Water Heater Venting Types

A gas water heater burns fuel, and that combustion releases byproducts that must leave the home safely. The venting method decides how those gases exit and what pipe material is allowed. This guide walks through the main venting types and the safety rules tied to each, with every claim kept next to the source it came from.

Atmospheric Venting Uses Natural Draft

Atmospheric venting relies on hot air rising. A. O. Smith states that atmospheric gas water heaters vent their exhaust through a vertical or upward-sloping metal duct vent to exit the home. The National Water Heater Authority adds detail, noting that combustion gases in conventional storage water heaters can exceed 400 degrees Fahrenheit, are less dense than ambient air, and rise naturally through a flue pipe or B-vent.

Because the draft is natural rather than fan-driven, the vent path has to stay vertical or upward-sloping without interruption. A. O. Smith warns that any blockages or misalignments in the venting may result in excess heat and fumes being released back into the ambient air.

This is the oldest and simplest configuration, and it is also the one most sensitive to a blocked or poorly pitched flue.

Power Venting Uses a Blower

A power vent adds a fan so the exhaust does not depend on natural draft. A. O. Smith states that power vent gas water heaters are equipped with an electric blower fan installed on top of the water heater, which requires an electrical outlet close to the water heater, and that the venting can run either vertically or horizontally with a longer line to help with space and sizing.

The National Water Heater Authority describes the same setup as an electrically powered inducer blower that pushes exhaust gases through flexible PVC or CPVC vent pipe to the exterior, operating under positive pressure that enables horizontal runs and longer vent lengths, up to 100 linear feet in some manufacturer specifications. A. O. Smith notes the lower exhaust temperatures permit PVC piping, which is a less expensive solution than atmospheric vent metal piping.

The National Water Heater Authority also flags the failure mode: if the blower loses power or fails, the appliance must shut down via a pressure switch interlock.

Direct Venting Draws Combustion Air From Outside

A direct vent takes its combustion air from outdoors rather than from inside the home. A. O. Smith states that a direct vent water heater draws air for combustion from outside your home rather than inside, then vents the exhaust gases and excess heat back outdoors, using special coaxial venting that has separate chambers for intake air and exhaust in a single assembled vent piece.

The National Water Heater Authority describes this as a concentric or two-pipe arrangement that draws combustion air directly from outside through one pipe while exhausting flue gases through a second, and notes that horizontal termination is permitted, which expands installation flexibility significantly relative to atmospheric configurations. It adds that only the verified concentric or paired components specified by the manufacturer and evaluated under UL 307A should be used.

Because the combustion air comes from outside a sealed path, State Water Heaters states that direct vent designs prevent potential backdrafting issues within your home, thereby enhancing safety.

Condensing and Tankless Venting Use PVC

High-efficiency condensing units run cooler exhaust, which is why they can vent through plastic. State Water Heaters describes a power direct vent that uses a blower to draw in outside air for combustion and can use PVC piping for cost-effective installation, along with a concentric pipe-in-pipe configuration where both intake air and exhaust pass through a shared assembly with just one wall or roof penetration.

For tankless specifically, Rheem states that all indoor tankless gas water heaters must be vented vertically or horizontally to the outdoors using factory approved venting material, and that no other venting material is permitted. Rheem also notes that outdoor tankless water heaters do not require venting materials, and advises selecting an indoor location with the shortest venting distance to the outdoors.

The consistent theme is that plastic venting is tied to lower exhaust temperature, and that the specific approved material must come from the manufacturer rather than be substituted.

Safety: Backdrafting, Carbon Monoxide, and Material Rules

The reason venting is not optional is carbon monoxide. Bob Vila states that most water heaters burn fuel such as natural gas, oil, or propane, and that fuel burning releases byproducts like carbon monoxide, so without proper ventilation of the noxious gases created by combustion, modern water heating would be a dangerous proposition. A. O. Smith defines backdrafting as exhaust gases flowing backward into living space instead of going outside, which is a significant safety concern because those gases may contain carbon monoxide.

Material selection is part of that safety picture. The National Water Heater Authority states that Type B vent is prohibited for appliances in Categories III and IV, and that misuse causes interior corrosion and voids certification. That is the technical reason a hot atmospheric flue material cannot simply be swapped onto a cooler positive-pressure appliance.

The safe path across these sources is to match the venting type and material to the appliance, keep the flue clear and correctly pitched, and rely on manufacturer-approved components rather than improvised substitutions.