The Side Effect Nobody Mentions at Installation

What changes on the day it is fitted

Before: water in a building could move slightly back toward the main as it expanded. After: it cannot, because that is precisely what the assembly exists to prevent.

The building has become a closed system. Nothing has gone wrong; the device is doing exactly its job.

Why that matters

Water expands as it is heated and it does not compress. In an open system that expansion is absorbed by the supply. In a closed one it has nowhere to go, so it raises pressure instead.

EPA’s manual carries a case in which installing a backflow preventer created a closed system and thermal expansion in solar heating tanks generated dangerous backpressure, and notes that closed systems require pressure relief.

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

What it looks like in a building

Pressure that climbs when nothing is being drawn and falls back as soon as a tap is opened. A relief valve that weeps or discharges with no obvious fault. Fittings and flexible connectors failing earlier than they should. Occasional noise as pressure releases.

All of it is easy to blame on the plumbing, and the cause is a change made at the service connection.

What to raise, and when

Ask before the assembly goes in whether the building will be a closed system afterwards and how expansion is going to be accommodated. Asking afterwards is a second visit.

It is also worth raising at any change that adds a check valve, a pressure reducing valve or a meter with one built in, because those create the same condition without anybody calling it backflow prevention.