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Where a water softener drains, and the air gap the code requires

Updated October 30, 2026 · 2-minute read · Old House Sewer Lines

A water softener drain must end through an air gap: open space between the end of the softener's discharge hose or pipe and the drain that receives it. Section 608.4 of the Plumbing Code of New York State applies that rule to waste discharge pipes from water softeners and similar treatment equipment. The hose may not be pushed down into the drain or sealed to it.

The code text is short. "Overflow, relief valve and waste discharge pipes from such appliances and devices shall terminate through an air gap," §608.4 reads (NYSPC 2020, Chapter 6). The 2025 edition of the state plumbing code has been in force since December 31, 2025; the summary of what changed for sewer work is in the 2025 plumbing code and sewer rules.

How big the gap must be

Chapter 8 sets the size. Indirect waste must discharge through an air gap or air break into a trapped and vented receptor, and under §802.3.1 the air gap must be not less than twice the effective opening of the indirect waste pipe (NYSPC 2020, Chapter 8). For example, a discharge line with a half-inch opening needs at least an inch of open air between its end and the flood rim of the receptor. The receptor itself must have a trap and a vent, which rules out an untrapped hole in the floor.

Reverse osmosis drinking-water units follow a parallel rule. Section 611.2 requires their discharge to enter the drainage system through an air gap, or through an air-gap device that meets NSF 58 or CSA B483.1.

Receptors and smells

A trapped floor drain or laundry standpipe is a common receptor. Floor drains that see little water can dry out, and a dry trap lets sewer gas into the basement. North Dakota State University Extension suggests refilling rarely used drains with roughly a quart of water once every three or four weeks (NDSU Extension). If a softener's drain point smells like sewage, the trap is the first thing to check; see sewer gas smell in the house. A rotten-egg odor that comes only from the hot tap is a different problem with a different cause, covered in rotten-egg smell: sewer, gas or water heater.

What does not belong in the sanitary line

A softener's regeneration water goes to the sanitary drain through its air gap. Storm water does not: §1101.3 of the state code says storm water shall not be drained into sewers intended for sewage only. That is the rule behind town inspections for sump pump and roof leader hookups; see roof leaders and sump pumps in the sewer.

Homes on septic systems raise a separate question about brine. University of Missouri Extension, citing older University of Wisconsin and NSF studies, found no harm to a normally working septic system from softener regeneration (University of Missouri Extension, archived). Most homes in the towns this site covers are on public sewer.

Key facts

  • Softener waste discharge pipes must terminate through an air gap under §608.4 (NYSPC).
  • The air gap must be at least twice the effective opening of the waste pipe, §802.3.1 (NYSPC).
  • Indirect waste goes into a trapped and vented receptor (NYSPC).
  • Reverse osmosis units drain through an air gap or an NSF 58 or CSA B483.1 device, §611.2 (NYSPC).
  • NDSU Extension: a quart of water poured in every three or four weeks keeps a seldom-used trap sealed (NDSU Extension).

Sources