Here's a little conundrum for you. To get the right amount of heating and cooling to each room in your home, you need a load calculation. Rules of thumb don't work. But if you do a load calculation, the result isn't the size of air conditioner, heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump., furnace, or boiler you need. It's only the first step to sizing your system.
Do you know why? Let's take a look.
from Building Science http://www.greenbuildingadvisor.com/articles/dept/building-science/manual-j-doesn-t-tell-you-equipment-capacity