What Is a Cross-Connection in Plumbing?

A cross-connection is the point in the plumbing system where potable drinking water comes in physical contact with non-potable water. In other words, when safe water comes in contact with dirty water, it is referred to as a cross-connection in plumbing.

There are several reasons as to why cross-connection in plumbing may happen. The following sections will cover these reasons and also provide explanations for why cross-connection in plumbing is dangerous:

How Does a Cross-Connection Occur?

There are two primary ways in which a cross-connection in plumbing may occur. These are called backflow and back-siphonage in the system. Essentially, both processes occur because of a negative or positive pressure which causes the water to flow in the opposite direction than what was intended:


A cross-connection can occur when there is a backflow of water where it is not intended. The water in this case begins to flow in the opposite direction than what it was supposed to. This tends to happen due to positive pressure or excess in one place.

For instance, the water from the municipal distribution system is meant to flow to an individual residential unit. However, when a backflow happens, the reverse movement of the water can take place.

Cross-connection not only threatens to contaminate your own drinking water, but if left unattended, there is also a chance the public distribution system may become impacted.

Some common examples of a backflow occurring or a plumbing system being prone to cross-connection is the laundry tub, as well as some kinds of bathtubs. If the laundry tub has a plumbing fixture below the tub’s peripheral area, then chances are that when the tub fills up, even the faucet will get submerged.

This, in turn, might mean that the wastewater from the tub will flow back into the faucet and contaminate somebody’s drinking water, especially if the supply pipe is being drained.

Ideally, the faucet should be above the water tub and there needs to be an air gap between the tub and the faucet in question. Bathtubs have the same problem and it is advisable to provide an overflow outlet right below the faucets so there is no chance of any backflow.

In short, if there are washing machines, dishwashers and other appliances that draw water for cleaning or function, then ensure that there is enough air gap so that there is no possibility of a backflow.

Back Siphonage

Back siphonage is when the normal flow of water is reversed because of negative pressure or vacuum. If there is water stoppage due to other applications in the vicinity, the vacuum created in the pipe may cause a back siphonage where contaminated and non-potable water may be pulled back into potable or drinking water.

For example, when you are drinking out of a straw, you pull the liquid through the straw, creating a vacuum into which the water flows. The act of pulling on the straw causes the water to flow in the opposite direction.

When such a vacuum is created in a public water system or in an individual unit, it may cause contaminated and non-potable water to flow in the wrong direction. Essentially, the air pressure in the plumbing system is disbalanced, and the negative (or positive pressure, in the case of backflow) causes the water to flow in the direction opposite from what is intended.

Why Is Cross-Connection Dangerous?

Cross-connection is dangerous as it can lead to a severe health crisis if left unchecked. If the potable drinking water becomes contaminated with waste, it can lead to several diseases and may even cause an epidemic if the contamination is too severe.

For example, if there is a cross-connection between the wastewater coming out after the washing cycle ends, and the clean water supply, you could risk consuming water contaminated with detergent and whatever other bacteria and dirt may have come out of the clothes.

Additionally, cross-connection could also mean that the water may get contaminated with corrosive substances such as carbon dioxide and other types of acid. This makes the water entirely unfit to drink.

What is troublesome, however, is that if the contamination is not immediately detectable it may even go on for some time before you realize the quality of the water has been compromised.

What Is a Cross-Connection Control Device?

A cross-connection control device is a mechanical device that creates a physical barrier between potable and non-potable water. It will stop the backflow of dirty water into clean, potable drinking water.

Typically, devices of this kind create a vertical air gap. This is simply an open space with no moving parts that will help prevent accidental backflow. It is typically a mandate in public water systems to install an air gap between the water outlet and the flood level of a specific plumbing fixture.

As has been explained in the example of the washing tub above, if the faucet is above the flood level of the tub, then there is no chance of it getting submerged. This is simply an effective and inexpensive way to prevent backflow.

In places where an air gap cannot be created, a special backflow preventer valve may be installed. Sometimes there is no physical space to create air gaps, which is where a valve like this will be very useful to prevent backflow and control the cross-connection.

Final Thoughts

When dealing with plumbing systems, one has to be very careful about lines that deliver drinking water and lines that are meant for wastewater. Often, there may be a cross-connection where these lines might meet, but if adequate cross-connection control is not carried out, a negative or positive pressure may cause the two streams of water to be contaminated.

This can be extremely dangerous as you could be exposed to toxins and contaminants of all kinds through your drinking water. If you notice that the water coming out of the faucet is murky, then you need to call a plumber immediately to check if there has been a contamination.

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