Can bogong moths damage a gas hot water system?

They can stop it working. Moths that get inside a gas continuous-flow unit can collect in the combustion fan, which then cannot move enough air through the burner. The heater is designed to stop firing when that happens, so the hot water goes cold. Clearing and servicing the unit usually gets it running again.

Why is my gas hot water making a loud noise?

A new roaring or vacuum-like noise usually points to the fan. Something caught in the fan wheel, like insects or debris, unbalances it or restricts the airflow. A worn fan motor can sound similar. In either case, turn the hot water off at the controller and have a licensed gas fitter look at it.

Should I spray insecticide on moths around my hot water unit?

No. Spraying a gas appliance is not safe, and it will not clear moths already inside. Bogong moths are also listed as endangered, and conservation groups are asking people not to use insecticides on them. Switching off outdoor lights at night helps keep them from gathering around your home.

How often should a gas hot water unit be serviced?

Manufacturers generally recommend a service at regular intervals, and the owner's manual for your model gives the schedule. A service cleans the burner and flues, checks gas pressures and tests the safety devices. Units in roof cavities and other hard-to-see spots benefit most, because problems are less likely to be noticed early.

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Case study · 4 min read

Bogong moths in hot water unit, Clovelly

A Clovelly gas hot water unit roared like a vacuum cleaner, then stopped heating. Its fan was packed with bogong moths. Adam and Greg cleaned and serviced it.

Adam Norton · 8 October 2026

Combustion fan removed from a Rinnai gas continuous-flow hot water unit in Clovelly, packed with dead bogong moths

The call came from a top-floor apartment in a three-storey block in Clovelly. For four days the gas hot water unit had been making a loud noise, like a vacuum cleaner running in the roof. Then it stopped producing hot water altogether. The owner had already guessed the cause, because Sydney was in the middle of a bogong moth swarm.

They were right. When Adam and Greg opened the unit up, its combustion fan was packed with dead bogong moths.

What bogong moths do to a gas hot water system

The unit is a Rinnai Infinity, a gas continuous-flow (instantaneous) water heater, installed in the roof cavity above the apartment. Like all units of this type, it has a fan that pushes air through the burner and out the flue. The heater needs that airflow to burn gas properly.

With dozens of moths jammed into the fan wheel, the fan could not run smoothly or move the air it was meant to. That explains the roaring noise. Gas water heaters are designed to stop firing rather than burn without enough air, so once the fan could no longer do its job, the hot water stopped.

Rinnai Infinity gas hot water unit in a Clovelly roof cavity with the cover off and the moth-filled combustion fan removed — The Rinnai with its cover off in the roof cavity. The fan is out and you can see the moths packed inside the wheel.

Cleaning and servicing the hot water unit

Adam and Greg removed the fan and cleared out the moths. Because insects had got inside the unit, they did a full service rather than only cleaning the fan. That meant cleaning the burner assembly, thermocouple and flues, checking the injectors and gas pressures, and cleaning the line strainer and water filters. They then tested and recommissioned the unit and checked it was working correctly.

If your gas hot water is playing up for other reasons, our hot water troubleshooting guide covers the common causes and what you can safely check yourself.

Why bogong moths are turning up in Sydney homes

Bogong moths migrate every spring, travelling up to 1,000 kilometres to caves in the Australian Alps, according to the Australian Museum. Strong westerly winds can push them off course toward the coast, and they are drawn to artificial light. The species was listed as endangered in 2021 after its numbers crashed, and this season it has been back in big numbers across NSW.

Once the moths reach Sydney, they look for dark, sheltered spots to rest during the day. Roof cavities, eaves and the inside of outdoor appliances all qualify. A hot water unit in a roof space, with openings for air and a flue, is exactly that kind of spot.

Signs moths have got into your hot water unit

A new roaring, rattling or vacuum-cleaner noise when the hot water turns on. That is often the fan struggling against something caught in it.
Hot water that cuts out or will not heat at all, especially if it started with a strange noise first.
An error code on the hot water controller that was not there before.
Lots of moths around the unit, the flue or your eaves. If you can see them gathering outside, some may be getting in.

Do not try to open a gas appliance yourself to clear it out. Turn off the hot water at the controller and call a licensed gas fitter. Also, do not spray insecticide into or around the unit. It will not fix the problem, and conservation groups are asking people not to use insecticides on bogong moths.

Hot water units in roof cavities

Plenty of Clovelly and Coogee apartment blocks have gas hot water units tucked into the roof space above the top floor. It saves room, but it also means the unit is out of sight. Problems like this one can build for days before anyone works out what is going on, and getting to the unit means a ladder and a trip into the roof.

If your hot water lives in the roof, it is worth having it serviced regularly, so a gas fitter sees inside it before something like a fan full of moths stops it.

Contact Norton Plumbing

Norton Plumbing has repaired, serviced and replaced gas and electric hot water systems across the Eastern Suburbs since 2019, from Clovelly and Coogee through to Bondi, Randwick and Maroubra. Adam Norton is the primary plumber on the tools and holds NSW plumbing licence 397768C. Phone 0477 858 951. Based at 10/11a-15 Berwick Street, Coogee NSW 2034. See our hot water service page or our Clovelly plumber page.

Frequently asked

Common questions

Can bogong moths damage a gas hot water system?: They can stop it working. Moths that get inside a gas continuous-flow unit can collect in the combustion fan, which then cannot move enough air through the burner. The heater is designed to stop firing when that happens, so the hot water goes cold. Clearing and servicing the unit usually gets it running again.
Why is my gas hot water making a loud noise?: A new roaring or vacuum-like noise usually points to the fan. Something caught in the fan wheel, like insects or debris, unbalances it or restricts the airflow. A worn fan motor can sound similar. In either case, turn the hot water off at the controller and have a licensed gas fitter look at it.
Should I spray insecticide on moths around my hot water unit?: No. Spraying a gas appliance is not safe, and it will not clear moths already inside. Bogong moths are also listed as endangered, and conservation groups are asking people not to use insecticides on them. Switching off outdoor lights at night helps keep them from gathering around your home.
How often should a gas hot water unit be serviced?: Manufacturers generally recommend a service at regular intervals, and the owner's manual for your model gives the schedule. A service cleans the burner and flues, checks gas pressures and tests the safety devices. Units in roof cavities and other hard-to-see spots benefit most, because problems are less likely to be noticed early.

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