a burner 1 of a heater , not shown specifically , has a variable - amount fuel feed means 2 as well as a controllable blower 3 , so that the burner output can be changed by varying the fuel and air throughputs . both the fuel feed means 2 and the blower 3 are usually actuated or driven electrically . the fuel feed means 2 as well as the output of the blower 3 are controlled by means of a control device 4 , which makes it possible to switch on a plurality of , usually very many power stages of the burner 1 . different power stages differ , in general , in terms of both the air throughput and the fuel throughput of the burner 1 . at the time of a changeover between two power stages , the control device 4 may initially initiate a phase of transition , during which the amount of fuel being fed to the burner 1 by the fuel feed means 2 , on the one hand , and the output of the blower 3 , and consequently the air throughput of the said burner 1 , on the other hand , are changed or adjusted continuously or &# 34 ; gradually &# 34 ; from the actual values of the power stage set previously to the values of the power stage set last . the values of the air and fuel throughput set after the end of the phase of transition are such that at a relative optimum of the fuel consumption is obtained at least approximately compared with the actual heat output . according to a first embodiment of the present invention , the different power stages can be set arbitrarily with a switch 5 . after the end of the corresponding phase of transition , the control device will then adjust the fuel feed means 2 as well as the blower 3 to the standard values of the air and fuel throughput , which are associated with the corresponding power stage , and maintain this setting . a handle ( setting element ) 6 is additionally provided , with which the amount of fuel fed to the burner 1 can be changed within predetermined limits without changing the blower output preset by the switch 5 . a power stage with relatively low blower output and corresponding with a relatively low blower noise can thus be set by means of the switch 5 within limits predetermined by the design , and the heat output can be increased and optionally also reduced by means of the handle 6 as needed . according to another embodiment , provisions may be made additionally or alternatively to control the room temperature in the driver &# 39 ; s cab , etc ., of a motor vehicle . a controller 7 with a thermostat 8 detecting the actual value of the room temperature and with a setting member 9 for arbitrarily presetting a temperature set point is used for this purpose . the controller 7 will send a signal reflecting the variance of the temperature to the control device 4 , which will then switch on different power stages of the burner in a predetermined manner , optionally taking into account the outside temperature , which may be detected by an additional thermostat 10 , until the temperature set point set with the setting member 9 is reached and is finally maintained at an extensively constant value by the output of the burner 1 being changed over alternately between two adjacent power stages . depending on the setting of a changeover switch 11 , the room temperature then continues to be maintained within a preset range of variation by continuously alternating between the adjacent power stages in the manner of a high - low control , or a changeover is made to a special mode of operation , in which the power stage with the lower blower output is switched on among the adjacent power stages , and that blower stage remains on , while the output of the burner 1 is otherwise controlled by varying the fuel throughput , i . e ., by intervening in the fuel feed means 2 . the changeover switch 11 may also be optionally eliminated , and a changeover is always made to the latter mode of operation when the temperature set point is reached . should the room temperature in the interior space of the vehicle deviate from the set value selected to a greater extent due to external effects , e . g ., due to a door of the vehicle being left open for a longer time , there is an automatic changeover , at least temporarily , to the mode of operation in which the control device 4 selects different power stages of the burner 1 depending on the temperature variance . if the control device makes it possible to switch on a plurality of regulating steps with a ratio of fuel throughput to air throughput predetermined according to a characteristic , it is possible , in a special mode of operation with constant air throughput , i . e ., constant blower output , to raise or lower the heat output by one power stage compared with a regulating step simply by changing the fuel throughput . the blower output must be changed only if the second regulating step is needed . however , this hardly ever happens during a prolonged stop of the vehicle with the doors and windows open , because a quasi - steady - state will now become established after a certain time , and only minor changes in the heat output are needed to maintain this . while a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention , it will be understood that the invention may be embodied otherwise without departing from such principles .