Smoke alarm cut hours before Hobart fire

A smoke alarm was disconnected just hours before a house fire broke out, threatening the lives of a Hobart family.

Fire crews were called to the blaze at the northern suburb of Bridgewater about 8am on Monday after a heater sparked nearby materials.

"Had that one member of the family not woken to find the room alight, firefighters would have been confronted with a very different type of incident," regional fire investigator Adam Doran said.

"The reality is this family is very lucky to be alive."

Mr Doran confirmed the property was fitted with smoke alarms, but added they had been disconnected on Sunday night while food was being cooked.

The fire caused about $250,000 damage.