What is a plumbing cross connection?

Sunday, October 7, 2018

It’s a very bad thing. In fact, the evolution of modern plumbing is largely the history of developing safeguards to avoid it. A cross connection is a point in a faulty plumbing system where polluted water (such as sewage) can flow into the drinking water piping and contaminate it. It was the bane of ancient plumbing systems dating back to the Roman Empire.

   And, until plumbing codes began to be rigorously enforced well into the twentieth century, cross contamination continued to be a public health problem. One famous example was the 1933 Chicago World’s Fair: thousands of visitors from all parts of the county came to enjoy the “Century of Progress” exhibits, but contaminated drinking water ended up spoiling the party. Before the fair closed that fall, 98 persons had died and over 1,400 became seriously ill from amoebic dysentery.

   Investigators found that plumbing installed by unlicensed and unqualified persons at two major hotels had allowed cross connections and back-siphonage. Polluted water caused the people who drank it to become critically ill or die.

   Back siphonage is a particular kind of cross connection in which a partial vacuum is created in water supply piping, typically by a temporary loss of water pressure, which can suck the contents out of a plumbing fixture, or other container of contaminated water, if preventative measures are not in place.

   A typical example in a Gainesville home of a situation that could cause a cross connection would be an exterior hose faucet without a “vacuum breaker” installed. If the homeowner is adding water to a backyard pool or pond with a hose inserted into the water, and pump serving the municipal water system fails, the lack of water pressure can cause the contents of the pond or pool to be sucked up into the home’s water supply piping. 

   An even worse  contamination would be created if the hose was connected to a spray-jar containing insecticide or liquid fertilizer--which is why vacuum breakers (the round brass attachment screwed onto the end of a hose faucet, as in the diagram at right) have been required by the
building code for a number of years now.


   Another cross contamination scenario would be created by a sink with a faucet arm that extends below the rim of the sink. If the sink is full to the rim and the water pressure fails, siphoning of the sink contents into the water supply will begin. The modern plumbing code addresses this possibility by requiring faucet arms to terminate safely above the rim of the sink, creating what is called in the plumbing trade an “air gap.”

    Yet another is the drain piping for a water softener system that terminates in the bottom of a laundry sink, as shown below. The lack of an air gap in this case means that a sink full of contaminated water could be sucked back into the water softener and the house water supply.


   Professional plumbers are always on the lookout for piping/fixture configurations that might cause cross contamination. And so are we.

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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 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?

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

Why is there mold inside my toilet tank?  

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

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

How can I tell if a house is connected to a septic tank system or sewer?

Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water? 

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? 

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? 

How can I tell what type of plumbing pipe I have? 

  What causes low water pressure in a house?

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 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? 

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.  

Photo - 1933 World’s Fair Poster - Wikipedia Commons

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