A smoke detector has a housing, a smoke sensor in the housing capable of detecting smoke therein, and an alarm connected to the sensor signaling when the smoke sensor detects smoke. A partition subdivides the housing into separate first and second chambers, and the housing is formed with first and second smoke holes allowing air from the exterior into the first and second chambers. The partition has a sensor port provided with a sensor for detecting air flow through the sensor port and a fan port having a fan for drawing air from one of the chambers and forcing it into the other chamber. A controller connected to the fan and to the air-flow sensor operates the fan means to move air from the one chamber through the fan port into the other chamber and generates an output when air flow through the sensor port exceeds a predetermined limit.