Reduced pressure principle assembly (RPZ) testing

What a reduced pressure principle assembly is

A reduced pressure principle assembly (called RP, RPZ, or reduced pressure zone device) is the highest level of mechanical backflow protection. Like a double check, it has two spring-loaded check valves in series. Between them sits a hydraulically operated relief valve. The relief valve holds the pressure in that middle zone lower than the supply pressure at all times. If either check leaks, or supply pressure drops, the relief valve opens and dumps the zone water out to a drain instead of letting it move backward toward the municipal main.

That discharge is the point. An RPZ fails safe: if something inside is wrong, water visibly comes out of the bottom of the device rather than silently flowing the wrong way. It is also why the device has strict rules about where it can be installed.

Where an RPZ is required

Durham Region’s By-law 24-2018 classifies hazards using the CSA B64 series. Severe hazards are connections where backflow could carry something dangerous to health at any concentration. The Region’s program page names the RP assembly as the required device for that class. In practice that means:

Setting	Why it is severe hazard
Boiler and heating plants with chemical treatment	Corrosion inhibitors, glycol, biocides on the water side
Medical, dental, veterinary	Bodily fluids, sterilisation, dental unit water lines
Restaurants and commercial kitchens	Carbonators (CO2 and copper), dish machine chemicals
Car washes	Detergents, waxes, recycled wash water
Food and beverage processing, industrial process water	Process chemicals, non-potable loops
Buildings with an auxiliary water supply	Well, cistern, or reclaimed water on the same premises

If the Region’s survey classified your building as severe hazard, the premise isolation device on the incoming service is an RPZ, and it is tested every year.

Installation rules that affect testing

An RPZ must not be installed in a pit, vault, or anywhere it could be submerged, because the relief valve port must always be open to air. It also needs a drain that can take the full relief valve flow, and it must be protected from freezing. When we schedule a test, the location matters: a device in a heated mechanical room with a floor drain is straightforward. A device that was installed in a pit before the rules were applied, or in an unheated space, may be flagged as an installation issue on top of the test result. That is a plumbing correction, and we will tell you plainly if we see it.

What the annual RPZ test checks

The test uses a calibrated differential pressure gauge on the assembly’s test cocks and works through four things:

1. Relief valve opening point. The tester lowers the zone pressure and confirms the relief valve opens at or above the minimum differential in CSA B64.10.1, before the first check would allow leakage.
2. Check valve 1. Holds tight against reverse flow and maintains its minimum differential.
3. Check valve 2. Holds tight, tested independently.
4. Shut-off valves and test cocks. Both shut-offs close fully. Test cocks are free and not leaking.

Readings, device make, model, serial number, size, and location go on the report, which is filed through BSI Online. The Region reviews it and issues the test tag. The $25 BSI filing fee applies per report, subject to annual increases.

Why RPZs fail

• The relief valve discharges constantly. Almost always the first check is fouled or worn, so the zone pressure creeps up and the relief valve keeps opening. Sometimes it is debris under the relief valve seat itself.
• The relief valve does not open when it should. A stuck diaphragm, a blocked sensing line, or scale. Dangerous because it removes the fail-safe.
• Check discs, seats and springs wear out exactly as they do on a double check.
• Freezing and flooding. Unheated rooms and old pit installations account for a share of winter failures and cracked bodies.

A failed RPZ is reported as a failure. The Region has to be notified within the short window the by-law sets, a licensed plumber does the repair or replacement, and the assembly is retested. See what happens after a failed test. Most repairs are a rebuild kit rather than a new assembly.

Testing an RPZ in Durham Region

RPZs are common in the buildings that most need after-hours scheduling: restaurants, clinics, and plants that cannot lose water mid-shift. Tell us where the device is and when the water can be off for an hour, and we book around that. Same program, same by-law, same filing whether the building is in Pickering or Port Perry. Details on the visit itself and pricing.