"A system that is too large will cool or heat your house quickly, but you may not feel comfortable. That's because it will satisfy the thermostat before it can adequately remove sufficient moisture from the air during the cooling mode, leaving you feeling sticky and humid. This could even lead to moisture and mold problems. And, the stress of short-cycling (too many starts and stops) will shorten the life of your equipment and increase your heating and cooling bills. On the other hand, a system that is too small just cannot get the job done, especially in extreme weather conditions. The air conditioner will run constantly in the Summer and the furnace will do the same in the Winter. But a correctly sized system isn't just based on the size of the structure. Many factors go into determining the size of the system. Including type of house and walls, type and size of windows, insulation, basement and attic conditions, house orientation, and so on. A Salesman must visit the house and take detailed measurements and notes while conducting the survey."