Glossary · Heating, cooling & ventilation
Sealed-combustion (direct-vent) appliance
Also called: direct-vented · sealed-combustion furnace
A furnace, boiler, water heater or fireplace that brings in combustion air from outdoors through its own pipe and vents exhaust straight outside.
What it means
Because the burner is sealed off from indoor air, house pressure from exhaust fans or a tight building envelope can't pull exhaust back inside the way it can with a natural-draft appliance. Many condensing furnaces, newer water heaters and gas fireplaces are built this way. Fan-assisted appliances that still draw air from the room sit in between: they resist spillage better than natural-draft ones, and sealed combustion restricts it further.
Why it matters in an audit
Switching spillage-prone equipment to direct-vent or sealed-combustion models is the surest defense against combustion gases, which makes deeper air sealing safer.