Air conditioners that are too large for a space tend to stop and start frequently, which is called short cycling. They use excessive energy and do not remove humidity effectively. Air conditioners achieve optimum efficiency when they cool a space sufficiently while running continuously, which is the goal of choosing an air conditioner to fit a 1,900 square foot house.
Air Conditioner Ratings
Air conditioners are rated by "tons" of BTUs (British thermal units) of heat that they can remove in an hour. One BTU is enough energy to raise the temperature of one pint of water 1 degree Fahrenheit. One pint of water weighs one pound. An air conditioner ton is 12,000 BTUs, enough heat to melt 1 ton of ice in one day. An average central air conditioner can remove 36,000 to 72,000 BTUs per hour or 3 to 6 tons.
The Environmental Protection Agency says the best air conditioner size for a house from 1,500 to 2,000 square feet is an air conditioner capable of producing 30,000 BTUs an hour. Reduce the BTU capacity 10 percent if an area is heavily shaded. Increase the capacity by 10 percent if the area is in the sun. Add 600 BTUs for each person above two who regularly occupy the area. Increase the capacity by 4,000 BTUs if the unit is used in the kitchen.
An air conditioner is least efficient when it starts. Air conditioners that are too large for a space are prone to short cycling. Air conditioners that run on long cycles are cheaper to operate than those that stop and start. Air conditioners condense indoor humidity on cooling coils and drain it as water outdoors. In short cycles the moisture on the coils evaporates back indoors. In humid climates air conditioners that run in long cycles will remove indoor humidity better than air conditioners that are too large.
The standard method of calculating the sizes of air conditioners necessary for residential cooling is Manual J developed by the Air Conditioning Contractors of America and the Air Conditioning and Refrigeration Institute. Manual J calculates all the variables of location, size, shape and construction of your house plus other environmental and energy calculations necessary for choosing air conditioners that run efficiently and save energy on long cycles.