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What Is Backflow?

A loss in pressure in the mains potable system from sudden demand or a pipe break, can allow used or contaminated water to flow back into the potable water system and could end up in people’s homes and drinking water.

Clean, potable water is, and always has been essential.  For more than 5000 years, humans have worked to ensure that their public water supply was safe to drink.  Ancient Egyptian records show that they had officials who inspected the water supply system every 10 days to confirm quality.

As toilets became common place in the 19th century, risk of cross-contamination of the water supply became more and more of an issue.

The author remembers the TV garden show, “Dig This” from the late 1970’s, where the presenter, Eion Scarrow, made up a holiday watering and fertiliser device based on a bottle with holes, up high in a plant pot, connected to a hose.

But the following week had to retract this suggestion as he’d failed to think of backflow prevention and the potential existed for fertiliser laden water to siphon back into the potable water system.

The type of backflow device (e.g. Reduced Pressure Zone, Double Check Valve etc) depends on how much risk ones work presents to the potable water system as per this example for the Christchurch City Council

Risk Activity Examples Device Testing
  Uses or produces toxic or bacterial matter that may cause death or serious illness if leaked into the main water supply Hospitals, mortuaries, chemical plants, cooling towers, air conditioners, hairdressing salons, commercial laundries Reduced pressure zone Annual
  Produces backflow that can endanger health Public swimming pools, garden irrigation systems Double-check valve Annual
  Could cause a nuisance from colour, smell or taste but does not endanger health Most private homes Air gap separation or hose vacuum break valve Annual

The video link here is from the US, but is a great example of how backflow events can occur from either

  1. Siphoning backflow events
  2. Pressure backflow events

Obviously, it’s crucial to prevent this occurrence.  Backflow preventers are devices designed to do just this via air-gaps and check valves with built in redundancy.

We’re simply putting a differential pressure (DP) gauge across points 1&2 and 2&3 etc to check for the absence of leaks under varying conditions.

Line drawing 1

If you’re a process person (like most of the Homersham staff) with no plumbing experience, when one first looks at backflow testing it can appear daunting.  But it’s actually very simple.

The backflow meter is nothing but a differential pressure gauge put across the various valves to check for leaks and tightness.  The diagrams below might be foreign to plumbing but will make sense to process people.

Line Picture 2 Procedures vary between jurisdictions, (but must conform to NZ Backflow Testing Standard) but the basics are the same.  Whether digital with a memory, like our fabulous back flow digital test rig – the Mako, or analogue gauge based, that’s all we’re doing i.e. testing DP across valves.

The New Zealand standard for testing may be found at Water New Zealand’s web site here


For the more practical, here’s a diagram of a typical backflow-preventer (this one a dual check valve).


Backflow Diagram chopped-leek
Oh No! A leek!
845-5 Picture 51ViwuN4otL

                            Analogue                                                                                           Mako - Digital
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