What is a Cross Connection?
A cross connection is a place where dirty water can get into the clean drinking water (“potable” water). When you turn on your faucet, you expect clean water to flow out of your faucet. But under some conditions, dirty water can flow back into the clean water supply, contaminating the water you drink. People have died from drinking water contaminated by a cross connection.
Two situations can cause cross connections: siphonage and back pressure.
Siphonage
Siphonage occurs when there is a loss of water pressure in the water supply. This can happen for a municipal water system during a firefighting emergency or during maintenance. It can happen on a home well system if there is a loss of power. During the loss of water pressure, dirty water from a bucket, tub, or sink can be drawn up into the water supply.
Siphonage can occur under many common circumstances in a home. I’ve listed the most common places where siphonage can occur, with the most dangerous at the top.
The water softener discharge hose goes directly into the floor drain, a sewer pipe, or even a laundry tub. Dirty water from the floor drain (or sewer, laundry tub, etc.) could siphon back into the clean water supply through the water softener. To prevent the cross-connection, the discharge hose needs an air gap (a vertical space of an inch or more) between it and the floor drain, the sewer pipe, or the top of the laundry tub.
This water softener drain tube goes directly into the floor drain - unsafe!.
This water softener drain tube has an air gap between it and the floor drain - safe!.
A hose bib or utility faucet does not have an anti-siphon valve. A garden hose attached to the hose bib or the utility faucet could allow dirty water to siphon back into the clean water supply. Make sure that each hose bib on your house has an anti-siphon valve. Some utility tubs (“laundry sinks”) will not have threads on the faucet that allow a garden hose to attach - these are safe. You can attach an anti-siphon valve to utility faucets with threads. These anti-siphon valves are also called vacuum breakers.
This modern hose bibb has a built-in anti-siphon valve (vacuum breaker)
A toilet tank overflow tube is not at least 1 inch below the “critical line” (CL) marking on the fill valve. Ensure that the toilet tank fill valve is marked with “ASSE 1002”, and that the fill valve and overflow tube have been correctly installed. The “ASSE 1002” marking is often in a tiny circle, barely readable, on the top of the fill valve or on the side of the fill tube. The bottom of the fill tube must be at least 1 inch above the top of the overflow tube.
A washing machine drain hose is hooked over the edge of a utility sink. If the utility sink clogged, and water pressure were lost, the dirty water from the sink could be sucked back into the clean water supply through the washing machine. The clothes washer drain hose needs an air gap (vertical space) of 1-1/2 inches between the tube exit and the top rim of the utility sink.
(1)The washing machine drain hose is hooked over the utility sink. The drain hose needs an air gap (vertical space) between it and the top of the sink.
(2) The faucet is threaded, allowing a garden hose to be attached. The faucet needs an anti-siphon valve attached.
The dishwasher does not have an air gap in the drain hose. Without an air gap, dirty water from the kitchen drain can siphon into the clean water supply through the dischwasher. An air gap designed specifically for dishwashers is often installed on top of the kitchen sink. Other arrangements such as a standpipe under the sink can also provide the necessary air gap.
Dishwasher air gap
The kitchen sink sprayer can be immersed in dirty water. Dirty water can siphon from the kitchen sink into the clean water supply through the sprayer. Some sprayer attachments have built-in back-flow prevention. These will be marked with ““ASSE 1025” or “ASME 112.18.1”. You should add a back-flow prevention device to a sink sprayer if it does not have one of these stamps. At a minimum never leave it dangling in the sink.
This kitchen sink sprayer can be left immersed in dirty water, creating a cross-connection.
A shower attachment can be left in dirty water. Dirty water can siphon from the tub or shower floor into the clean water supply through the shower attachment. Some shower attachments have built-in back-flow prevention devices. These will be stamped “ASSE 1014” or “ASME 112.18.1”. You should add a back-flow prevention device to a shower attachment if it dangles below 1 inch above the flood rim of the shower or tub and it does not have one of these stamps. At a minimum never leave it dangling in the tub or on the shower floor.
This shower attachment can be left on the floor, creating a cross-connection.
The bathtub faucet is below the rim of the tub. This is usually only seen on very old bathtubs, like claw-foot tubs. The solution here is to replace the faucet with a faucet that extends like a goose neck above the tub so that the faucet opening is above the rim of the tub. You can also install backflow preventer valves on both the cold and hot water lines to the tub.
Back Pressure
Back pressure can occur on boilers if the pressure from the boiler is greater than that of the water supply. A boiler should have a one-way valve (“backflow preventer”) that prevents this.
Summary
When I inspect a home, I look for cross-connections and let the client know how to address them. Cross connections are very common and are usually easy to fix.
Resources
Protect Your Water by Preventing Cross Connections
Wisconsin statutes regarding cross connections
