a 2 b is according to the invention connected to at least one diluting air input line 5 a 5 b 5 c 5 d that is connected to the surrounding atmosphere atm at one end and connected at its other end to the ventilation channel 2 b through a valve 4 a 4 b 4 c and 4 d a control unit cpu is connected to the temperature sensor 3 and to the relevant valves 4 a 4 b 4 c and 4 d in the dilution lines 5 a 5 b 5 c and 5 d which control unit cpu opens and closes the relevant valves when the temperature exceeds pre determined threshold values that are set and stored in the control unit four dilution lines 5 a 5 d are shown in the drawing but it is preferable that at least two dilution lines 5 a 5 b are connected to the ventilation channel 2 b with first 4 a and second 4 b valves in the associated dilution lines 5 a and 5 b and where the control unit opens the relevant valve when a first or second threshold value is exceeded the first threshold value is a pre determined first temperature t level1 and the second threshold value is a pre determined second temperature t level2 where t level1 lt t level2 the system can.