Gas tankless water heater venting is not one-size-fits-all. The correct setup depends on the heater model, where it will be installed, and how the vent will reach the outdoors.
Quick answer: Some indoor models use direct vent, while others use power vent. Outdoor-rated models typically discharge exhaust directly at the appliance. Always follow the exact model’s installation manual rather than choosing vent components by appearance or diameter alone.
What Do Direct Vent and Power Vent Mean?
These are common venting terms, but manufacturers may use them differently. Always follow the terminology and approved configuration for the exact model.
The main difference is where the heater gets the air needed for burning gas and how it sends exhaust outside.
Direct vent: The heater brings in air from outdoors and sends exhaust back outdoors through a closed system. It does not depend on the room for its combustion air.
Power vent: A built-in fan pushes exhaust outdoors. Where the heater gets its combustion air depends on the model, so the product manual must be checked before choosing the installation space.
How Indoor and Outdoor Setups Differ
| Installation type | Where air comes from | How exhaust leaves | Check before installation |
|---|---|---|---|
| SetupIndoor direct vent | Air sourceDrawn from outdoors | ExhaustSealed route to outdoors | CheckApproved pipe size, material, length, bends, and exit location |
| SetupIndoor power vent | Air sourceDepends on the model; some use air from the room | ExhaustFan-assisted route to outdoors | CheckAir-supply needs, approved vent parts, route limits, and room requirements |
| SetupOutdoor-rated heater | Air sourceAvailable at the outdoor installation location | ExhaustDischarges at the appliance | CheckRequired distance from openings, weather exposure, freeze protection, utilities, and service access |
Our tankless water heaters do not all use the same venting setup. The GEN 3 10.3 GPM model is designed for indoor direct-vent installation, while the 4.0 GPM 90,000 BTU model uses a power-vent configuration. Always choose vent components according to the specifications of the exact model.
What Do Concentric and Two-Pipe Mean?
These terms simply describe how the intake and exhaust pipes are arranged. They are not two additional types of water heater.
Concentric venting
A concentric vent places two air passages inside one assembly—like a pipe within a pipe. One passage brings air in and the other sends exhaust out, so the installation may need only one opening through the wall or roof.
Two-pipe venting
A two-pipe system uses one pipe to bring outdoor air to the heater and another to carry exhaust outside. The required pipe material, length, bends, and exit locations still depend on the model.
Can You Reuse the Existing Water Heater Vent?
Do not assume so. A traditional storage water heater may use a chimney or older vent system that is not approved for the new tankless unit. The replacement heater may need a different pipe size, material, seal, route, or exit location.
The installer should inspect the complete path—not only the opening in the wall. Reusing an old route may still require replacing the pipe and fittings or closing an abandoned chimney connection.
What Shapes the Vent Route?
The shortest route is not automatically acceptable, and the longest route allowed on paper is not automatically the best choice. The layout must work within the exact model’s limits.
Pipe length and bends
Every bend makes it harder to move air through the vent, so adding elbows can reduce the permitted pipe length. Measure the complete route and check the model’s limits before ordering parts.
Distance from openings
The vent outlet must be kept a safe distance from windows, doors, air intakes, ground level, meters, and building corners. The exact distances come from the model instructions and local code.
Cold-weather placement
In snowy climates, intake and exhaust openings must remain above expected snow accumulation. Exposed water lines and model-specific freeze-protection requirements also need separate planning.
Moisture and drainage
Condensing models produce condensate during operation and require an approved way to drain it. Some vent configurations may also have model-specific condensate or drainage requirements, so follow the installation manual for the exact heater.
Choosing Vent Pipe and Accessories
The correct pipe material depends on the heater. Some models use an approved stainless-steel system, while others may allow specific plastic vent materials. Terms such as “gas tankless” or “condensing” are not enough to determine the correct vent material or component system.
Do Outdoor Tankless Water Heaters Need a Vent Pipe?
An outdoor-rated gas tankless water heater typically releases combustion exhaust directly at the appliance, so it does not require a vent pipe routed from an indoor unit through a wall or roof. That does not mean it can be mounted anywhere.
The location still needs the required distance from windows, doors, and other building openings, along with access to gas or propane, water, electricity, and future service. Weather, freezing temperatures, and protection of outdoor water lines also matter. An indoor-only model should never be installed outdoors simply to avoid installing a vent.
If you are still deciding between the two locations, see Indoor vs. Outdoor Propane Tankless Water Heaters: How to Choose.
What to Confirm Before Ordering
- The exact heater model and approved installation location
- The approved vent type and pipe material
- Pipe diameter and component compatibility
- Maximum permitted pipe length and number of bends
- Sidewall or roof exit requirements
- Clearances from windows, doors, grade, and air intakes
- Condensate, drainage, and cold-weather requirements
- Local permits, inspections, and code requirements
Venting is also one of the factors that can change the final project price. For a broader planning view, read Gas Tankless Water Heater Installation Cost: What Affects the Price?
Frequently Asked Questions
Does every indoor gas tankless water heater need venting?
Yes. Combustion exhaust must be discharged outdoors through the system approved for that heater. The exact intake and exhaust arrangement varies by model.
Can a gas tankless water heater use the existing chimney?
Do not assume it can. Many tankless models need their own approved vent system rather than an existing storage-heater chimney. Have the complete route checked before installation.
Can I use PVC to vent a tankless water heater?
Only when the exact heater’s listing and installation instructions approve the specific plastic vent material and configuration. Some systems require stainless steel or another listed vent product.
Can different brands of vent parts be mixed?
Not unless the appliance and vent instructions explicitly approve the combination. Matching diameter does not establish compatibility.
Can a tankless water heater vent through a side wall?
Many models can vent through a side wall, but the parts, pipe length, slope, and required distance from nearby openings must follow the exact heater’s instructions and local code.
Plan the Vent Before Choosing the Final Location
Venting should be planned while choosing the heater—not after the unit is mounted. The vent type, route, required distances, and accessories can all affect where a gas tankless water heater can be installed.
Once the venting and installation location are clear, you can compare models with fewer surprises during installation. See our guide to sizing a natural gas tankless water heater, or browse current Mizudo gas tankless water heaters.
Technical references: U.S. Department of Energy Building America guidance on tankless water heater retrofits and gas-fired tankless water heaters. Always follow the installation manual supplied with the exact appliance and local installation requirements.
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