Rain Harvesting 90mm Air Gap Backflow Prevention


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Rain Harvesting 90mm Air Gap Backflow Prevention

Protect your rainwater from contamination by preventing stormwater backflow – and mosquitoes – entering your tank through your outlet pipes.

The Air Gap prevents stormwater backflow from entering a rainwater tank, by creating a physical air break on the outlet of the tank overflow and before the stormwater drainage system.  The Air Gap also acts as a visual inspection point to detect trickle top-up system failure.

Prevent stormwater contamination

By creating a physical air break in your tank overflow outlet pipe, the Air Gap stops stormwater surges from back-flowing into your tank and contaminating your rainwater. Importantly, it still lets overflowing water flow out of your tank outlets at up to 2.9 litres per second.

Keep mosquitoes out

With dual, mosquito-proof 955 micron stainless steel screens, the Air Gap also protects the rainwater in your tank against contamination from mosquitoes and larger insects and animals.

Install and clean with ease

The Air Gap is straightforward to install and maintain. Paint the exterior as desired and easily remove both mosquito-proof screens for cleaning when required.


  1. Select an installation point for your Air Gap. This must be a vertical section of pipe after your tank overflow. Where practical, we recommend installing your Air Gap at shoulder height to provide more convenient access for maintenance. Consider whether your stormwater is a wet or “charged” line to determine the appropriate minimum installation height.
  2. Remove a 142mm section of pipe at your desired installation point. The Air Gap incorporates two 90mm female sockets that glue directly to 90mm pipe. If you’re using adaptors to fit the Air Gap to your existing, non-90mm pipe, you may need to remove an additional length of pipe to allow for these adaptors. Ensure all cut edges are clean and smooth.
  3. Orient your Air Gap so it is the right way up. To do this, refer to the “Install this side facing up” instructions on the product.
  4. Apply solvent weld glue to inside the sockets of the Air Gap or adaptors and also outside of each pipe.
  5. Fit the Air Gap into position and insert both pipes into the sockets and hold until glue sets.


How does backflow occur, and how does an air gap help?

In an extreme rainfall event there is the possiblity that water can back up through the stormwater overflow pipe and towards the rainwater tank.

If your local stormwater infrastructure (beyond your property boundary) becomes blocked, the system can “charge”, causing dirty stormwater to surge back through your stormwater pipework. If your rainwater tank overflow is connected to stormwater, this “charged” stormwater can backup into the tank.

If the pipes connected to the overflow of the rainwater tank are crushed, excess water from any pipework connected to the same system could flow back towards the tank, taking with them whatever debris is in the system.

If your tank is going to a soakage pit, in an extreme rain event the pit could become saturated and water can flow back up towards the

An air gap prevents this problem by creating a physical gap between your rainwater tank and stormwater lines. With an air gap, overflowing stormwater is prevented from backing up into your tank. Instead, it will spill out through the front of your air gap.

My screens keep falling out. What should I do?

Check that the bayonet fitting is engaged correctly. Push and turn the top filter to secure the screen in place.

Should I install my air gap at the top or bottom of the overflow?

You can install your air gap at the top or bottom of the vertical overflow pipe. Installing your air gap closer to the top of the overflow will generally make it easier to maintain.

We recommend ensuring you don’t have a charged line which could effect the height of installation.

Can I plumb 100mm pipe into a 90mm air gap?

You can connect the downstream (bottom) side of your air gap to a 100mm pipe with the use of a 100×90 socket reducer. Please consult a licenced plumber for suitability in reducing the upstream (top) connection to your air gap.Can I run two or more pipes into one first flush diversion chamber?

The overflowing water from my tank is spilling out the front of my air gap. How can I fix this problem?

Overflowing rainwater from your tank can begin leaking from the air gap if the bottom screen becomes dirty or obstructed. To prevent this problem, regularly check and clean your bottom screen.


Rain Harvesting

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