Venting is where most tankless water heater installation mistakes happen. Wrong material, wrong configuration, wrong termination clearances, wrong vent length — any one of these creates a callback, a failed inspection, or a carbon monoxide hazard. This guide covers every venting decision on a gas tankless job — condensing vs. non-condensing material requirements, direct vent vs. power vent configurations, concentric vs. two-pipe systems, termination clearances, vent length calculations, and the most common mistakes that show up on inspection.
PlumbersCrib.com carries tankless water heaters from Rheem, American Water Heaters, Stiebel Eltron, and Intellihot. Contact our team with any technical questions.
The single most important venting rule
Always use the venting material specified in the manufacturer's installation manual for the specific unit you are installing. This is not optional and it is not interchangeable between brands or models. A venting material that is approved for one unit may not be approved for another — even from the same manufacturer. When in doubt, pull the manual before you pull the pipe.
The second rule: the type of unit — condensing or non-condensing — determines the venting material category. Using the wrong category of vent pipe is the single most common venting installation error and the one with the most serious consequences.
Condensing vs. non-condensing — why venting material is different
Non-condensing units exhaust at 300°F to 500°F. At these temperatures, only high-temperature metal venting is safe — typically Category III stainless steel concentric pipe. PVC, CPVC, and polypropylene cannot handle these temperatures and will fail — potentially catastrophically. Category III stainless steel is more expensive than plastic, both in material cost and installation time, but it is the only acceptable option for non-condensing exhaust.
Condensing units extract so much heat from the exhaust gases — including latent heat from water vapor condensation — that the exhaust exits at approximately 90°F to 120°F. At these temperatures, plastic venting materials (PVC, CPVC, polypropylene, or ABS solid core) are appropriate and significantly less expensive than metal. Category III stainless steel vent pipe costs approximately three to five times the per-foot price of Schedule 40 PVC — the material cost savings on a condensing installation can be substantial on a longer vent run.
Never use PVC on a non-condensing unit. PVC softens at approximately 140°F and will fail under 300°F to 500°F non-condensing exhaust — creating a fire hazard and a carbon monoxide risk. This is not a code nuance — it is a physical material limitation. If you are replacing a non-condensing tankless and the existing vent is metal, confirm the replacement unit type before reusing the venting.
Venting material summary
| Unit type | Exhaust temperature | Required venting material | Examples on PlumbersCrib |
|---|---|---|---|
| Non-condensing | 300°F – 500°F | Category III stainless steel — Metal Fab, ProTech, UBBINK | Rheem RTG Series (0.82 UEF) |
| Condensing | 90°F – 120°F | PVC, CPVC, polypropylene, or ABS solid core | Rheem IKONIC, American Adapt SC X3, Intellihot (0.93 – 0.96 UEF) |
Venting configurations — direct vent vs. power vent
Gas tankless units use one of two combustion air configurations — direct vent (sealed combustion) or power vent (indoor air combustion). Understanding the difference is essential for planning the installation and meeting code.
Direct vent — sealed combustion
A direct vent unit draws combustion air from outside the building through a dedicated intake pipe and exhausts through a separate exhaust pipe. Both pipes terminate at the exterior. Because the unit draws all of its combustion air from outside, it does not depressurize the mechanical room or compete with other combustion appliances for indoor air — eliminating one of the primary carbon monoxide risks associated with atmospherically vented appliances.
Most modern residential condensing tankless units are direct vent. The IFGC combustion air requirements of Section 304 do not apply to direct-vent sealed combustion appliances — a meaningful code simplification on jobs where multiple combustion appliances share a mechanical space.
Power vent — indoor air combustion
A power vent unit draws combustion air from the indoor space where it is installed and exhausts through a single pipe to the exterior. Only one pipe penetration through the exterior wall is required. On installations where running a second pipe for combustion air intake is difficult or expensive, power vent is the simpler configuration.
The trade-off is that power vent units depend on adequate indoor air volume for combustion. In mechanical rooms that share space with furnaces, boilers, or other combustion appliances, the aggregate combustion air demand must be calculated per NFPA 54 Section 9.3. Failure to do this is a named deficiency in many inspection findings. If the mechanical room is small or shared, confirm adequate combustion air before speccing a power vent unit.
Concentric vs. two-pipe venting
On direct vent installations, there are two ways to route the intake and exhaust pipes to the exterior — concentric (co-axial) or two separate pipes.
Concentric venting combines both the intake and exhaust into a single double-wall pipe — typically 3"/5" diameter — requiring only one penetration through the exterior wall. The exhaust runs through the inner pipe and the combustion air intake runs through the outer annular space. Concentric venting is faster to install, requires a single hole through the wall, and is visually cleaner on the exterior. Most manufacturers offer concentric vent termination kits for sidewall and roof termination.
Two-pipe venting runs the intake and exhaust as two separate pipes to the exterior — typically 2" or 3" pipe depending on the unit and run length. Two separate holes through the exterior wall are required. Two-pipe configurations allow longer individual vent runs and more flexibility in routing intake and exhaust to different wall locations. When using two-pipe, the intake and exhaust terminations must be at least 12 inches apart both vertically and horizontally to prevent the unit from drawing its own exhaust back in as combustion air.
Vent length — how to calculate maximum run
Every tankless manufacturer publishes a maximum equivalent vent length for each model — the total length of straight pipe plus the equivalent length added by each elbow. Always calculate the equivalent vent length before starting the job to confirm the planned route is within spec.
