What is a plumbing dye test?

It shows whether water is leaking from the tank into the bowl, usually past a worn flapper. The EPA's WaterSense program gives the method: "Identify toilet leaks by placing a few drops of food coloring in the toilet tank. If any color shows up in the bowl after 10 minutes, you have a leak." It adds to flush right after the test so the color doesn't stain the tank.

Step by step:

1. 1Take the lid off the tank and add a few drops of food coloring or a dye tablet.
2. 2Don't flush. Wait 10 minutes.
3. 3Look in the bowl. Any color means water is getting past the flapper or seal.
4. 4Flush to clear the dye.

This leak is easy to miss because it's silent. The EPA says "An old or worn flapper can cause your toilet to flush on its own or silently leak thousands of gallons a year." In a building with dozens of toilets, a dye tablet in every tank is a fast way to find the bad ones. See how to replace a toilet flapper, and if the dye test passes but the toilet still runs, why a toilet leaks for reasons besides the flapper.

To find pipes connected to the wrong system. Most cities have two separate networks: the sanitary sewer, which goes to a treatment plant, and the storm drain, which often goes straight to a wash, river, or retention basin. A sink or shop drain tied into the storm drain sends sewage or chemicals into the environment. A roof drain or sump pump tied into the sanitary sewer floods the sewer during storms.

A guidance manual prepared for the EPA says "Dye testing is an excellent indicator of illicit connections and is conducted by introducing non-toxic dye into toilets, sinks, shop drains and other plumbing fixtures." Crews then check the manholes downstream. The manual is blunt about what a positive result means: "The discovery of dye in the storm drain, rather than the sanitary sewer, conclusively determines that the illicit connection exists."

The results can be striking. In a Michigan program where more than a thousand buildings were dye tested, the manual reports "Illicit connections were found at 60% of the automobile-related businesses tested." That's why commercial buildings with shop drains, wash bays, and floor drains get tested more often than homes.

Cities also dye test in the other direction. The City of Solon, Ohio, describes it as "a method used to locate points where rain or ground water improperly enters the sanitary sewer system." Dye goes into a downspout or yard drain, and the city says "The downstream sanitary sewer manhole is then monitored for the presence of dyed water, which, if found, indicates a cross-connection between storm water drains and sewer pipes." Our page on how commercial roof drains and overflow drains must be piped covers where roof water is supposed to go.

### What about Phoenix?

Phoenix uses dye testing to check sewer taps. City Code 28-28 says "A customer who has been connected to the public sewer and questions whether a sewer tap is in service may apply to the Department for a sewer dye test to be performed." That's useful when a building's sewer connection is in doubt, such as an older property or a lot that may once have been on septic. Our page on whether you're on septic or city sewer in Phoenix covers other ways to tell.

Dye goes into the house drains, then someone watches the yard, the drainfield, and nearby ditches or water for color. If dye surfaces, sewage is reaching the surface. That's a clear failure.

The catch is that a clean result proves very little. Pennsylvania's environmental agency, in its training for sewage enforcement officers, says "Dye is a tracing agent; it will only identify a surface malfunction." It also says "Dye testing cannot confirm the absence of a malfunction." A drainfield can be failing underground, backing up, or leaking into groundwater without dye ever appearing at the surface. Thurston County, Washington, makes the same point: "While dye tests do not find all possible problems, they are especially useful for locating seeping sewage on properties located on or near the shoreline."

So a dye test is not a septic inspection. In Arizona, a property on septic needs a full inspection by a qualified inspector before it's sold. See what a septic inspection covers when you buy a home in Arizona.

Mostly for tracing drains inside and around a building:

• Which drain goes where. In an old building with several sewer laterals, dye in a fixture shows which cleanout or manhole it reaches.
• Confirming a repair. The EPA-prepared manual says "For discharges resulting from internal plumbing and lateral connections, dye testing can confirm the correction."
• Finding where a leak surfaces. Dye in a suspect drain can show whether water appearing in a wall, ceiling, or yard came from that line.
• Tenant improvements. Before a new tenant ties in, dye confirms that floor drains and sinks reach the sanitary sewer and not a storm line.

When the dye never shows up, that's information too. The manual says "The investigation is not complete until the dye is found," and one of the reasons it gives for missing dye is that "The building is actually hooked up to a septic system."

### Dye test or smoke test?

They answer different questions. A dye test follows water and proves a flow path. A smoke test fills the drain and vent system with smoke and shows where air and sewer gas escape: cracked pipes, dry traps, open connections. For a sewer gas smell with no obvious source, smoke testing usually finds it faster. For "does this drain go to the sewer or the storm drain," dye is the tool.

The tracer dyes made for this work are. One major maker, Kingscote Chemicals, says its Bright Dyes "are safe, non-toxic, biodegradable, and certified by NSF International to ANSI/NSF Standard 60 for use in drinking water." Not every colored product qualifies, though. Pennsylvania's handout warns "Some dyes may be toxic so only use dyes that have NSF certification (NSF.org)." Food coloring is fine for a toilet tank at home. For tests that reach storm drains or the ground, use a certified tracer dye. The product's safety data sheet also notes the tablets "May cause an allergic skin reaction," so wear gloves.

HQ Plumbing & Air traces drains with camera inspections and liquid-smoke testing, finds leaks and sewer gas, and repairs what the tests turn up for businesses and homes across the Phoenix area. We're licensed and insured, with free estimates and upfront pricing. See our commercial plumbing services.

Dye in, watch for color. It tells you where the water goes, and nothing more, which is often exactly what you need to know.