A cross-connection is any temporary or permanent connection between a public water system or consumer's potable (i.e. drinking) water system and any source or system  containing non-potable water or other substances.  An example is the piping between the public water system or consumer's potable water system and an auxiliary water system, cooling system, or irrigation system. 

Backflow is the undesirable reversal of flow of non-potable water or other substances through a cross-connection and into the piping of a public water system or consumer's potable water system.  There are two types of backflow - backpressure backflow and back siphon. 

A backflow preventer is a means or mechanism to prevent backflow.  The basic means of preventing backflow is an air gap, which either eliminates a cross-connection or provides a barrier to backflow.  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. 

Backflow into a public water system can pollute or contaminate the water in that system (i.e. backflow into a public water system can make the water in that system unusable or unsafe to drink) and each water supplier has a responsibility to provide water that is usable and safe to drink under all foreseeable circumstances.  Furthermore, consumers generally have absolute faith that water delivered to them through a public water system is always safe to drink.  For these reasons, each water supplier must take reasonable precautions to protect its public water system against backflow.

Water suppliers usually do not have the authority or capability to repeatedly inspect every consumer's premises for cross-connections and backflow protection.  Alternatively, each water supplier should ensure that a proper backflow preventer is installed and maintained at the water service connection to each system or premises that could poses a significant hazard to the public water system.  Generally, this would include the water service connection to each dedicated fire protection system or irrigation piping system and the water service connection to the building domestic water supply.

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.  Therefore, all backflow preventers have to be tested periodically to ensure that they are functioning properly.  A visual check of air gaps is sufficient however, mechanical backflow preventers have to be tested with properly calibrated gauge equipment.

A residential dual check is similar to a Double Check 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 Double Check.  A residential check is effective against backpressure, backflow and back siphon but should be used to isolate only non-health hazards and is intended for use only in water service connections to single family homes. 

When water is heated it expands.  For example, water heated from 90º F to a thermostat setting of 140º F in a 40 gallon hot water heater will expand by almost one-half gallon.  This is because when water is heated, its density decreases and its volume expand.  Since water is not compressible, the extra volume created by expansion must go someplace.  During no-flow periods in a system, pressure reducing valves, backflow preventers and other one-way valves are closed, thus eliminating a path for expanded water to flow back to the system supply.  Hence, system pressure increases. Potable water expansion tanks are designed to absorb the increased volume of water created by thermal expansion and to maintain a balanced pressure throughout the potable water supply system.  They are used to prevent plumbing system and/or water heater damage and unnecessary relief valve discharge caused by excessive pressure from thermal expansion.  Most water heater manufactures now require expansion tanks to be installed when the water heater is installed.  If the expansion tank is not installed the warranty may be voided.