If your notice lists an RP or a DC, it is describing one of the two workhorse assemblies used on commercial service lines, fire lines and many irrigation systems. They look similar from a distance: a bronze or stainless body, two shutoff valves and a row of small test cocks.
The difference is what happens in the space between the two check valves. An RP keeps that zone at lower pressure and vents it to the outside when needed. A double check simply relies on the second valve to hold if the first one leaks.
Side by side
| Feature | RP assembly | Double check assembly |
|---|---|---|
| Internal parts | Two checks plus a relief valve | Two independent checks |
| Hazard level | High hazard (health risk) | Low hazard (pollutant, not a health risk) |
| Protects against | Backpressure and backsiphonage | Backpressure and backsiphonage |
| Visible sign of trouble | Water discharging from the relief port | Usually none until tested |
| Installation | Above ground, with room for discharge to drain | Above ground or in a valve box, depending on local rules |
| Test points | Relief valve, check one, check two | Check one, check two |
Which one is usually required
The water supplier makes the call based on what is downstream. These are typical patterns, not rules for your address.
- IfChemicals, medical equipment, car wash systems or chemical injection in irrigationThenAn RP is the usual requirement because the downstream water could make people sick.
- IfA standard fire sprinkler system without added chemicalsThenA double check, often a detector version with a small meter, is common. Fire rules may add steps.
- IfPlain landscape irrigation at a homeThenA pressure vacuum breaker, double check or RP may be accepted. Check with the city before choosing.
- IfYou want to downgrade from an RP to a double checkThenDo not swap on your own. Ask the water supplier first, in writing.
Living with each one
RP: what owners notice
- It can spit or dump water, which is it doing its job.
- It needs drainage space below the relief port.
- It should not be buried in a pit where the port could flood.
- Repair kits cover three parts: two checks and the relief valve.
Double check: what owners notice
- It runs quietly, so a failing check is not obvious.
- Underground boxes fill with dirt and roots over time.
- The test cocks must stay reachable for the tester.
- Repair kits cover two check modules and seals.
Ask before installing or replacing either type
- What type and size does the water supplier require for this connection?
- Does the new assembly need to be an approved model on the supplier's list?
- Where will an RP's discharge go, and will it flood landscaping or a walkway?
- Will a permit or inspection be needed? (Check with the city.)
Common questions
Is an RP always better than a double check?
It gives more protection, but it also discharges water and needs space to drain. The right device is the one your water supplier requires for that hazard.
What does DCDA mean on my notice?
Double check detector assembly. It is a double check with a bypass meter, often used on fire sprinkler lines to detect small unauthorized flows.
Can a double check be installed in an underground box?
In some places, yes, as long as the test cocks are accessible. Local rules vary, so ask the water supplier.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.