Phoenix Hard Water Guide

Water softener drain line: size, air gap and where it can go

Every salt-based softener rinses itself with water that has to go somewhere. The drain line is a small part of the install, but it is where code questions and many leaks come from.

Does a water softener need a drain?

Yes. A salt-based softener flushes brine through its resin and rinses the hardness away down a drain. Our guide to how often a softener regenerates explains the stages. Morton says a floor drain close to the softener is preferred, with a laundry tub or standpipe as other options. Pentair lists a floor drain, sewer trap, utility sink, vent stack or dry well, “depending on local plumbing codes”.

Where should a water softener drain?

Close to the unit, and into a receptor your code accepts. If you are planning an install, the drain is one of the three things the spot needs, with the main line and a power outlet; see water softener installation in Phoenix. On a pre-plumbed water softener loop, check whether the builder left a standpipe or drain next to the stubs.

Drain line size, length and height

Figures vary by brand, so follow your own manual. What the manuals we read say:

Maker Minimum size Longer runs Height
Pentair whole-house softener 1/2 inch inside diameter Avoid lines over 25 feet —
Fleck and Clack control valves 1/2 inch 3/4 inch beyond 20 feet, or if backwash is over 7 gallons a minute —
WaterBoss 1/2 inch inside diameter, “cannot be reduced” 3/4 inch at 25 feet or longer Up to 8 feet with at least 40 psi; the end must be no higher than the valve
Morton 1/2 inch tubing for a rigid run Avoid runs over 30 feet No more than 8 feet above the floor

Kinks, crushed tubing and a line raised too high all restrict the flow. WaterBoss lists a restricted drain line as a cause of salty water and of a brine tank that fills with water, and Morton says it can stop the softener drawing brine, so route the tubing where it won’t be stepped on or pinched. Where local code bans flexible hose, Morton says to use a rigid drain run.

The air gap for water softener discharge

The drain line must not go into the drain. It ends above it, with open air between, so that dirty water can never be siphoned back into the softener and your supply. Manufacturers’ figures:

  • Morton, Whirlpool and GE: an air gap of 1-1/2 inches between the end of the hose and the drain.
  • WaterBoss: at least 2 inches above the flood rim of the drain.
  • Fleck: twice the drain line’s diameter, and never less than 1 inch.
  • Pentair: “Most local codes require an air gap.”

As an example of a code rule, the Minnesota Department of Health explains that its state code requires an air gap of twice the discharge pipe’s diameter, or at least 1-1/2 inches. That is Minnesota’s code, not Arizona’s, but it shows what an inspector looks for: no line piped straight into a drain pipe, and no line pushed into a floor drain or sink.

How much water does a water softener discharge?

Penn State Extension and University of Nebraska–Lincoln Extension both estimate about 50 gallons per recharge, which Penn State says may or may not overload a septic system. Newer, more efficient softeners can use less: the EPA’s WaterSense program notes that the NSF/ANSI 44 standard has a voluntary efficiency rating requiring 5.0 gallons of water or fewer per 1,000 grains of hardness removed.

A hardness setting that’s too high, or a softener that’s too small, means more recharges and more water down the drain; check the hardness setting for your city and work out your size.

Water softener and septic system

If your home is on a septic system, the discharge goes into it unless it is routed elsewhere. The sources don’t fully agree:

  • The EPA says softeners and similar systems “may overload your septic tank with salt water”, which can disturb how solids settle and push them into the drainfield. It advises checking that the two are compatible.
  • University of Minnesota Water Resources Center says older studies found harm, but more recent work shows discharge from more efficient softeners doesn’t harm septic tanks. Timer-based softeners and ones set wrongly may.
  • Texas A&M AgriLife Extension advises against routing softener backwash into the pretreatment part of the system, suggesting the final pump tank, the drainfield, or another method your local permitting authority suggests.

We found no official definition of a “septic-safe water softener”. What the sources point to is efficiency: a demand-initiated (metered) softener set to the right hardness recharges less often. Ask your septic permitting authority before connecting a softener to a septic system.

Water softener drain line outside or into a dry well

Running the line outdoors, onto the ground or into a dry well, is a local code question. Keep in mind what’s in the water: University of Arizona Extension says sodium from softeners can build up in soil and damage its structure. Outdoors, the drain line also has to be protected from freezing, which WaterBoss stresses. Our guide to installing a water softener outside in Arizona covers the rest.

A discharge pump

If no drain is within the height limits above, a pump can lift the water to one. None of the manuals we read covers discharge pumps, so ask a licensed plumber.

Phoenix rules on softener discharge

We found no ordinance in Phoenix or other Valley cities that restricts softener discharge. Even so, local plumbing code and your city decide where a drain line may end, whether you need a permit, and what’s allowed outdoors. Check with your city’s building department before you run a new drain line.

Valley cities do pay attention to the salt. Scottsdale says high salinity in its wastewater is “caused in part by salt-based water softeners” and is not removed by traditional treatment, and it ran a two-year pilot from July 2014 offering rebates to remove salt-based softeners. Its later removal rebate, $125 a year for two years, closed to applications on March 31, 2025. At a 2011 Arizona Legislature study committee on water salinity, City of Phoenix Water Services told members that salt from softeners made up about 10 percent of the dissolved solids at wastewater treatment plants, according to the meeting minutes.

Where these facts come from

Common questions

Do you need a drain line for a water softener?
Yes, for a salt-based softener. Each recharge sends rinse water and the removed hardness down the drain, and every manual we read asks for a drain close to the unit: a floor drain, laundry tub, standpipe or similar, as local code allows.
What size drain line should I use?
At least 1/2 inch inside diameter, as Pentair, Clack, Fleck and WaterBoss all say. Go up to 3/4 inch for long runs: Clack and Fleck say beyond 20 feet, WaterBoss says at 25 feet or more. Follow your own model's manual.
Can a water softener drain into a septic tank?
Extension services disagree on how much harm it does. The EPA says softeners may overload a septic tank with salt water and advises making sure the two are compatible. Ask your septic permitting authority before connecting one.
Can you drain a softener outside?
Only where your local plumbing code and city allow it. Pentair lists a dry well among possible drains, depending on local codes. Salty rinse water on the ground can also harm soil and plants, so check with your city's building department first.