the insulation which faces the screen is preferably coated as for example by spraying a very thin reflecting aluminum layer 20 the air gap 7 between the screen and the insulation is about 10 mm wide towards the center the conical metal screen 2 is affixed to a cylinder 8 which may be made of the same material as the screen 2 a fastening member 9 designed for fastening the cylinder 8 and the conical screen 2 together and anchors the entire assembly to a hook 10 in the ceiling a control unit 11 is preferably secured to the outside of the cylinder 8 the control unit 11 is connected to an electric terminal 13 in the ceiling through a crown contact plug 12 the resistance element 1 is also connected to the control unit 11 above the entire apparatus a thermistor 14 is provided near the ceiling the thermistor senses the indoor air temperature and is electrically connected to the control unit 11 near the center of the apparatus a sensor e g an infrared sensor or ir detector 15 is provided and at the rear or upper side of which a potentiometer 16 is centrally attached the tip of the potentiometer is secured near the center of the cylinder through a snap lock 17 the sensor or detector and the potentiometer 16 are electrically connected to the control unit 11 with this arrangement one can change.