5.4 When to Use RP vs DC
Key Takeaways
- Select RP vs DC by degree of hazard first (health vs non-health), then confirm hydraulic conditions (backpressure, backsiphonage, continuous pressure)
- Health-hazard applications with continuous pressure and possible backpressure require RP (or air gap)—never a DC, regardless of cost
- Common exam trap: placing a DC on a toxic chemical feed, medical waste line, or treated boiler makeup because “two checks should be enough”
- RPs can discharge water and need drain capacity; DCs avoid normal relief discharge but offer a lower protection level and silent failure mode
- The ASSE 5110 tester reports measured condition and installation observations; the AHJ/water purveyor sets the required assembly type for the hazard
5.4 When to Use RP vs DC
Quick Answer: Use an RP (ASSE 1013) for health hazards under continuous pressure with backpressure and/or backsiphonage risk. Use a DC (ASSE 1015) only for non-health hazards when the AHJ allows it. Never “save money” with a DC on toxic chemical feed or other contamination risks. Testers report condition; purveyors/AHJs specify required protection.
Decision order that prevents field mistakes
Selection questions on ASSE 5110 and real program design follow a consistent logic:
- What is the degree of hazard? Health (contamination) vs non-health (pollution).
- What hydraulic conditions apply? Backpressure, backsiphonage, or both? Continuous pressure?
- What does the local program / plumbing code require for that use (containment at the meter vs isolation at the fixture)?
- Which listed assembly families meet that combination?
- Installation constraints (drainage for RP relief, freeze protection, orientation, accessibility for testing).
If you reverse the order—starting with price or “what fits in the pit”—you will fail written items and, worse, leave a premises under-protected.
Decision matrix by hazard and hydraulics
| Scenario (typical) | Hazard | Hydraulics | Usual mechanical choice |
|---|---|---|---|
| Chemical feed / toxic process water | Health | Continuous pressure, possible backpressure | RP (or air gap) |
| Hospital lab / medical equipment with contaminants | Health | Continuous pressure | RP |
| Boiler with chemical treatment | Health | Backpressure from pump/thermal | RP |
| Sewage ejector / reclaimed water interface | Health | Variable | RP / air gap per code |
| Fire sprinkler without chemicals (program-dependent) | Often non-health | Backpressure from fire pump | DC or DCDA if allowed |
| Fire sprinkler with antifreeze/chemical | Health | Backpressure | RP / RPDA family |
| Irrigation without chemical injection (AHJ-dependent) | Often non-health | Continuous pressure, backsiphonage risk | DC or PVB/SVB per elevation rules |
| Irrigation with chemical injection | Health | Pump/backpressure possible | RP |
| Hose bibb only, no continuous pressure | Varies | Backsiphonage | AVB (not RP/DC topic) |
Vacuum breakers (AVB/PVB/SVB) remain backsiphonage-focused tools with elevation and continuous-pressure limits; they are not substitutes for RP on health-hazard backpressure lines. This section keeps the spotlight on RP vs DC.
Feature comparison table (study sheet)
| Feature | RP (ASSE 1013) | DC (ASSE 1015) |
|---|---|---|
| Checks | 2 independent | 2 independent |
| Relief valve | Yes (zone) | No |
| Shutoffs | 2 resilient-seated | 2 resilient-seated |
| Test cocks | 4 typical | 4 typical |
| Health hazard | Approved | Not approved |
| Non-health hazard | Allowed | Intended use |
| Backpressure | Yes | Yes (non-health) |
| Backsiphonage | Yes | Yes (non-health) |
| Continuous pressure | Yes | Yes |
| Visible failure dump | Relief discharge | None |
| Key USC test extras | Relief ≥ ~2.0 psid; CK1 ≥ 5.0 & > relief | Each check ≥ 1.0 psid |
| Drain / air gap at relief | Required | N/A |
| Common fire detector form | RPDA (1047) | DCDA (1048) |
| Typical complaint | Water discharge / drain needs | Silent leakage risk if failed |
Memorize this table for rapid elimination on multiple-choice items.
Common exam traps
Trap 1: “Two checks are enough for chemicals”
A stem describes a chlorine, acid, pesticide, or antifreeze feed and offers a DC because it is cheaper and will not spit water. Wrong. Degree of hazard controls. Answer RP (or air gap if presented as an option for that premise).
Trap 2: Confusing product numbers
- 1013 = RP
- 1015 = DC
- 1047 = RPDA
- 1048 = DCDA
- 1020 = PVB
- 1056 = SVB
Questions love swapping 1013 and 1015.
Trap 3: Mixing pass/fail numbers across assemblies
Applying 1.0 psid to RP check #1, or 5.0 psid to a DC check, or looking for a relief test on a DC.
Trap 4: Assuming the tester chooses the assembly type alone
The stem asks what the tester should do when a DC is installed on a toxic line. Correct role language: perform/observe, document, report to purveyor/AHJ—not “approve the DC because both checks held 1.2 psid.”
