Photo of a home water softener systemPlumbing cross-connections
Definition of a cross connection & why you should care: unsanitary, unsafe

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Plumbing cross connections: this article gives a definition of a plumbing system cross connection and explains why plumbing or piping cross connections can be unsanitary or even dangerous.

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Plumbing CROSS CONNECTIONS - Health & Safety Risks from Plumbing Cross Connections

Examples of Plumbing Cross connections (C) Carson Dunlop Associates

Definition of plumbing cross connection: a plumbing cross-connection is defined as any physical connection or arrangement between potable water and any source of contamination. Examples of cross connections are shown at left.

Plumbing codes define a cross connection as follows:

Any physical connection or arrangement between two otherwise separate piping systems, one of which contains potable water and the other either water of unknown or questionable safety or steam, gas or chemical, whereby there exists the possibility for flow from one system to the other, with the direction offlow depending on the pressure differential between the two systems. - UPC 2006 

[Click to enarge any image]

Also see:

A cross connection risks contamination of building water piping or municipal water supply with bacteria. As Carson Dunlop's Home Reference Book points out, a cross connection is a dangerous situation where waste water may enter and contaminate the supply water.

A cross connection can occur in many places. One example is where a laundry tub has a faucet below the top of the laundry tub. If the faucet enters the tub through the wall, it is possible that when the tub is filled, the faucet will be submerged.

If this happens, the waste water in the tub may back up into the drinking water through the faucet if the supply piping is being drained.

The solution to plumbing cross connections is to raise the faucets above the top of a tub or basin, creating an air gap between the faucet and water in the tub. Cross connections are avoided through the use of an overflow.

In bathtubs, for example, where the faucets may enter through the wall of the tub below the top, an overflow provided below the faucets will prevent a cross connection.

Examples of Plumbing Cross connections (C) Carson Dunlop Associates

Other examples of cross connections in buildings include water softeners (see HEALTH RISKS & WATER SOFTENERS), washing machines, and dishwashers connected to a building drain without an air gap and water powered backup sump pump systems that use municipal water pressure and a venturi to evacuate water from a building or its sump pit (see Sump Pump Types).

Carson Dunlop's sketch (left) shows how an air gap at a typical sink protects against cross connections.

Bathroom fixture cross connections

Cross connection at bath tub (C) Daniel Friedman

Older bath sinks and bath tubs (photo above) may also form a cross connection if the tub spout is below the flood rim of the fixture.

In our photo at left the original tub spout is below the top edge of the bath tub (the fixture flood rim).

(A newer water spout has been installed through the tile tub surround wall.)

Bidets Cross Connections - Vacuum Breakers

Some plumbing fixtures necessarily create a situation which could lead to a cross connection. A bidet is a good example of this

A bidet has a water supply at the bottom of the bowl. This allows water to be directed up in a spray from the bottom center of the bowl.

Bidet cross connection protection Carson Dunlop Associates

There is, of course, the potential for waste water in the bowl to get into this supply water.

Bidet photo (C) Daniel Friedman

A special device (vacuum breaker) prevents water from flowing back into the supply plumbing.

Dishwasher Cross Connections & Air Gap Devices

Dishwasher photograph (C) Daniel Friedman

The air gap device shown in the sketch (below-left) demonstrates how we avoid a plumbing cross connection at dishwashers. The dishwasher in our photograph (below right) was installed with no air gap device and forms a potential cross-connection in this home.

Dishwasher cross connection details Carson Dunlop Associates

Garden Hose Cross Connections

Cross connection at garden hose (C) Carson Dunlop Associates

While cross connections are normally avoided during original plumbing work, home handy man changes can create them. Careless use of the house plumbing system can also create a cross connection.

Care should be taken, for example, to avoid placing a garden hose in a position to allow waste water to flow back into the supply plumbing system. For example, if the hose is left in a pail of water, it is possible for the contaminated water to flow back through the hose into the drinking water. This may happen if the house water supply is shut off and partially drained for some reason, while the hose is in the pail. Back-flow preventers (like the atmospheric vacuum breaker in the illustration) on the hose bib can prevent this.

Cross connections by garden-hose plumbing also occur when someone improperly connects a garden hose directly to a sewage waste pipe for any reason (photo, below right). We have found garden hoses connected in this fashion as building piping drains, water softener drains, boiler drains, etc.

Garden hose plumbing (C) Daniel Friedman

Back-flow preventers (like the atmospheric vacuum breaker in the illustration at above-left) on the hose bib can prevent this. We show an example of a garden hose backflow preventer at FAUCETS, OUTDOOR HOSE BIBBS and another back flow preventer at Plumbing Cross Connections at Heating Boilers

- Adapted with permission from Carson Dunlop's Home Reference Book

Clothes Washer Plumbing Cross Connection leads to Sewage Backup into Washing Machine

Cross connection and bacterial sewage backup at a washing machine (C) Daniel Friedman

It's not just technical theory. Sewage backups into appliances do occur. One of our clients in Cold Spring NY awoke at 2AM to hear their basement clothes washer running unattended.

