One of the major concerns in plumbing is how much protection of potable water is enough? This seems to have become more prevalent with the increasing demand by water utilities to require a backflow preventer on the inlet water supply to a building. This backflow preventer is known as the point-of-supply backflow preventer. Point of supply means it is intended to protect the water supply, not the water in the building.

The first thing you should question is why you need a point-of-supply backflow preventer? Your immediate answer should be you don’t need one. That is the premise of every model plumbing code in this country.

Going back to the 1920s and 1930s, when backflow protection was first mandated, the concept was point-of-use protection, which is protection at the fixture or connection to a nonpotable source. That is the concept still used in today’s plumbing codes.

For example, every kitchen faucet or lavatory faucet has an air gap. For pull-out spray faucets, backflow preventers are built into the faucet. A connection to a boiler as well as connection to a fire sprinkler system require a backflow preventer. Hence, every connection and outlet is protected against back siphonage and back pressure.

So why are water utilities asking for a reduced-pressure principle backflow preventer on the inlet water supply to every commercial building? Basically because they don’t know any better.

Many years ago, water utilities misinterpreted the federal law regarding protection of the potable water supply. Someone came up with the idea that if a reduced-pressure principle backflow preventer is installed on the inlet to every commercial building, you don’t have to worry about the quality of the water inside that building. Of course, it isn’t true.

If you follow the logic of installing a backflow preventer on the water supply, you can easily conclude that water utilities don’t care about the people inside the building, only “their” water in the public main. Think about it. If the water is contaminated inside the building because there is inadequate point-of-use protection, everyone inside the building could die from drinking contaminated water. You cannot draw any other conclusion. The people inside the building don’t matter.

If you try to point out this logic to the water utilities, you may not win. Sometimes they are just stuck in a funk. Other times, they will listen to you. The key is whether a water utility has the right to require such a backflow preventer.

In a number of states, the water utility can suggest a backflow preventer but it cannot mandate the valve. However, in some of those states, the utility can install the valve, provided it pays for it and installs the valve with its meter.

Other states grant the water utility the right to mandate a backflow preventer on the water supply to a building. While this is not right, the lobbying efforts by the water utilities managed to get such a law passed. This may appear to be in conflict with the plumbing code, but the elected officials in those states don’t believe so.


Discharge and floor drainage

The problem with the installation of a reduced-pressure principle backflow preventer on the inlet water supply to the building is not with the backflow preventer. That is the easy part. The problem is, once you have a it installed, you need to maintain it and provide for the discharge from the relief valve or relief port.

The backflow preventer requires annual testing. This includes an annual expense to the building’s owner. Let’s say a small town has 100 commercial buildings; that is 100 additional, unnecessary tests of backflow preventers. Then imagine a city like Chicago, where you have thousands of additional backflow preventers to be tested annually.

A bigger concern is the discharge from the backflow preventer when discharging at the maximum rate. A 4-in. backflow preventer will require a 6-in. or 8-in. floor drain to handle the discharge. If you only have a 4-in. building drain, that becomes a problem. If you undersize the floor drain in the room, flooding will occur and lawsuits will follow.

When a backflow preventer is installed in a basement, this adds to the difficulty of removing the water when the valve fully discharges. It is sometimes better to install the backflow preventer in an outside heated box so the water will simply discharge to the yard. When installed on the first level, a relief opening can be provided directly to the outdoors.

The little funnels installed under the backflow preventer are only there to handle the daily nuisance discharges from the valve. This occurs when the supply pressure, upstream of the backflow preventer, drops 2 psi or more in pressure. The relief valve opens up and discharges water. The funnels can easily handle this small amount of discharge. What they cannot handle is a complete discharge from the valve. That is when a properly sized floor drain is required.

Before you install a reduced-pressure principle backflow preventer, be sure to download the discharge table from the manufacturer’s website. This will tell you exactly how much water to expect when the valve opens up completely to protect the water supply. Make sure you size the drainage system to handle the full amount of discharge water.

One of the complaints I constantly hear is how high the water bill is after the water utility required the installation of a backflow preventer. Yes, the water bill will increase. Most commercial buildings are not occupied during periods of peak demand. If the water distribution system is sitting idle, every time the pressure drops 2 psi, water will be wasted discharging out the relief port. Then the pressure goes up and drops another 2 psi and water discharges. This happens all during peak demand periods when the commercial building is unoccupied.

Many utilities are unaware of how much water is discharged because the reduced-pressure principle backflow preventer is doing its job, dumping water. That is another reason why point-of-supply protection makes no sense. Whenever possible, don’t install a point-of-supply valve and rely instead on proper point-of-use backflow protection.


 This article was originally titled “Why point of supply?” in the September 2015 print edition of Plumbing & Mechanical.