Trap 5: Treating discharge as automatic failure without diagnosis
An RP that discharges during the relief test when you intentionally collapse differential is supposed to open. Continuous drip under normal static conditions is abnormal. Context matters.
Trap 6: Using PVB logic on backpressure health hazards
A PVB may be fine for certain backsiphonage-only cases; it is not the answer when the stem includes a booster pump and toxic fluid.
Cost and maintenance tradeoffs (honest comparison)
| Factor | RP | DC |
|---|---|---|
| Purchase cost | Usually higher | Usually lower |
| Installation | Needs drain / air gap for relief | Often simpler drain needs |
| Operating “nuisance” | Possible relief spit (expansion, supply swings, first-check weep) | Rare external water loss |
| Annual testing time | Relief + two checks (more steps) | Two checks |
| Failure visibility | Discharge can alert owners | Failures may be silent |
| Protection level | Higher (health hazard) | Lower (non-health) |
| Flood risk if drain poor | Real if relief opens hard | Lower from the assembly itself |
Owners sometimes pressure testers or plumbers to “put in a DC so we stop wasting water.” That is a program and design conversation with the purveyor—not a free upgrade path for health hazards. Thermal expansion tanks, pressure management, and proper drain design solve many nuisance RP discharges without illegal under-protection.
Tester vs AHJ / purveyor boundaries
Clear role language protects you ethically and on exam wording:
| Role | Responsibility |
|---|---|
| Water purveyor / AHJ | Sets required protection type for hazard class; enforces cross-connection ordinance; accepts or rejects assemblies |
| Surveyor (e.g., ASSE 5120 path) | Identifies cross-connections and recommends protection types within program rules |
| ASSE 5110 tester | Field-tests assemblies, records values, notes installation defects (plugged relief, missing cocks, wrong orientation), reports pass/fail of the assembly’s condition |
| Repairer (e.g., ASSE 5130 path) | Rebuilds or replaces internals when authorized |
| Owner / plumber | Installs listed assemblies as required; maintains freeze protection and drains |
If you find a DC on a health-hazard service, you still test it if asked, but your report must not imply that a passing 1.0 psid reading satisfies a health-hazard requirement. Note the assembly type and the program’s required type when your jurisdiction’s form allows; notify the purveyor per local procedure.
Selection walkthroughs
Walkthrough A — Car wash reclaim. Reclaim tanks and soaps can introduce non-potable, potentially contaminated water. Continuous pressure and pump-driven backpressure are likely. Select RP for containment/isolation as the program specifies. A DC “to avoid mess from relief water” is an exam fail.
Walkthrough B — Dry fire sprinkler without chemicals. Many programs accept DC or DCDA for non-health fire lines, with detector metering preferred for theft/leak detection. If antifreeze loops are added later, protection often upgrades to RP/RPDA.
Walkthrough C — Apartment domestic service with no special hazards. Some purveyors require containment RPs on all multi-family services regardless of internal hazards; others allow DCs or none at the meter with isolation devices inside. Follow the written program—selection is not pure physics in isolation.
Walkthrough D — Softener with salt only. Sodium chloride brine is often treated as non-health by some programs and health-related by others depending on local classification and concentration risks. When a stem labels the softener brine non-health and offers DC vs RP, DC can be correct; when it emphasizes contamination of the public supply and requires highest protection, RP appears. Read the hazard wording carefully.
Integrating Chapters 3–5 for exam readiness
- Chapter 3 taught backpressure vs backsiphonage and why differentials matter.
- Section 5.1–5.2 taught how the RP uses differentials with a relief.
- Section 5.3 taught the DC without relief.
- This section forces the selection skill: match assembly capability to hazard + hydraulics, avoid traps, respect authority roles.
When you reach field-test procedures, you will execute different gauge sequences for RP and DC, but selection logic stays the same: right device first, then right test.
Closing checklist before the next chapter
Before moving to PVB, SVB, and detector assemblies, confirm you can:
- Sketch RP and DC flow paths and name every major part.
- State USC-style numbers: relief ~2.0, RP CK1 5.0 + above relief, RP CK2 tight (no psid), DC checks 1.0.
- Explain continuous RP drip vs intentional relief opening during test.
- Reject DC on health hazards.
- State that testers report; AHJs require.
Those five skills dominate assembly-operation items in the 20% components-and-operation domain and reappear throughout field-test and installation questions.
A premises has a toxic chemical feed pump that can raise process pressure above city pressure. Which assembly type is appropriate among the common mechanical choices?
Which statement best describes the ASSE 5110 tester’s role when a DC is found on a service the local program classifies as a health hazard?
Compared with a DC, which maintenance or operational tradeoff is characteristic of an RP?
Which pair correctly matches device standard numbers?