Quick Answer: Tankless water heater venting comes down to one fact: condensing units (0.90+ UEF) exhaust at roughly 100–140°F and vent in PVC, CPVC, or polypropylene; non-condensing units exhaust at 300°F+ and require Category III stainless steel (commonly AL29-4C-rated). Rinnai’s own condensing lineup is rated for up to 65 equivalent feet in 2-inch PVC, with each 90-degree elbow counted as 6 equivalent feet and each 45-degree elbow as 3 feet. Concentric (single-penetration) and two-pipe (twin-pipe) are the two routing options, and termination needs to clear windows and doors — commonly 12 inches for a direct-vent sidewall termination, more for non-direct-vent setups. Get the material or clearance wrong and you have a code violation and a carbon monoxide risk, not just an ugly install.
Every brand roundup on this site talks about GPM, BTU, and UEF. Venting rarely gets the same attention, but it decides how much your install actually costs and whether an inspector signs off on it — and unlike most tankless decisions, it isn’t really a matter of preference. The manufacturer’s manual for your specific model sets the material, diameter, and length limits; this guide explains why those rules exist and what the real hardware looks like.
Why venting material depends on the heater, not your preference
A non-condensing tankless heater extracts heat from combustion gas once, then sends it up the vent still hot — commonly 300°F or higher. Plastic pipe at that temperature would soften or deform, so non-condensing units are vented in metal, almost always Category III stainless steel rated to the AL29-4C spec used across the industry.
A condensing tankless heater runs the exhaust through a second heat-exchange stage, wringing out extra efficiency (that’s the difference between roughly 0.81–0.82 UEF non-condensing and 0.90–0.98 UEF condensing) and dropping flue-gas temperature to around 100–140°F in the process. That’s cool enough for PVC, CPVC, or polypropylene, all of which cost meaningfully less than stainless pipe and fittings.
| Unit type | Flue-gas temp | Required vent material | Typical UEF |
|---|---|---|---|
| Condensing | ~100–140°F | PVC, CPVC, or polypropylene (Schedule 40, solid-wall) | 0.90–0.98 |
| Non-condensing | 300°F+ | Category III stainless steel (AL29-4C) | 0.81–0.82 |
That efficiency gap is exactly why our cost breakdown shows condensing gas units running a bit higher upfront but cheaper to vent than the metal-vent alternative over a full install.
How far you can actually run it
There’s no single “max vent length” for tankless water heaters — it’s a function of pipe diameter, material, and how many elbows are in the run, and it’s spelled out in a manufacturer equivalent-length table specific to each model.
Rinnai’s published guidance for its condensing lineup, run in 2-inch Schedule 40 PVC, allows up to 65 equivalent feet on both the combustion-air intake and the exhaust pipe. Elbows count against that total: a 90-degree elbow equals 6 equivalent feet, a 45-degree equals 3. A run with four 90-degree elbows has already used 24 of those 65 feet before accounting for a single foot of straight pipe. Stepping up to 3-inch or 4-inch PVC/CPVC extends the allowable distance considerably and is also what’s approved for common venting — routing multiple units into one shared vent stack, common in multi-unit or commercial installs.
| Configuration | Typical diameter | Elbow cost | Notes |
|---|---|---|---|
| Single-unit condensing (PVC) | 2 in. | 90° = 6 ft, 45° = 3 ft | Up to 65 equivalent ft per pipe on Rinnai's condensing series |
| Common-vented condensing (PVC/CPVC) | 3–4 in. | Manufacturer table | Multiple units sharing one vent stack |
| Non-condensing (stainless) | Model-specific | Manufacturer table | Generally shorter runs than condensing PVC systems |
Treat every number above as an example, not a rule to copy onto a different brand or diameter — pull the actual venting table for your model before you buy pipe.
Concentric vs. two-pipe: which routing to use
| System | Wall/roof penetrations | Best for | Tradeoff |
|---|---|---|---|
| Concentric (coaxial) | 1 | Heater close to an exterior wall | Cleanest install, but shorter effective run |
| Two-pipe (twin-pipe) | 2 | Longer runs, flexible routing | More material, two penetrations to seal and flash |
Concentric venting nests the exhaust pipe inside a larger intake pipe so both combustion air and exhaust exit through one opening — the simplest install when the unit sits right against an exterior wall. Two-pipe venting splits intake and exhaust into separate parallel runs, which costs a little more in fittings and requires sealing two penetrations, but it’s the better choice when the heater sits farther inside the house or the intake needs to pull air from a different spot than the exhaust exits. Most current condensing models support either configuration on the same unit — this is an installer’s routing decision, not a reason to choose one brand over another.
