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We’ve mentioned indirect waste piping, open site drains, and floor sinks in quite a few of our articles. However, never once have they received a detailed explanation of what they are. The following piece will end this blank once and for all. We will go into depth regarding definitions and applications. We will also explain why all the above are so important to the health and safety of our nation.

Indirect Waste / Indirect Drain

Let’s start with indirect waste or an indirect drain. To some degree, the name defines its function. Nonetheless, you need context to understand its main purpose. An indirect waste is a drainage piping that runs from a plumbing fixture or appliance and discharges into a separate waste pipe or receptacle. That waste piping connects to the building drainage system. The indirect waste line is not hard piped to the separate waste piping or receptacle. Instead, it discharges by way of an air gap or break. Please see below diagram to get a better understanding of how each is piped.

The indirect waste drainage piping begins at the fixture. It is being drained to its discharge point above the plumbing appurtenance that is directly piped into the drainage system. The end point of discharge from the indirect waste must be piped above the overflow rim of the fixture. The space between the overflow rim of the fixture and the end of the indirect discharge piping is called the air gap. This air gap allows any back-up waste water to spill out over the overflow rim of the plumbing fixture without ever touching the indirect waste discharge.

Indirect Drain’s Purpose

Indirect waste drain for commercial dishwasher

Which plumbing fixtures, appliances or equipment need to be discharged indirectly? Any drainage from the above referenced appurtenances used to store, prepare or serve food or equipment used to sterilize medical instruments should be discharged indirectly. Why? Those units must always be protected from a potential sewage backup even if there is a building main back-up.

Here are some examples of plumbing fixtures that can receive the discharge of indirect waste. These can be service or mop sinks, mob basins, floor drains, funnel drains, and floor sinks. Others such as a washing machine drain hose have their own appointed drain for direct waste.

Air Gap: Definition

An air gap used for an indirect waste is the separation between the outlet of the indirect waste and the overflow rim of the waste receptacle. Let me give you an example of how not to pipe an indirect waste. You have an AC unit with a condensate line above the floor and a service sink nearby. If there isn’t a floor sink close to the unit (we will explain the function of a floor sink further down the post), and you don’t have the space or budget to install one, you can pipe it to the nearby service sink.

Some definitions call for you to pipe the outlet of the indirect waste line above the drainage inlet. Although this is technically correct, it can be confusing. I’ve seen many indirect waste lines piped to service sinks where the outlet is piped 4” into the sink. The entire rim of the fixture is the “inlet” not the drain line opening at the bottom of the fixture.

If the indirect waste is piped below the overflow rim of the fixture, you run the risk of a backup contaminating the water supply. How you ask? If there is a back-up in the sink that rises higher than the outlet of the indirect waste and a negative pressure situation occurred, it is possible for the indirect waste to act as a siphon. This will pull waste up through the plumbing fixture to which it is attached.

Open-Sight Drains and What Is a Floor Sink?

Open sight drains or funnel drains are very similar to floor drains. The difference is that their overflow rim terminates above the floor. In fact, funnel drains are usually standard floor drains with an optional strainer that adds a funnel.

MiFab Floor Sink

MiFab Floor Sink

The reason they terminate above the floor line is to stop their use as a floor drain. If you were to terminate an in-direct waste to an open-site drain that was also used as a floor drain, that drain could easily become clogged with dirt and trash. An indirect waste producing a fair amount of discharge can create a dangerous if not toxic environment.

Jr. Smith Funnel Drian

Jr. Smith Funnel Drian

A floor sink is a receptacle usually square. It is usually lined with epoxy or even porcelain that receives the discharge of one or more indirect waste lines. It is piped flush with the floor, and it contains a strainer in the center which captures any potential debris. The existence of these plumbing systems is in place to prevent nightmares such as basement flooding from happening.

Final Word

When piping indirect waste lines to either an open site drain or a floor sink, an air gap is required to again provide positive separation from the directly connected drainage inlet. Hopefully, this article helped you to understand the functions of some crucial elements in the plumbing world. If you have anything to add up, you are welcome to leave a comment below.

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