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Tankless water heater venting is the most safety-critical part of a gas install. Here’s how it works and what installers check.
Gas and propane tankless water heaters produce exhaust containing carbon monoxide and water vapor. The vent carries it safely outdoors. Because tankless units use a fan, the vent is under positive pressure, so it must be sealed and made of approved materials. Mistakes can cause carbon monoxide leaks, error codes or a failed inspection. Always install CO alarms; see our safety disclaimer.
| Type | How it works | Where used |
|---|---|---|
| Direct vent, two-pipe | Separate intake and exhaust pipes to outside | Most indoor condensing units |
| Direct vent, concentric | Exhaust pipe inside an intake pipe, one wall or roof penetration | Many indoor units, tidy installs |
| Power vent (single pipe) | Exhaust to outside, combustion air from the room | Rooms with enough combustion air |
| Outdoor (no vent pipe) | Exhaust from the front of the cabinet | Outdoor models |
The tankless water heater exhaust vent type determines the vent material and cost. An example is the Rheem Prestige indoor direct vent tankless water heater.
Each manual lists the allowed vent diameter, maximum equivalent length and how much each elbow subtracts. Condensing vents slope back toward the unit so condensate drains. Non-condensing horizontal vents often slope slightly away from the unit. Supports and joints must follow the instructions.
The vent terminal must be a set distance from:
The manual and the fuel gas code specify the exact distances.
Sidewall venting is usually shorter and cheaper. Roof venting works when the unit is far from an exterior wall or sidewall clearances can’t be met. Concentric kits are available for both.
In mild climates, an outdoor tankless water heater avoids vent pipes entirely, which can lower installation cost.
Most manufacturers sell vent kits, such as concentric terminations and wall plates, for their units. Using the brand’s approved kit simplifies inspection. See installation requirements.
Outdoor tankless water heaters vent at the unit, no vent pipe needed.
Usually not. Most tank water heaters use a natural-draft metal vent, while tankless units use a sealed, positive-pressure vent with specific materials. Some manufacturers offer kits to run a tankless vent through an existing chimney chase. Follow the manual.
A unit that draws combustion air from outside and exhausts outside through a sealed system, either two pipes or one concentric pipe. It doesn’t use indoor air, which improves safety and efficiency.
It varies by model and pipe diameter, often from about 40 to over 100 equivalent feet, with each elbow counting as several feet. The manual lists the exact limits.
Only condensing models that the manufacturer approves for PVC, CPVC or polypropylene. Non-condensing units produce exhaust hot enough to damage PVC.
This guide is general information for U.S. homeowners, not professional advice. Gas, electrical and venting work must follow the manufacturer’s instructions and local codes, and in most areas requires a licensed plumber, electrician or gas fitter and a permit. Specifications and prices change; always confirm details on the product page and in the installation manual.
