When Eileen Kelly's doctor saw the levels of carbon monoxide in her patient's blood, she wondered if the 74-year-old had taken up smoking.

Key points: Energy Safe Victoria says if you have a Vulcan Heritage or Pyrox Heritage heater, you should not use it until it is tested by a qualified gasfitter

Energy Safe Victoria says if you have a Vulcan Heritage or Pyrox Heritage heater, you should not use it until it is tested by a qualified gasfitter If you have one of these heaters, call the manufacturer, Climate Technologies on 03 8795 2462 to arrange testing

If you have one of these heaters, call the manufacturer, Climate Technologies on 03 8795 2462 to arrange testing If you live in DHHS housing, call 1800 148 426

She hadn't.

Had she been spending time around people who smoked? No, she hadn't.

Eventually, Eileen's doctor asked her an unusual question: Did she have a gas heater?

This time, the answer was yes.

She had been running her Vulcan Heritage heater almost around the clock in her small Greensborough public housing unit to keep her and her dog Odis warm.

It had been installed 10 years before by the Department of Health and Human Services (DHHS) around the same time Eileen's health started failing.

Eileen's doctor told her to get the heater checked immediately. So she did. Twice.

Both times, the gasfitter who checked the carbon monoxide levels in her flat told her there was "nothing to worry about".

But the ABC understands he did not do the test properly.

If he had, he would have found dangerous levels of carbon monoxide.

And it is very possible that Eileen's neighbour, Sonia Sofianopoulos, would still be alive.

Sonia Sofianopoulos with her grandson Adam, aged eight. ( Supplied )

'I knew as soon as the heater went on, Mum's here'

Sonia Sofianopoulos loved spending time with her friends and family, almost always over a Greek coffee.

"She drank a lot of coffee," her eldest daughter, Eleni Kontogiorgis, said with a laugh.

Most mornings before the sun rose, Sonia would let herself into her daughter's home, while Eleni and her family were still sleeping, put the heater on, and start preparing the family's breakfast.

"I didn't even put an alarm on, because I knew as soon as the heater went on, Mum's here, so I'd get ready and come down," Eleni said.

"We enjoyed it — just sitting there and having a chat. That was our catch-up time."

In July 2017, when Sonia's family had not heard from her for a couple of days, her daughter Stella went to check the Greensborough public housing unit.

What she found still haunts her.

"When Stella got there and she opened the door, all she got was a 'whoof' of air, as if she had been opening an oven," Eleni said, as her sister Stella sobbed beside her.

"The unit was really hot, she was struggling to breathe, she saw Mum on the ground."

Sonia Sofianopoulos was dead.

Her Vulcan Heritage heater was on, and there was a pot of chickpeas simmering on the stove.

Emergency services told the sisters Sonia had most likely had a heart attack.

"It just didn't make sense to me, that it was a heart attack," Eleni said, shaking her head.

"She had no hereditary heart disease. She saw the doctor very regularly, she was very careful about what she ate."

It was not until three-and-a-half months later, they learned the truth in a letter from the Coroners Court: Sonia had died from carbon monoxide poisoning.

"That just threw me," Eleni said.

"I was like, how?"

Sonia Sofianopoulos' grandson Adam and daughters Eleni Kontogiorgis and Stella Sofianopoulos. ( ABC News: Danielle Bonica )

'A vulnerable technology with potential fatal outcomes'

Sonia Sofianopoulos was not the first death caused by an open-flue gas heater.

In 2010, six-year-old Tyler Robinson and his eight-year-old brother Chase died of carbon monoxide poisoning while they were sleeping in their home in Mooroopna, near Shepparton.

The Victorian coroner found the carbon monoxide in their systems came from an IXL Finesse gas heater.

Energy Safe Victoria estimates there are hundreds of thousands of open-flue gas heaters in homes around Australia.

Most are in Victoria, where many people grew up with them.

Many like the one that killed Sonia are brown and rectangular, with a metal cage covering the gas-fed flames at the front.

A Vulcan Heritage gas heater. ( ABC News )

They are only ever meant to be used in big, draughty spaces, where the carbon monoxide can dissipate.

But over the years, they have ended up in a lot of places they should not have.

Using exhaust fans in bathrooms and kitchens while the heater is running can increase the danger by drawing the carbon monoxide into living areas.

Heaters that have not been serviced regularly or have been installed incorrectly also pose a much higher risk.