Each 90-degree elbow typically adds five feet of equivalent length to the vent run — confirm the exact value in the unit's installation manual, as this varies by manufacturer. Each 45-degree elbow adds less — typically two to three feet equivalent. The sum of all straight pipe runs plus all elbow equivalent lengths must not exceed the manufacturer's published maximum.
As an example using the Rheem RTG Series non-condensing units we carry — the maximum vent run is 43 feet with zero elbows. Each 90-degree elbow reduces the available run by 1.6 feet equivalent. Plan the vent route on paper before ordering pipe and fittings to confirm the run is within spec.
Condensing units allow longer runs. Because condensing units use lower-cost plastic pipe that is easier to work with and because the exhaust pressure requirements are different, most condensing manufacturers allow significantly longer maximum vent runs than non-condensing units. The Rheem IKONIC and American Water Heaters Adapt SC X3 both allow up to 150 feet of 3" pipe — more than three times the RTG's 43-foot maximum. On jobs where the mechanical room is far from an exterior wall, this is a meaningful installation advantage for the condensing spec.
Termination clearances
Vent termination clearances are specified in both the manufacturer's installation manual and the IFGC. The most common standard IFGC minimums are:
- 12 inches minimum from windows, doors, and other openings — to prevent exhaust from entering the building
- 12 inches minimum between intake and exhaust terminations on two-pipe systems — to prevent exhaust recirculation
- 3 feet minimum from inside corners — exhaust can accumulate in corners and re-enter the building
- Above grade — terminations must be above the highest anticipated snow line in cold climates; a blocked termination shuts down the unit
- Not above a walkway or public area — condensing unit exhaust produces visible vapor plume in cold weather and can deposit mineral residue below the termination
Always confirm manufacturer-specific clearances in the installation manual — some manufacturers specify clearances that exceed the IFGC minimums and those take precedence.
Vent slope
Horizontal vent runs must slope continuously toward the unit — typically a minimum of 1/4" per foot — so that condensate in the vent pipe drains back to the unit's condensate drain rather than pooling in the pipe. A vent run with a low spot that traps condensate will block the vent, cause nuisance shutdowns, and eventually damage the pipe. Check the slope with a level on every horizontal run before commissioning.
Most common venting mistakes
Wrong venting material for the unit type. Using PVC on a non-condensing unit or Category III metal on a condensing unit where the manual specifies plastic only. Both are wrong — pull the manual before pulling the pipe.
Exceeding maximum equivalent vent length. Not accounting for elbow equivalent lengths in the total run calculation. Always calculate equivalent length before starting the job.
Insufficient termination clearances. Terminating too close to windows, doors, or other openings — particularly common on retrofit jobs where the termination location is dictated by an existing hole in the wall.
Blocked termination in cold climates. Not accounting for snow accumulation at the termination point. A termination buried under snow shuts down the unit — use an approved above-grade termination kit rated for cold climate installation.
Inadequate combustion air on power vent installations. Not calculating aggregate combustion air demand on shared mechanical rooms with multiple appliances. This is a named NFPA 54 deficiency and a carbon monoxide risk.
Incorrect slope on horizontal runs. A horizontal run with inadequate slope or a low spot traps condensate, blocks the vent, and causes nuisance shutdowns. Check every horizontal run with a level.
Intake and exhaust too close together. On two-pipe systems, placing intake and exhaust terminations less than 12 inches apart allows the unit to draw its own exhaust back in — causing combustion problems and eventually triggering a fault code.
Frequently asked questions
Can I reuse existing metal venting on a tankless replacement?
Possibly — if you are replacing a non-condensing unit with another non-condensing unit and the existing metal venting is in good condition and compatible with the new unit's specifications. Confirm vent diameter, maximum run length, and manufacturer compatibility before assuming the existing venting can be reused. Never reuse metal venting for a condensing replacement — the manufacturer will specify plastic, and metal is not appropriate.
Can I vent a tankless unit through the roof?
Yes — both sidewall and roof termination are typically approved, subject to manufacturer specifications and local code. Roof termination requires an approved vent cap rated for the exhaust temperature and must be above the roof surface per the manufacturer's minimum clearance. Sidewall termination is more common on residential installations because it is easier to route and inspect.
What is the difference between 2" and 3" vent pipe?
Vent pipe diameter is specified by the manufacturer for each model and is not interchangeable. Larger diameter pipe allows longer maximum runs — many manufacturers specify 2" pipe for shorter runs up to 50 feet and 3" pipe for longer runs up to 150 feet. Confirm the required diameter and maximum run for your specific unit in the installation manual before ordering pipe.
Do I need a permit to install or replace a tankless water heater?
In most jurisdictions, yes — a permit is required for any water heater installation or replacement, including tankless. A permit triggers an inspection that verifies venting, gas connections, and electrical are correct. Pulling the permit protects you, the homeowner, and the warranty. Always confirm local permit requirements before starting the job.
How far apart do intake and exhaust terminations need to be on a two-pipe system?
A minimum of 12 inches both vertically and horizontally — to prevent the unit from drawing its own exhaust back in as combustion air. Confirm the manufacturer's specific requirement in the installation manual, as some specify larger minimum separations than the IFGC standard.
Shop tankless water heaters at PlumbersCrib.com
- Rheem IKONIC Series — 0.96 UEF condensing, PVC venting, up to 150 ft run
- Rheem RTG Series — 0.82 UEF non-condensing, Category III stainless steel venting, 43 ft max run
- American Water Heaters Adapt SC X3 — 0.95 UEF condensing, PVC venting, up to 150 ft run
- Intellihot commercial tankless — condensing, multiple venting configurations including common venting
Contact our team or call 877-881-5501 Monday through Friday with any venting or spec questions before you order.