Sometimes you might find that your AC develops an unpleasant odour. The mildew or stale smell could mean that the water isn’t draining away from the indoor evaporator properly. A build-up can cause bacteria to form. The Air Conditioning Unit fan blows the odour this causes out – regular servicing can prevent this problem. In rare and extreme cases, the water in air conditioning units for large buildings can contain a type of bacteria called Legionella, so it’s vital to maintain and service your AC regularly.