Central air conditioning has only become common in homes in the last few decades. If your home was built before the 1990s, there's a good chance the builder did not construct it with air conditioning in mind. You may have a boiler heating system or baseboard heat, which does not lend itself well to installing central air. But that does not mean you can't have air conditioning. Here are a few AC options that work well when central AC is not an option.
How many rooms in your home do you actually need to stay cool? If you live in a rather moderate climate and really only need your bedroom to be a few degrees cooler in the summer, then you might be able to get away with just a window unit. This type of air conditioner mounts inside a hung window. You put it in the window in the spring and remove it in the fall. The downside to window units is that they are a bit cumbersome to install and are not terribly energy-efficient. However, they are cheap, and you can put them in yourself; you don't need to hire a pro.
Ductless AC Units
Ductless air conditioning systems have two components. There is an outdoor condensing unit, much like the unit you would have with central air conditioning. Then, there is an air handler, which is a device that mounts through your wall. The cool air comes from the condenser and is blown into your home via the air handler. If you have a larger home, you could have two or three air handlers hooked up to the same condenser unit.
Ductless AC units need to be professionally installed, and they take up quite a bit of space on the wall -- although they are often mounted near the ceiling where they are more out of the way. However, they cool more thoroughly than window units, are much more energy-efficient, and do not need to be removed and stored each fall.
Portable AC Units
If you do not have hung windows and therefore cannot use a window air conditioner, another option is a portable AC unit. This is a freestanding air conditioner that sits on the floor. It vents out a window. Often, portable air conditioners are on wheels so you can move them around a little -- although you cannot move them too far because the vent tube needs to stay attached to the window.
You typically still need to put a portable AC unit away at the end of cooling season. They're not terribly efficient, but they don't require professional installation and are a good quick fix for a hot home without hung windows.
For more information on air conditioners, contact an appliance store.