A heat flow detector is described which measures a predetermined rate of heat flow in its surroundings. The surroundings being in the proximity of a recessed incandescent fixture which is designed to operate safely in the designated environment. Should the operating environment change for the fixture it will be sensed by the heat flow detector. If the operating environment becomes more dangerous so that a fire or other damage may occur the heat flow sensor will turn the incandescent lamp off and on at a slow rate to indicate the changed operating environment.The heat flow detector will cause a slow turn off and on if an oversize lamp is inserted in the incandescent fixture. The heat flow detector will then prevent an overheated condition due to an incorrect bulb.