A smoke detector would be placed (magnetic mounts) on the central duct, the vena cava if you will, of a home furnace's air return system. (Or the air return duct, if the house has only one air return).
When smoke is detected, it would trigger an alarm in the air ouput duct that would then reverberate
throughout the house, waking all therein. Would be an add-on, by drilling a hole in the air return ducts and placing the detector on the outside. Alarm unit wouldn't need a hole drilled, as metal conducts sound just fine. Just stick it right on there and wait for a fire.
Communication between the two units via radio signals or wire.
Not for use solo - it's additional to existing ceiling-mount smoke detectors.
Inspiration: I was brushing my teeth this morning when, through the ducts in my house, I heard fairly clearly the jangling of my cell phone's alarm, even though it was on another floor and across my house.