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Two Causes, One Outcome, Different Devices

This is the distinction the whole trade rests on, and it is the one almost nobody outside the trade has ever been given.

Key point

Backflow happens two ways. Backsiphonage is the supply falling to a vacuum so that atmospheric pressure pushes a pollutant into it; backpressure is a connected system at higher pressure forcing something in against a supply that is working normally. Most device families address only one of the two.

Source: U.S. EPA, Cross-Connection Control Manual, EPA 816-R-03-002, February 2003.

CrossConnect is a referral service, not a contractor. We connect property owners and managers with independent contractors. Backflow assembly testing is normally restricted to testers holding a specific certification and using calibrated equipment, and the report usually has to reach your water purveyor — ask to see the certification and ask who files the report. We carry out no testing, we certify nobody, we do not set prices, and we never charge callers anything. Nothing here selects a device for any installation or states that any assembly is passing.

What a cross-connection is

EPA defines it as the link or channel connecting a source of pollution with a potable water supply. That is broader than it sounds: a hose lying in a bucket is one, a boiler feed is one, an irrigation line is one.

A cross-connection on its own does nothing. It needs an event to make water travel the wrong way along it, and there are exactly two.

Source: U.S. EPA, Cross-Connection Control Manual, EPA 816-R-03-002, February 2003.

Backsiphonage: the supply lets go

EPA describes it as atmospheric pressure exerted on a pollutant liquid, forcing it toward a potable water supply system that is under a vacuum.

The supply loses pressure — a main breaks, a hydrant opens, a pump stops — and the column of water in the pipe falls back. Anything the pipe is in contact with at the far end goes with it. Nothing has pushed; the supply has stopped holding.

Backpressure: the other side pushes harder

EPA describes this as flow produced by the differential pressure between two systems both of which are above atmospheric pressure.

Here the supply is working normally. What has happened is that something connected to it — a boiler, a booster pump, an elevated tank, a gas cylinder being purged — has developed a higher pressure than the supply, and water flows from high to low as it always does.

Backsiphonage and backpressure comparedTwo drawings of the same arrangement: a water supply pipe running across, with a branch dropping into a vessel of liquid. In both, an arrow on the supply pipe points back toward the source, because the outcome is the same. In the first the supply has lost pressure and atmospheric pressure pushes the liquid into it. In the second the supply is still pressurized but the connected system is at a higher pressure and forces liquid in against it.BACKSIPHONAGEthe supply has fallen to a vacuumatmospheric pressure, pushinga main break, a hydrant in use, a pumpshut down — the supply lets goBACKPRESSUREthe supply is still pressurized, at 65 to 70 psithe connected system, at 85 to 90 psia boiler, a pump, an elevated tank, a gaspurge — the other side pushes harder
The same reversed arrow, from opposite causes. The pressures in the second panel are those of a documented incident in which propane at 85 to 90 psi entered a main carrying 65 to 70 psi.

Why the distinction decides everything

Because the device families are not equivalent. An atmospheric vacuum breaker and a pressure vacuum breaker address backsiphonage and are not for backpressure. A device chosen against one event gives no protection at all against the other.

So an installation protected by the wrong family is not partly protected. It is unprotected against whichever event it was not chosen for. What each family is approved against.

And why this is not academic

The propane case in EPA’s manual is a backpressure event with numbers attached: gas at 85 to 90 psi against water at 65 to 70, and about 2,000 cubic feet of it into an 8-inch main.

Three incidents, as the manual records them.

Know what is on the tag before you call.Make, model, size and the date of the last test are usually stamped or tagged on the assembly itself, and they decide which test kit and which paperwork the visit needs. We will route you to somebody who covers your area, selected on the ZIP code of the property, and you are never charged. Backflow is our specialty, though the number will take any plumbing request on the property.

Call (888) 764-9890
Call (888) 764-9890