A washing machine drain hose in our photo (left) had been connected directly to the sewer line without an air gap. We pointed out the cross connection and risk of sewage backup into the washing machine, potentially providing a path of bacterial contamination back through the washing machine into the house water supply piping.

Our client thought that the risk of sewage backup into the washing machine was not a concern. The new homeowner called us two weeks later with an amazing house noise and sewage backup story.

The home occupants woke at 2: AM, hearing a noise in their basement. Walking downstairs into the basement the occupants found that mysteriously the clothes washing machine had begun to agitate. This was an old washing machine that would start agitating when a sensor determined that the liquid level in the washer had reached a sufficient level.

Clothes washer proper piping to avoid cross connection (C) Carson Dunlop Associates

During the night a municipal sewer backup had occurred in the street, forcing raw sewage back into the house sewer line, and upwards into the tightly-connected washing machine hose, filling the washing machine with sewage. The washing machine had been cross-connected by a tight plumbing connection to the building main drain line.

A sewer line backup had pushed waste into the washer where its pressure switch started the agitator. Plumbing cross connections are unsanitary and is a health risk. The clothes washer was merrily agitating away its full load, not of laundry, but of sewage.

Carson Dunlop Associates' sketch (left) shows a proper indirect-connection washing machine drain hookup.

Water Heater Cross Connections

If the temperature & pressure discharge tube of a water heater relief valve (RELIEF VALVE, WATER HEATER) has been connected to a building drain without use of an air gap or similar safety device (TUNDISH used in PLUMBING) the connection is unsafe and unsanitary, forming a cross connection between the water heater TP valve disharge tube piping back through the valve into the water heater tank and thus into the building's potable water supply system.

Watch out: Such installations are both unsanitary and unsafe, risking BLEVE EXPLOSIONS as well as sewage or bacterial contamination.

Water Softener Cross Connections Between Drains and Water Supply Piping

a plumbing cross connection at the water softener - this is unsanitary

Plumbing cross Connections such as those we show in our water softener backwash/recharge drain tube connections at page top and just at left deserve a mention while we're discussing water softeners. A "cross connection" is a direct physical connection between a building water supply pipe and a drain pipe.

This may sound like an unlikely event in buildings but in fact it is common in a few instances: installers often make a tight connection between the water softener backwash/discharge drain (usually a small diameter plastic tube) and a building drain.

While it is not unique to water softener installations, this plumbing error is often made when these devices are installed. It is unsanitary and is a health risk.

What's the problem? In the unlikely event of a sewer line backup, sewage may be forced backwards up the drain and into the supply fixture, in this case the water softener - a very unsanitary event. There is also a possible bacterial growth path up such drains even when a sewer backup is not occurring.

a plumbing cross connection at the water softener - this is unsanitary

The proper way to make this connection is with a plumbing drain which provides an air gap between the water discharge line and the drain line [photo].

You can review this detail with your plumber or, if installing a new water softener, with the installer. Also see  HEALTH RISKS & WATER SOFTENERS for details.

While they are not unique to water softener installations, cross connections are a plumbing error which is often made when water softeners are installed.

Plumbing Cross Connections at Heating Boilers

Backflow preventer on a hot water heating boiler (C) Daniel Friedman

On hot water heating systems, the plumbing is connected to the boiler so that water can be added to the boiler.

Modern hot water boiler and steam boiler installations have a back-flow preventer (BACKFLOW PREVENTER VALVE, HEATING SYS) to prevent the boiler water from coming back into the drinking water supply piping system. This is another potential cross connection. Older systems may not have back-flow preventers to protect against this.

At a heating boiler lacking a backflow preventer the combination of a defective pressure reducing water feed valve or use of a manual water feed valve combined with a drop in building supply pressure can cause unsanitary boiler water to enter the building potable water supply piping system.

In addition a heating boiler itself may become back-contaminated by sewage if its temperature / pressure relief valve (RELIEF VALVE, TP VALVE, BOILER) was connected directly to a building drain without a visible air gap.

Watch out: Such hot water heating boiler or steam heating boiler installations are unsafe, risking BLEVE EXPLOSIONS as well as sewage or bacterial contamination.

Sump Pump Plumbing Cross Connections

Water powered sump pump

Water powered sump pumps can form an unsanitary cross connection between ground water or basement water leaks and the building municipal water supply piping - and thus are illegal in some plumbing jurisdictions.

See Sump Pump Types for details about water powered sump pumps.

Cross connections are defined with further examples at Home Inspection Definitions & Terms.

- Adapted with permission from Carson Dunlop's Home Reference Book


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