"There are many, many factors that can come together that can make [open-flue gas heaters] a vulnerable technology with potentially fatal outcomes," said Paul Fearon, chief executive of the regulator Energy Safe Victoria.

"There is a growing consensus that open-flue heaters are incompatible with modern energy efficient homes."

Despite this, there are still 19 open-flue gas heaters being sold in Australia, from modern-looking faux log fires to wall heaters.



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'That made me angry, that this hadn't been picked up earlier'

If Eleni Kontogiorgis had not started investigating how her mother had died from carbon monoxide poisoning, it is hard to say whether authorities would have acted.

When she received the autopsy results in November last year, she looked on the internet and saw that faulty gas heaters could lead to carbon monoxide poisoning, and called the department.

"They assured me that they checked all the heaters and everything was fine at the time," Eleni recalled, shaking her head.

"And I said, 'Well, how did my mum pass away from carbon monoxide toxicity if there was nothing wrong?'"

She said the DHHS employee questioned whether her mother may have "caught" the toxicity while she was on an outing.

"I didn't know what to say. I just looked at the phone thinking, 'Who am I talking to?' I was quite frustrated and angry."

Eleni went to her mother's former unit, and told the new tenant as well as the tenants of 16 other units not to use any of their gas appliances until they had been tested.

That was when her mother's neighbour, Eileen Kelly, told Eleni that she had been suffering carbon monoxide poisoning the previous year, and had her home tested.

"That made me angry, that this hadn't been picked up earlier," said Eleni.

"What sort of testing had been done in the past?"

Almost four months after Sonia's death, Energy Safe Victoria visited the Greensborough units and carried out its own carbon monoxide testing.

They found high levels of carbon monoxide emanating from the residents' Vulcan Heritage heaters, and decommissioned them all on the spot.

Eileen Kelly had been using an oxygen tank to breathe for the previous year because of her pulmonary fibrosis.

She was waiting outside her unit for the test to be done, when the representatives from Energy Safe Victoria suddenly came out of her apartment to join her.

"You're lucky you're on oxygen," she says they told her.

"We had to stop testing because it was too dangerous for us."

Energy Safe Victoria has issued advice on heater safety. ( Energy Safe Victoria )

Heaters must be regularly serviced and tested

Unlike other professions, Victoria's 6,000 gasfitters do not have to undergo ongoing training to maintain their licence to operate.

Some in the industry have told the ABC they believe potentially hundreds of gasfitters are conducting out-of-date carbon monoxide tests.

Eileen Kelly said the DHHS-contracted gasfitter who carried out her test did not follow the new procedure of testing, which requires the home's exhaust fans to be on.

It is only now she realises that she was in a type of "fog" when the gas heater in her home was on — feeling dizzy, weak, and confused.

All are symptoms of carbon monoxide poisoning.

She said her doctor told her that her health issues were definitely exacerbated by the near-constant exposure to carbon monoxide.

"Recently he said to me how much better he thought I was. He couldn't believe the difference," she said.

Do you know more about this story? Email vicindepth@abc.net.au

For now, Energy Safe Victoria has issued a safety alert for the Vulcan Heritage and Pyrox heaters, urging customers not to use them until they have them checked by the manufacturer.

If they are found to be faulty, they will be replaced by the manufacturer, Climate Technologies, at its own cost.

It has also forced Climate Technologies to stop making the Vulcan Heritage and Pyrox heaters.

Despite this, Energy Safe Victoria's Paul Fearon said the appliances were generally safe if serviced and tested every two years.

"Irrespective of whether it's a ban, recall or voluntary undertaking, it still requires the public to have this critical safety testing and servicing done."

But as Eileen Kelly discovered, it is extremely difficult for consumers to know whether the gasfitter is doing the job properly.

Mr Fearon said he supported the carbon monoxide training becoming mandatory for gasfitters, but avoided saying whether he believed many of them were conducting the test incorrectly.

"Gasfitters have a very substantial civil exposure if they have done that work and not received the training," he said.

A candle lit in memory of Sonia Sofianopoulos. ( ABC News: Danielle Bonica )

'We don't want this to ever happen again'

Given all the variables, Eleni Kontogiorgis wants open-flue gas heaters banned altogether.

"There's too many things that can go wrong," she said.

"You hear about it, things like this happening in third world countries … but here?"

A coronial inquest into Sonia Sofianopoulos's death will begin on Monday, and Eleni and Stella are hoping it will give them some answers.

"We don't want this to ever happen again," Eleni said.

DHHS declined to comment, saying it would be inappropriate to do so due to the pending inquest.