The main types
| How it works | Can it be tested? | |
|---|---|---|
| Air gap | A clear gap between the water outlet and the highest level the water below can reach, so nothing can be drawn back | Nothing mechanical to test; it is checked by looking |
| Reduced pressure zone device (RPZ) | Two check valves with a relief valve between them that dumps water to the outside if either check valve leaks | Yes, through its test points |
| Double check valve assembly | Two independent check valves in series, with isolating valves and test points | Yes |
| Dual check valve | Two check valves in a compact body, often fitted at or near a water meter | No test points, so it is replaced rather than tested |
| Vacuum breaker | Lets air in to break a siphon when supply pressure drops; guards against back-siphonage only | Some pressure-type models have test points; simple hose-tap models do not |
How a plumber chooses one
The question is what could get back into the supply if the device failed, and how harmful it would be. Garden hose water is a small risk; a chemical dosing system or medical equipment is a serious one. The more serious the hazard, the more protective the device a plumber will fit, with the reduced pressure zone device or an air gap at the top end.
Where a device can be protected and reached also matters. Testable devices need clear access to their test points. An RPZ device releases water from its relief valve when it operates, so it needs somewhere for that water to drain and should not sit where it could be flooded.
What a plumber will do
For a new installation, a plumber will assess the connection and choose the device. They fit it with isolating valves, and usually a strainer to keep grit out, then test it before handing over.
For a device that has failed a test, the plumber will usually strip and clean it and replace the rubber parts and springs. If the body is damaged, the parts are no longer made, or the device type no longer suits the hazard, they will recommend a replacement. Our backflow testing page explains the test itself.
Living with a device
You can
- Keep the device and its test points easy to reach, and never box it in.
- Write down the make, model and serial number and keep them with your test reports.
- Tell a plumber if an RPZ device starts dumping water: it is warning you that a valve inside is not sealing.
- Leave the device alone otherwise. For what homeowners may do in your state, see what you can do yourself.
Leave to a licensed plumber
- Choosing, installing and commissioning a device.
- Repairs, servicing and replacement parts.
- Moving or removing a device, which should only happen with advice on what protection replaces it.
- Paperwork for installing or replacing a device.
Licences and certificates for device work
Plumbing licences come in classes, so ask which one covers the job. In the ACT, for example, the plumber classes include water supply plumber and sanitary plumber, and NSW wants a contractor licence for plumbing no matter how small the job.
Installing or replacing a device brings paperwork in some states. Western Australia says installing or replacing a backflow device is not minor work, so expect a certificate of compliance within 5 working days of the job. South Australia names backflow device work in its electronic certificate of compliance rules, with your copy due within 7 days. For other states, see our compliance certificate guide.
Questions to ask the plumber
- What hazard did you assess here, and why this device?
- Will the test points be easy to reach for future tests?
- Where will the relief valve discharge go?
- Will you test the device after installing it, and give me the result?
- Which certificate or record will I get for this work?
What we would do
If a device has failed a test, ask whether a service kit will fix it before agreeing to a new one; many failures are worn rubber parts. If you are adding equipment, irrigation or a tank, raise backflow protection at the planning stage so the device goes somewhere it can be reached and drained. Keep the device details and every test report together. A plumber who works on backflow devices regularly will know which parts are still available for older models.