Four fire engines and around 25 firefighters were called to a house fire on Cromwell Road in Hounslow.
Part of the ground floor of a three-storey terraced house was damaged by fire. There were no reports of any injuries.
A London Fire Brigade spokesperson said: “This incident shows just how important it is to test your smoke alarms regularly. There were smoke alarms fitted inside the property, but they failed to raise the alarm, so crews fitted new smoke alarms before they left.
“The residents were asleep at the time of the fire and were awoken by the smell of smoke. Upon investigation they could see black smoke outside the bedroom door on the first floor and quickly left the building.
“Smoke alarms give the earliest possible warning when a fire starts, providing time to escape. We encourage everyone to have smoke alarms fitted in every room where a fire could start, plus a heat alarm in the kitchen. It's important to test them regularly."
The Brigade was called at 1023 and the fire was under control by 1056. Fire crews from Heston, Feltham, Twickenham and Southall fire stations attended the scene.
The Brigade’s Fire Investigators believe the fire was accidental and caused by an electrical event in a bathroom extractor fan.