Termination clearances: the part inspectors check first
Vent termination location is one of the most common reasons a tankless install fails inspection. The general rule manufacturers publish: a direct-vent installation (sealed combustion, pulling its own outside air through the intake pipe) needs roughly 12 inches of clearance from an operable window or door. A configuration that instead draws combustion air from inside the mechanical room needs much more — commonly 4 feet of clearance below or beside any opening that can be operated.
Rheem RTG20151D-1 Concentric Vent Kit
- Combines intake and exhaust into one wall or roof penetration — the simplest termination when the unit sits against an exterior wall.
- Purpose-built for Rheem's RTG-95DV Series condensing tankless heaters; check your model's approved-vent list before ordering, since concentric kits aren't universal across brands.
- Reduces the number of penetrations to flash and seal to exactly one, which is the single biggest source of inspection callbacks on a two-pipe install.
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2. Schedule 40 PVC Vent Pipe & Fitting Kit — Best for Condensing Units
2-Inch Schedule 40 PVC Vent Pipe and Fitting Set
- The material Rinnai and most other condensing-unit manufacturers specify by name — confirm solid-wall Schedule 40, not cellular-core (ASTM F891), which looks similar but isn't rated for the same service.
- Works for a straightforward two-pipe run; budget your equivalent-foot count against your model's table, remembering that each 90-degree elbow costs roughly 6 equivalent feet.
- Meaningfully cheaper than stainless steel vent pipe of the same length — one of the real cost advantages of buying a condensing unit in the first place.
- Never substitute cellular-core CPVC, Radel, or ABS pipe even if it fits the same fitting sizes — manufacturers explicitly exclude those materials.
3. Z-Flex Z-Vent Stainless Steel Vent Pipe — Best for Non-Condensing Units
Z-Flex Z-Vent AL29-4C Stainless Steel Vent Pipe
- The required material class for non-condensing tankless units, whose flue-gas temperature would damage plastic pipe.
- AL29-4C stainless is the industry-standard alloy for pressurized, high-temperature gas-appliance venting — the same rating you'll see called out on manufacturer-specific stainless vent kits.
- Costs more per foot than PVC, which is part of the real total-cost tradeoff between condensing and non-condensing units beyond the sticker price of the heater itself.
- Confirm diameter and length against your specific model's venting table — non-condensing systems typically allow shorter total runs than a comparable condensing PVC install.
4. Rheem RTG20210 Horizontal Termination Kit — Best for Sidewall Venting
Rheem RTG20210 Horizontal Termination Kit
- A dedicated termination fitting for horizontal (sidewall) venting rather than a straight-through wall penetration — matters for hitting the 12-inch window/door clearance manufacturers specify for direct-vent sidewall terminations.
- Purpose-matched to Rheem's RTG-95DV Series; check compatibility before ordering if you're on a different brand or series.
- A mismatched or generic termination cap is a common reason inspectors flag an otherwise-correct vent run — using the manufacturer-listed termination avoids that entirely.
How to choose
- You have (or are buying) a condensing unit (0.90+ UEF): Schedule 40 PVC, CPVC, or polypropylene — confirm solid-wall, not cellular-core.
- You have a non-condensing unit, or aren’t sure: stainless steel, AL29-4C-rated, Category III. Check your manual — this isn’t optional.
- The heater sits right against an exterior wall: a concentric kit gives you one penetration and the simplest install.
- The heater sits farther inside, or needs intake air from elsewhere: a two-pipe setup, sized against your model’s equivalent-length table.
- You’re terminating near a window or door: confirm your exact clearance in the installation manual — 12 inches for direct-vent sidewall is common, but non-direct-vent configurations need roughly 4 feet, and local code can require more than either.
- Still choosing the heater itself: see our best gas tankless water heater roundup — condensing vs. non-condensing is called out on every pick, and it’s the decision that determines everything in this guide.
The bottom line
Tankless water heater venting isn’t a style choice — it’s set by whether your unit is condensing or non-condensing, full stop. Condensing units vent in PVC, CPVC, or polypropylene and, per Rinnai’s published tables, can run up to 65 equivalent feet in 2-inch pipe with elbows costing 6 (90°) or 3 (45°) equivalent feet apiece. Non-condensing units require Category III stainless steel rated for 300°F+ flue gas. Concentric kits simplify a wall-adjacent install to one penetration; two-pipe setups handle longer or more flexible runs. Whichever you’re installing, pull the manufacturer’s specific venting table before buying pipe, and don’t skimp on termination clearance — it’s the detail inspectors check first, and the one that matters most for carbon monoxide safety.