Tankless Water Heater Venting, Explained

A non-condensing tankless unit exhausts flue gas while it is still hot, well above the temperature at which water vapor in the exhaust condenses back into liquid. That flue gas is hot enough, and corrosive enough over time, that only Category III stainless steel vent pipe is rated to handle it safely at the positive pressure these units push it through. Plastic pipe simply is not rated for that combination of heat and pressure, and using it on a non-condensing unit is a certified path to vent failure and combustion gas leaking where it should not.

A condensing unit works differently: a secondary heat exchanger extracts additional heat from the flue gas before it exits, cooling it enough that water vapor in the exhaust does condense, which is also where the unit gets its extra efficiency. Because the exhaust leaving a condensing unit is cool enough, PVC, CPVC or polypropylene vent pipe is rated for it, materials that would fail immediately on a non-condensing unit's hotter exhaust. That condensate itself is mildly acidic and has to be collected and neutralized before it reaches a drain, which is why condensing installations include a condensate neutralizer as standard equipment rather than an accessory.

Both approaches are sealed combustion, meaning the unit draws its combustion air from outside the building rather than from the room it sits in, which is safer and more efficient than an atmospheric unit pulling combustion air from indoor space. Concentric venting achieves this with a single pipe-within-a-pipe assembly through one wall or roof penetration: exhaust travels out through the inner pipe while fresh combustion air is drawn in through the outer annular space around it, in the same physical opening. Two-pipe venting instead uses two entirely separate pipes, one dedicated to exhaust and one to intake, which can terminate at different points if the installation calls for it.

Concentric venting is generally simpler to install where a single clean penetration is available and the termination location works for both intake and exhaust together. Two-pipe venting offers more routing flexibility when the intake and exhaust need to terminate in different locations, at the cost of two penetrations instead of one. Which is appropriate for a given installation depends on the unit's certification and the specific building layout, decisions that belong to the installing gas fitter.

Every unit's installation manual publishes a maximum equivalent vent length, expressed in feet, that the total vent run cannot exceed while still moving exhaust and, on sealed combustion units, intake air reliably. Straight pipe counts against that budget at something close to its actual length, but every elbow in the run counts for meaningfully more than its physical length, because a bend adds resistance to flow that a straight run of the same length does not. A run with several elbows to route around framing or clear an obstruction can hit its maximum equivalent length with far less physical distance covered than a straight run would.

This is why counting elbows matters just as much as measuring feet of straight pipe when a vent run is being planned, and it is also why substituting more elbows for a straight path someone thought was cleaner can silently push a run past its rated maximum even though the physical footprint looks shorter. Exceeding the manufacturer's maximum equivalent length is not a minor inefficiency; it can prevent the unit from venting reliably at all.

A vent termination cannot simply exit the nearest exterior wall wherever is convenient. Manufacturer installation manuals, working from requirements in the International Fuel Gas Code and similar plumbing and mechanical codes, specify minimum clearances from a vent termination to windows and doors that open, since combustion exhaust exiting near an operable opening can be drawn back indoors. Similar clearances apply to soffits and other openings into the building envelope, to adjacent property lines, to the ground or a walking surface to avoid exposure at head or hand height, and to other vent terminations or air intakes on the same or an adjacent building.

These clearances vary by unit and by local code amendment, which is exactly why they belong in the installation manual and to the professional doing the installation rather than to a general guide. A vent that technically reaches outside is not the same thing as a vent that terminates in a location the manufacturer's listing and local code actually permit.

Vent parts must be listed for the specific unit. These are common accessory pieces referenced by installation manuals; verify compatibility against your unit's own manual before buying.