Commercial properties, multi-family residential buildings, and just about any other type of building that isn’t a single-family residential home is required to have a backflow prevention device installed. These devices are tasked with an extremely important sanitation function: prevent accidental reverse flow of water back into the main line, bringing the potential for bacteria and other types of contamination with it. By law, backflow prevention devices must be inspected every year to make sure they are still capable of accomplishing this goal, and any failures or faults need to be fixed as soon as possible.

Nearly every backflow prevention device will fall into one of four categories, and while they might all perform essentially the same function, they do their job in very different ways.

Air Gap Backflow Prevention

We’re putting this one first because this is simultaneously the most common type of backflow prevention and the only type that doesn’t require any sort of an annual inspection. The air gap method of backflow prevention is simple: create a gap of air between a point between the water supply and a sewage collection point. Perhaps the most common example of this is your typical bathroom sink: the air gap is the space between the tap itself and the drain below it. Because water can’t flow back upwards into your faucet from the drain below, backflow is virtually impossible. Thus, this is considered a type of backflow prevention. This is one of the reasons why your average house is not required to have a secondary backflow prevention system installed—with almost all faucets, taps, and fixtures employing some type of an air gap, contamination is almost impossible.

Double Check Backflow Prevention

Double check backflow prevention is commonly used in buildings and properties that have features that require a pretty large amount of water or that have a fairly high chance of accidental contamination. This includes fire sprinklers, irrigation sprinklers, and boilers systems used for either hot water or heating purposes.

A double check valve system is essentially two check valves that are designed to engage in the event of a sudden, drastic change in water pressure. For example, should water pressure from the source suddenly and drastically fall off, the pressure in your own water lines could force water to flow back into the main water system itself. A double check valve would then engage, preventing this from happening. The benefit to a double check valve is that it has two different valves, and one can close off even if the other is stuck open for extra protection.

Reduced Pressure Zone Prevention

A reduced pressure zone backflow preventer is not all that different from a double check system. Both have two different check valves that they use to provide protection. Both valves can operate independently from one another (allowing one to close if the other malfunctions or seizes up). And both are frequently used in irrigation and boiler systems. However, a reduced pressure zone system has one key feature: a third valve that allows a technician to alleviate pressure in water lines without contaminating the water supply. This is critical for irrigation systems in particular, where water contaminated by pesticides or other toxic chemicals might find its way back to the public water system.

Pressure Vacuum Breaker Prevention

A pressure vacuum breaker system uses a combination of water and atmospheric pressure to provide protection against backsiphonage (the process of water flowing backward into the potable water supply). As water flows into this type of check valve, it forces a spring-loaded valve to open while simultaneously closing an atmospheric air valve to close, thus enabling the water to flow normally. When water flows backward, the spring-loaded water valve shuts and the atmospheric valve opens, creating pressure that stops the water being able to reach your supply line again. Because this type of breaker system uses atmospheric pressure to assist in stopping water flow, they are also sometimes referred to as “atmospheric vacuum breakers.”