Seals.
A window air conditioning unit sits inside of an open window and cools the air it draws in from the exterior. The window air conditioning unit must then be sealed into the window so that the only air coming through is the air it pulls and sends out through the cooling unit. If the window doesnt have seals around its edges and theres light poking through, hot air is then able to come around the sides of the unit. Cool air may be coming through the window air conditioning unit, however, if hot air is coming through, youre not going to have a really cool house. So see to it those seals are tight. If you can see light, you need new seals. Also, see to it the window air conditioning unit is sitting in the window properly, as an odd angle could also allow air to escape into your house.
Debris Company .
As the window air conditioning unit pulls air into your house, its also pulling in dust, pollen and various other impurities. These can often obstruct the window air conditioning unit, so it will need to be cleaned every now and then to ensure youre getting nothing but cold air coming through.
Sound.
If you have used your window air conditioning unit for a while, you understand what a loud noise it makes. If it starts to make any odd noises or it gets even louder, thats an indication that something is wrong. Listen carefully to determine if its the fan or if something is possibly obstructing the airways.
If your window air conditioning unit is not functioning properly, it would be much better to get a new one rather than having a hot house or even run the risk of starting a fire. Naturally, central air would be much better, but if window air conditioning is all you can afford, at least see to it its running efficiently.