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Water Softener Drain Line and Air Gap Basics

What the drain line carries, why an air gap matters, and which parts are involved; local code governs the details.

By Softenry Editorial · Updated October 2026

The short answer

The drain line carries regeneration water to a drain; an air gap keeps drain water from being drawn back. Local plumbing code sets the requirements.

What it does

During regeneration the valve sends backwash and brine rinse water to the drain line.

Air gap

An air gap is a physical break between the drain line and the drain that prevents back-siphoning. Air-gap fittings and flow controls: AIR GAP INTERNATIONAL AG140-001 Drain Line Flow Control, AIR GAP INTERNATIONAL AG140-002 Drain Line Flow Control.

Code

SafetyDrain and air-gap requirements are set by local plumbing code and the maker's manual; use a licensed plumber where required.

Products mentioned

Softener part · AIR GAP INTERNATIONAL

AIR GAP INTERNATIONAL AG140-001 Drain Line Flow Control

Softener part

AIR GAP INTERNATIONAL
Softener part · AIR GAP INTERNATIONAL

AIR GAP INTERNATIONAL AG140-002 Drain Line Flow Control

Softener part

AIR GAP INTERNATIONAL

Questions readers ask

Does a water softener need an air gap?
Plumbing codes commonly require one on the drain connection; check yours.
How far can a softener drain line run?
Follow the maker's limits for length and height.