How Does the Relief Valve Work in a Backflow Preventer?
The relief valve is what separates a reduced pressure assembly from every other backflow preventer. It is also the part that is most often described incorrectly, including in a fair amount of material written by people who have never put a gauge on one.
Here is what it actually does.
What it is measuring
A common description says the relief valve monitors the pressure difference between the inlet and the outlet of the device. That is not right, and the distinction matters if you want to understand why yours is discharging.
An RP has two independent check valves in series. The space between them is the zone. The relief valve senses supply pressure ahead of the first check on one side of a diaphragm, and zone pressure on the other. It is watching the margin between those two.
Under normal flow, the first check holds a pressure drop across itself, which keeps the zone below supply pressure by a comfortable margin. The relief valve stays shut. When that margin narrows — because the first check is fouled and leaking backward, or because supply pressure fell away suddenly — the relief valve opens and discharges the zone to atmosphere.
The result is a physical gap that contaminated water cannot cross. Not a valve trying to hold against pressure, but an open path to the ground.
Why that design is the point
Two checks in series are already redundant. The relief valve exists because redundancy alone is not enough for a health hazard. If both checks foul, a double check assembly quietly passes contaminated water and nobody knows. An RP dumps to the ground where somebody sees it.
That is the trade. An RP wastes some water and needs a drain and clearance. In exchange, it fails loudly. For anything classified as a health hazard in California, that trade is not optional.
Consequence for installation: an RP cannot go in a pit or a below-grade vault where a discharge would submerge the relief port. It needs to sit above grade with an air-gapped path for full relief flow. A submerged relief port is not a working RP.
What it does in each scenario
Backpressure
Downstream pressure rises above supply — a boiler, a booster pump, elevation. The second check holds. If it does not hold perfectly and zone pressure climbs, the relief valve opens.
Backsiphonage
Supply pressure drops away, usually a main break or heavy hydrant draw. Now the margin the relief valve is watching disappears from the supply side. It opens, venting the zone to atmosphere and breaking the siphon.
A fouled first check
Debris on the seat lets water leak backward past it into the zone. Zone pressure rises toward supply pressure, the margin collapses, and the relief valve discharges — often continuously, which is the classic symptom.
What testing measures
The relief valve opening point is a required part of every RP test. A tester establishes the differential across the first check and finds the point at which the relief opens, and that number has to fall above the minimum. A relief valve that opens too low nuisance-discharges. One that opens too high, or not at all, is a device that has quietly stopped protecting anything.
You cannot see any of this from outside. It is the reason the annual test exists.
Common questions
Does a DCVA have a relief valve?
No. Two checks, no relief, no visible indication of failure. That is the fundamental difference between the two assemblies — more on that in our RP vs. DCVA comparison.
My relief valve discharges occasionally. Is it broken?
Not necessarily. Brief discharge during pressure fluctuation is normal behavior. Continuous or repeated discharge is worth diagnosing — we walk through the causes in why an RP relief valve discharges water.
Can the relief valve be rebuilt?
Yes. Manufacturer rebuild kits cover the relief valve along with the checks, and a rebuild plus a passing re-test resolves most problems short of a cracked body.