About the Author
Mr. Harrington is a general contractor with 35 years' experience who provides expert consulting services to attorneys, property owners and other contractors.
Most of us take our homes’ water and sewer systems for granted. We blithely run faucets, flush the toilet and take our showers assuming clean, safe water flows in and the yucky water is flushed out. In fact however, the physics of water systems operate so that without proper controls in place, it would be quite possible for our own sewage to contaminate the water we drink, cook and bathe in.
It’s all about positive and negative pressures. As long as clean water is being pushed in and dirty water is drained away by gravity everything’s fine. But if things go wrong, that is to say positive pressure in the domestic water supply goes negative, things can really suck, quite literally. There are many situations where the positive water pressure can be compromised or disrupted. The resulting vacuum will draw contaminated water back into the domestic supply.
Take for example an owner of a hillside home filling their swimming pool with a garden hose thrown into the pool. Now let’s assume the city’s water main is somewhere below the altitude of the pool. Were the city’s main to break, the positive water pressure pushing clean water uphill into the house would disappear, leaving gravity to suck the home’s good water right out of the house. Water in the pool would in turn be siphoned back through the garden hose, through the house's domestic supply lines and possibly back into the water main itself. This situation is known as “backflow,” and the potential contamination of the home and community's water supply can present very serious health hazards, not just for one house but every house served by that water main.
Due to the obvious public health hazards, rules and regulations have implemented a variety of controls to prevent backflow from happening. The oldest and most commonly prescribed preventative is called an air gap. Simply put, this failsafe involves a fixed air space (or disconnect) between the lowest opening of a house’s water supply line feeding a sink, basin, or other receptacle, and the flood level rim of that receptacle. This air gap can be observed in most sinks in the average home where the opening in the faucet spout is always some distance above the maximum level that water can achieve in the sink. Imagine how the situation would change if that hose I used in the example above wasn’t left in the pool water but was instead held above the level of water in the pool. The resulting air gap would prevent the hose from sucking water out of the pool no matter what. Noting the simplicity of this solution, the June 2013 issue of the ICC Building Safety Journal states, "An air gap is not a device and has no moving parts. It is, therefore, the most effective and dependable method of preventing backflow and should be used where feasible." And that’s why traditional sinks and tubs in residential construction incorporate this design.
The second most common control method is called an Atmospheric Vacuum Breaker or AVB. This type of control is installed downstream of a home’s shutoff /supply valve, and utilizes an inline air inlet that mechanically closes in response to the positive water pressure that occurs when the home’s shutoff/supply valve is opened, allowing water to flow throughout the system. Conversely, once the supply is shut off and there is no longer positive water pressure, this air inlet opens, preventing any backflow into the pipes. Unfortunately, the AVB solution will not prevent backflow if the water pressure drops because of something that happens outside the system while the shutoff/supply valve is open. Returning again to the example of the hose left in the pool water, without some sort of a backflow prevention device, a problem due to a failure in the city’s water main could allow the pool water to be siphoned back into the domestic system, and possibly into the main itself.
To prevent this, the law requires that a code-approved backflow prevention device be incorporated into any valve to which additional items are likely to be attached. In our example, an AVB is typically installed on the garden hose bib, the valve that would control the water flow into the hose used in my example with the swimming pool. Most people have seen the AVBs even if they don't understand their purpose. It’s that bothersome ring on the end of the bib where the hose attaches that often leaks water when the hose bib is turned off. And AVB’s aren’t failsafe. Unlike the air gap, AVB’s can be removed, and they can break. Because it has moving parts it needs to be maintained. Most people don’t know that.
Other backflow controls include a variety of check valves that use springs and/or pressure to prevent the siphoning of contaminated water back into the domestic system. For instance, modern pullout kitchen faucets can be extended and lowered into the sink well below the flood level rim of the sink. As a consequence, the traditional air gap solution won’t do. The code requires check valves to be incorporated into the design of these handy faucets. Most faucet manufacturers actually incorporate two separate check valves connected in series and conforming to ASME A112.18.1 and/or ASME A112.18.3, standards that establishes the performance requirements, physical characteristics, and testing for backflow protection devices in plumbing fixture fittings. While the versatility of the pullout faucet hose is desirable, their backflow prevention devices involve moving parts and will require additional attention to insure their long-term efficacy. Here again, this type of backflow control may not be understood by the product’s user. It falls on us and our industry to ensure the proper education of that consumer.
This concern for education is not without historic precedence, hand held shower wands with integral hoses are a perfect example of this. Early versions of these wands were often installed as aftermarket additions and did not incorporate anti backflow controls. Some of these are still in use today. I can remember an early version which was used to inexpensively convert a tub only bathing facility into a tub shower facility. It was nothing more than an open rubber hose that attached to the tub diverter or spout through friction. This condition is of concern because the hose allows the opening of the shower supply, the shower head, to extend well into the tub itself potentially allowing a large volume of bathwater to contaminate the domestic potable water supply.
The real danger comes when the product is modified after the fact by the end user. Because they can be messy when they leak and because their purpose is often misunderstood, home owners will often compromise the AVBs on hose bibs by removing them, or replacing them with hose bibs that don't have the AVBs on them at all. Aftermarket shower wands need to have the shower head attachment securely fastened for the anti backflow system to work properly. In all instances, all anti backflow devices that have moving parts will eventually wear out, and additional care must be taken to maintain or replace them as necessary. However, none of this will happen unless the public at large understands the processes of and the need for these systems. Just as care must be taken to insure all tub/sink wands that have this potential must have backflow controls when manufactured, care must be taken to insure that the public understands that these processes exist and must be maintained.