamber design is built into the unit rather than something that can be retrofitted onto an existing atmospheric appliance.
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