Through the wall air conditioners are installed like the name says, right though the wall of the house. They are installed higher up near the ceiling because cold air is heavy and will sink down towards the floor. They also can double as room heaters. The hot air can be directed downward by using the unit's air directional controls.
The power of an air conditioning unit is measured in BTU's, British Thermal Units. Through the wall air conditioners come in BTUs from 8,000 to 12,000. The size of the unit depends on the size of the room. For a room that is 350 square feet, choose 8,000 BTUs; for 425 square feet choose 9,000; for 500 square feet choose 10,000 BTUs; for 640 square feet, 14,000 BTUs; for 900 square feet, choose 15,100 BTUs; for 1,110 square feet choose 18,000 BTUs; for 1,170 square feet choose 18,500 BTUs; for 1,435 square feet choose 22,000 BTUs. If the room is not properly insulated, choose the next higher size.
Through the wall air conditioners have advantages over the in the window models. They are installed in a sleeve that gives a good tight fit, keeping unwanted drafts out when the unit is not in use. They are not as obvious as a window unit which can protrude out into the wall and be a danger to a small child or prevent people from opening and closing the window.
Make sure that the filter is easy to access. You will need to change it once a year. Check the energy efficiency rating. The higher the rating, the more money you will save. A rating of 10 is the highest. You will need to know the thickness of the wall the unit will be installed in before you buy it.
Installing a through the wall air conditioner will add to the value of you home. It is considered a permanent installation since you had to put a hole in the wall. It will be one feature that prospective home buyers will like. The manufacturers are always coming up with new and improved models and there is the potential that you will want to change the unit in the future. That is not a problem. They will always make the same sizes, and many models from one company will fit into a sleeve from another, so if your new choice is a different brand, chances are you will just have to put in the new unit.