Code Stories · why the rule exists
Backflow, Cross-Connections and RPZ Testing
The rule in Massachusetts today
Verbatim, 310 CMR 22.22(13)(d): "each reduced pressure backflow preventer will be inspected semiannually ... If the supply is used less than six months of the year, these devices shall be inspected and tested once each year. Each double check valve assembly shall be tested annually." ... "Each test shall be conducted by a Certified Backflow Prevention Device Tester". 248 CMR 10.14: "No plumbing permit shall be issued for cross connection installations that require Reduced Pressure Zone Backflow Preventers or Double Check Valve Assemblies until the application for a permit is accompanied by a letter of approval from the Massachusetts (DEP) or its designee."
- RPZ backflow preventers inspected/tested semiannually (every 6 months) (310 CMR 22.22(13)(d))
Why it exists
Water flows from higher to lower pressure, and it does not care which way is 'supposed' to be downstream. Backsiphonage happens when supply pressure drops, for example from a main break, a repair shutdown or firefighting draw. Any submerged hose or inlet can then become a straw. Backpressure happens when a connected system such as a boiler, pump or chiller runs at higher pressure than the supply. An RPZ is essentially two check valves with a relief valve between them. The zone between the checks is held at lower pressure than the supply, and if either check leaks, the relief valve dumps water to the floor instead of letting it pass backward. Check valves can fail silently, fouled by debris, so these assemblies are built with test cocks and must be tested. The EPA manual notes that even air gaps must be inspected.
How it came to be
- 1933 — Amebic dysentery outbreak linked to two Chicago hotels during the Century of Progress World's Fair. EPA's figures: epa.gov
- 1933 — Illinois Department of Public Health timeline (gives a higher case count). idph.state.il.us
- 1934 — TIME reports that the expert committee blamed the hotels' plumbing. time.com
- 1936 — Federal report published: NIH Bulletin No. 166, 'Epidemic Amebic Dysentery: The Chicago Outbreak of 1933' (House Doc. 74-376). govinfo.gov
- 1936 — Bulletin 166 finding on condenser cross-connections, as quoted by a secondary source. edmcdevitt.org
- 1978 — South Carolina church: chlordane back-siphons through submerged hoses into Kool-Aid for a vacation Bible school (EPA case file). epa.gov
- 2003 — EPA manual on why testable assemblies matter. epa.gov
Make it stick
August 1978, South Carolina: an exterminator left garden hoses submerged in buckets of chlordane. The water department shut off the line on a hillside to install a separate meter, and the pressure drop siphoned the insecticide into the church's pipes, where it ended up in the Kool-Aid for vacation Bible school. About a dozen children and three adults got dizzy and nauseous; none needed hospital care (EPA).
On the exam
Expect backsiphonage vs. backpressure; matching the device to the hazard (air gap, AVB, PVB, DCVA, RPZ; high vs. low hazard; continuous pressure or not, per 248 CMR 10.14 Table 11); vacuum breakers installed with the critical level at least 6 in. above the flood level rim; the MA test intervals (RPZ semiannually, DCVA annually, PVB at least annually by the owner, 310 CMR 22.22(13)(d)); who tests (a certified backflow prevention device tester); DEP approval before a permit for RPZ or DCVA installations; and the definition of cross-connection. For RPZ placement details such as clearance, drainage for the relief port, and not in a flood-prone pit, follow the manufacturer and the water supplier's cross-connection program; those details were not verified in the local 248 CMR text.
What people get wrong
- Backflow takes a dramatic event. Ordinary main repairs and shutoffs cause most of the EPA case histories.
- A single check valve is a backflow preventer. Single checks foul and fail silently and cannot be field-tested like an assembly. Hazard-rated devices are required.
- The 1933 outbreak happened because the World's Fair overloaded Chicago's sewers / the hotels shared a rooftop tank. UNVERIFIED and not used. Those details appear only in backflow-industry blogs. The documented finding is defective hotel plumbing and cross-connections (TIME 1934; EPA; Bulletin 166 as quoted).
- An RPZ that spits water is broken and should be plugged. Discharge from the relief port is the device doing its job. Diagnose the cause, and never plug the port.
Try it: 3 real practice questions
Q1. How often must an RPZ (reduced pressure zone) backflow preventer be tested in Massachusetts?
MassDEP's cross-connection rule requires every reduced pressure backflow preventer to be tested semiannually by a certified backflow tester. The annual interval applies to double check valve assemblies, or to an RPZ on a supply used less than six months a year. RPZs protect high-hazard connections, so they are tested twice as often.
310 CMR 22.22(13)(d)Q2. Which of the following is considered a HIGH hazard cross-connection?
A high-hazard (contaminant) connection is one where backflow could bring in something toxic or disease-causing. A chemical feed system is connected to concentrated treatment chemicals, so it needs an RPZ or air gap. A sprinkler system without additives holds only stagnant water, which is a low hazard normally protected by a double check.
248 CMR 10.14Q3. The two conditions that cause backflow are:
Backflow happens only when the normal pressure relationship reverses: either downstream pressure rises above supply pressure (backpressure, from a pump, boiler or elevated tank) or supply pressure drops below atmospheric and pulls water backward (backsiphonage, from a main break or heavy fire-flow demand). Every backflow preventer is selected for which of those two conditions it must stop.
248 CMR 10.14These come from the PressureTest bank. Start 7 days free for the full bank, timed exam simulations and drawn isometric problems.