In a closed hydraulic system, fluids will move from areas of high pressure to areas of low pressure. Municipal water towers provide a source of continuous high pressure in a water supply system, and the faucets in the home or business create a low-pressure release. Ordinarily, this establishes a one-way flow of water through the system.

However, if anything causes this pressure to be interrupted, pressure imbalances can be created beyond the point of interruption, which may, under certain conditions, reverse the flow of the water. And if the faucet does not release the water into the air, but is instead connected directly to a source of contamination, that contamination can flow back into the water line.

This reversal of direction is called backflow . There are two types of backflow... backpressure backflow and back-siphonage. Back-siphonage is the most common form.

Back-siphonage

Back-siphonage is backflow caused by a negative pressure (vacuum) in a potable water system. The effect is like drinking water through a straw.

Back-siphonage can occur when there is a broken water main, when there is a fire nearby where a Fire Department is using large quantities of water, or even when a fire hydrant is opened for testing. Any buildings near such a break or unusual fire hydrant usage will experience lower water pressure. This is when back-siphonage can occur.

Backpressure backflow

Backpressure backflow is backflow caused by a downstream pressure that is greater than the supply pressure.

Backpressure can result from a reduction in the supply pressure, an increase in downstream pressure, or a combination of both.

Pumps can create increases in downstream pressure, as can temperature increases in boilers, etc. Reductions in supply pressure occur whenever the amount of water being used exceeds the amount of water being supplied, such as during water line flushing, fire fighting, or breaks in water mains.

These would not be problems if everyone's plumbing systems were built and maintained according to the plumbing code, and if all plumbing systems were built and maintained by individuals trained in hydraulics.

Nor would there be a danger if homeowners always followed the code when making repairs, and if they used only products designed to safeguard the system.

Unfortunately, not everyone has had the proper training.

Cross-Connection

Every week, utility companies across the United States receive calls concerning blue water at the customers' kitchen sinks, caused by toilet bowl cleaner in an adjacent bathroom.

This is caused by cross-connection. A cross-connection is any connection between a drinking water supply and any source of non-potable water or other substances.

A potentially hazardous cross-connection occurs every time someone uses a garden hose sprayer to apply insecticides or herbicides to their lawn.

Any contaminant in contact with the end of a garden hose could end up in a home's water pipes if there is a drop in pressure in the water mains. Over half of the nation's cross-connections involve unprotected garden hoses.

Another cross-connection occurs when someone uses a garden hose to clear a stoppage in a sewer line.

Without a backflow prevention device between the hose and sill faucet, the hose becomes part of a continuous closed system, so the contents of the hose and anything it is connected to or submerged in can backflow into the plumbing system and contaminate drinking water.

Toilets can provide a backflow hazard as well. To prevent sewer gas and the germs associated with it from getting back into drinking water, it is essential that the ballcock be properly air-gapped from the water contained in the toilet tank.

A plumbing-code approved toilet ballcock will prevent hazardous backflows if installed correctly.

The air inlet on the ballcock MUST be located above the water level maintained in the tank by the float and the overflow pipe.

Allowing the refill tube to fall down into the toilet tank creates a cross-connection! It is important that the refill tube be attached to and properly air gapped above the over-flow pipe.

Water suppliers normally ensure that a proper backflow preventer is installed and maintained at the service connection to each premises. While this protects the municipal water supply from contamination, it does not prevent backflow in the home. To protect individual plumbing systems, several types of backflow preventers are available.

Residential Backflow Preventors
The basic means of preventing backflow is an air gap, which opens the system, thus either eliminating the cross-connection or providing a barrier to backflow.

AIR GAP

An air gap is a vertical, physical separation between the end of a water supply outlet and the flood-level rim of a receiving vessel. This separation must be at least twice the diameter of the water supply outlet and never less than one inch.

An air gap is considered the maximum protection available against backpressure backflow or back-siphonage but is not always practical and can easily be bypassed.

MECHANICAL BACKFLOW PREVENTOR

The basic mechanism for preventing backflow is a mechanical backflow preventer, which provides a physical barrier to backflow.

The principal types of mechanical backflow preventer are the reduced-pressure principle assembly, the pressure vacuum breaker assembly, and the double check valve assembly. A secondary type of mechanical backflow preventer is the residential dual check valve.
A reduced-pressure principle assembly (RP) is a mechanical backflow preventer that consists of two independently acting, spring-loaded check valves with a hydraulically operating, mechanically independent, spring-loaded pressure differential relief valve between the check valves and below the first check valve.

It includes shutoff valves at each end of the assembly and is equipped with test cocks. An RP is effective against backpressure backflow and back-siphonage and may be used to isolate health or non-health hazards.

A pressure vacuum breaker assembly (PVB) is a mechanical backflow preventer that consists of an independently acting, spring-loaded check valve and an independently acting, spring-loaded, air inlet valve on the discharge side of the check valve.

It includes shutoff valves at each end of the assembly and is equipped with test cocks. A PVB may be used to isolate health or non-health hazards but is effective against back-siphonage only.

A double check valve assembly (DC) is a mechanical backflow preventer that consists of two independently acting, spring-loaded check valves.

It includes shutoff valves at each end of the assembly and is equipped with test cocks. A DC is effective against backpressure backflow and back-siphonage but should be used to isolate only non-health hazards.

A residential dual check valve (RDC) is similar to a DC in that it is a mechanical backflow preventer consisting of two independently acting, spring-loaded check valves.

However, it usually does not include shutoff valves, may or may not be equipped with test cocks or ports, and is generally less reliable than a DC.

An RDC is effective against backpressure backflow and back-siphonage but should be used to isolate only non-health hazards and is intended for use only in water service connections to single-family homes.

Mechanical backflow preventers have internal seals, springs, and moving parts that are subject to fouling, wear, or fatigue. Also, mechanical backflow preventers and air gaps can be bypassed.

All backflow preventers have to be tested periodically to ensure that they are functioning properly. A visual check of air gaps is sufficient, but mechanical backflow preventers have to be tested with properly calibrated gauge equipment. This should only be done by licensed professionals.