Backpressure
To define backpressure in backflow prevention: it is reversed flow caused when the pressure downstream of a potable water connection becomes greater than the supply pressure, pushing water or other fluids back toward the supply. Booster pumps, boilers and heating equipment, pressurized vessels and piping that rises above the supply are the usual sources. Only a reduced pressure principle assembly or a double check valve assembly protects against it; vacuum breakers do not.
Updated 2026 · Researched from published standards, not professional or legal advice.
Backpressure at a glance
| Question | Answer |
|---|---|
| Definition | Downstream pressure higher than supply pressure, forcing flow backward |
| Pump back pressure | A booster, circulating or process pump discharging toward the potable connection |
| Thermal sources | Boilers, water heaters and heating loops where thermal expansion raises pressure |
| Elevation | Piping or tanks above the supply, where the static head exceeds supply pressure |
| Protection | Reduced pressure principle assembly (high hazard) or double check valve assembly (low hazard) |
| Not protection | Atmospheric, pressure and spill-resistant vacuum breakers, which handle backsiphonage only |
Backpressure backflow occurs when a downstream system generates more pressure than the potable supply that feeds it, pushing water or other fluids backward through the connection. Common sources of elevated downstream pressure include booster pumps, pressurized vessels, heating equipment that causes thermal expansion, and elevated piping where a column of water creates pressure by height alone. Unlike backsiphonage, which pulls, backpressure actively pushes fluid toward the potable system.
Backpressure situations are common in commercial and industrial settings where water is connected to boilers, carbonation systems, fire sprinkler systems with pumps, and process equipment that adds chemicals. These systems often contain treated or contaminated water under sustained pressure, so a failed or missing preventer can result in a steady, continuing push of contaminant into the supply rather than a brief event. That makes proper device selection especially important wherever pumps or heat sources are present.
For testers and property owners, the key point is that only certain devices resist backpressure. Assemblies built around spring-loaded check valves, such as double check valve assemblies and reduced pressure principle assemblies, are designed for it, while vacuum breakers are not. When surveying a property, always ask what is downstream: if anything can pump, heat, or elevate the water, the protection must be rated for backpressure conditions.
Common questions
What is pump back pressure in backflow prevention?
It is the backpressure created when a pump downstream of the potable connection, such as a booster pump, a chemical feed pump or a process circulating pump, can generate more pressure than the supply delivers. When supply pressure dips, the pump pushes its system backward through the connection unless an assembly rated for backpressure is in place.
Do vacuum breakers protect against backpressure?
No. Atmospheric, pressure and spill-resistant vacuum breakers are designed for backsiphonage, where supply pressure falls. Under backpressure the downstream side pushes the check closed against the air inlet and the device cannot relieve. Backpressure hazards need a double check valve assembly or, for high hazard, a reduced pressure principle assembly.
Sources
- U.S. EPA, Cross-Connection Control Manual (Office of Water): definitions of backflow, backsiphonage, backpressure and the assembly types.
- University of Southern California Foundation for Cross-Connection Control and Hydraulic Research, Manual of Cross-Connection Control: field test procedures for RP, double check and vacuum breaker assemblies.
Related terms
| Backflow | The undesirable reversal of water flow in a piping system that can carry non-potable water or other substances back into the potable supply. |
| Backsiphonage | Backflow caused by a drop or loss of pressure in the supply piping that siphons water backward through a cross-connection. |
| Thermal expansion | The pressure increase that occurs when heated water expands inside a plumbing system that has been closed off from the supply by a backflow preventer or check valve. |
| Reduced pressure principle assembly (RP) | A testable backflow prevention assembly using two independently acting check valves with a pressure differential relief valve between them, suitable for high hazard applications. |
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