Even with ventilation, the temperature will still rise if it is running continuously. This is similar to playing with your mobile phone in an air-conditioned room; the temperature will still increase after a long period of use.
Are you saying that if you spend a long time with your phone in an air-conditioned room, its temperature will rise?
Ventilation is needed to cool the ASICs with cold air from outside, which is why the temperature of the ASIC chips should remain virtually constant during operation.