The defining characteristic of an indirect waste flow receptor is a space, called an “air gap,” between the end of the pipe flowing into it and the receptor. The minimum air gap specified is usually twice the diameter of the incoming pipe, but a minimum of one inch. It is required by building codes to prevent the flow of sewage backwards up a pipe when a vacuum is created in the pipe by lack of water pressure.

    A common example of an indirect waste receptor in a home is a drain pipe for a water softener system that dumps into a laundry sink, with the pipe secured so that the end is elevated one inch above the overflow edge of the sink. Indirect waste receptors can also be used for the piping from a water heater drain pan, and typically consist of a one inch gap between the drain pipe pointed downward into a funnel-shape receptor beneath it that connects to a home’s drain/waste system. 

Here’s an example below of the wrong way to drain the flush water from a water softener system.

    The standpipe behind a washing machine can also be used as an indirect waste receptor, but maintaining the required one-inch gap can be difficult if the pipe is shared with a washing machine drain hose. The fitting shown at the top of the page is designed to solve the problem, and can be found at www.airgap.com. The one fixture that cannot be used for an indirect waste receptor is a toilet.

    The air gap device required in many jurisdictions for a dishwasher is another example of an indirect waste receptor, designed to prevent backflow of sewage into a dishwasher. It is mounted near the edge of the sink next to the faucet. A close-up of one is shown below. Some areas allow a loop in the dishwasher drain hose that is secured to the underside of the counter, called an “air gap loop,” as an alternative.

Here’s links to a collection of our other blog posts about PLUMBING:

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• Why is the vacuum breaker at my hose faucet leaking, gushing water, or making funny sounds?

• Why is the cover plate of some water meter boxes in the ground painted purple?

• Why is it a problem when a trap under a sink is installed backwards? 

• What are the code requirements for plumbing vent terminations?

• What are the code requirements for layout of drain piping under sinks?

• Why is there sand in the bottom of my toilet tank?  

• What causes a gurgling sound when a bathtub or sink drains? 

• Are drop-in toilet bowl cleaner tablets safe?

• What can I add to my septic tank to help it work better? 

• Why do septic tank contractors want you to get rid of your kitchen disposal?

• Why is there mold inside my toilet tank?  

• What are the pros and cons of a wall-mounted toilet?

• Does a refrigerator water supply line require a shutoff valve behind it?

• Which plumbing fixtures require water shut off valves in a home? 

• What do the ABS, PVC, CPVC, PB, and PEX plumbing pipe names mean?

• Why is a backflow preventer required on lawn sprinkler systems? 

• How can I locate my septic tank?

• Is a hot water faucet handle required to be on the left? 

• Can you live in a house while the plumbing is being replaced?

• What is the loose wire sticking out of the ground under the gas meter for? 

• Why is sunlight exposure bad for PVC pipe?

• Why is the European-style bottle trap not approved by the plumbing codes in the U.S.? 

• Why can't PVC be used for water pipe inside a house?

• What are the common problems to look for when the plumbing has been replaced in a house? 

• What's that powdery crust on the pipe connections at the water heater? 

• Should I call a plumber or septic tank contractor when my septic tank backs up into the house? 

• How do I get rid of the sewer gas smell in my house?  

• What are the pipes on my roof? 

• Should I wrap the water heater with an insulation blanket? 

• My water bill went way up last month. How do I look for a leak?

• Why are shower water valves all single-handle nowadays? 

• Why does the water have a rotten-egg smell in some empty houses? 

• What is an "S-Trap" under my sink? Why is it a problem? 

• What is a P-trap?

• Where is the septic tank? Are you going to inspect it? 

• What does polybutylene pipe look like? Why is it a problem?

• Which water pipes are an insurance problem and possibly uninsurable? 

• Does a home inspector check the plumbing under the floor slab?

• Is it alright to disconnect the washing machine drain from the septic tank and divert it to the ground in the yard?

     Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.