An elderly Sydney woman suffered carbon monoxide poisoning and three men were injured in another incident as the dangers of using unsafe heating in homes was underscored overnight.

The 81-year-old woman had been using an outdoor heater to warm the inside of her Surry Hills unit when she was found about 7.00pm yesterday.

Superintendent Paul Johnston from Fire and Rescue NSW said carbon monoxide readings showed she was lucky to have survived.

"Checking the reading, I think we came up with quite a large reading of carbon monoxide within [the] unit," he said.

"[That is] very dangerous, extremely dangerous and possibly even a lethal amount of carbon monoxide was found, [which] could have been fatal there and [that] could have been absolutely disastrous."

Superintendent Johnston said people should be aware of the dangers of bringing outdoor heaters inside their homes.

It follows an incident earlier this week in which a family of four suffered carbon monoxide poisoning from using an open-coal fire inside their Penrith home.

And earlier this month, a Sydney couple were killed by the fumes of a makeshift wood heater which was brought into a makeshift holiday cabin in Kurrajong near the Blue Mountains.

Meanwhile, a man suffered serious burns and two others were injured while trying to refuel a methylated spirits heater at Turramurra on Sydney's Upper North Shore overnight.

"A 43-year old male had burns to up to 18 per cent of his upper body and another two had some more minor burns than that," Superintendent Johnston said.

He said investigators will look at whether the heater was faulty, or it was not being refuelled correctly.