The first thing to determine when designing a central heating and cooling system is whether it will primarily be a heating or a cooling system. This may sound ridiculous at first, but here's what it means. A system installed in a house in Florida is primarily a cooling system, while a system installed in Vermont will be a heating system. The systems will supply both heating and cooling according to the season, but will operate most of the time in the primary mode due to the length and extremes of their particular climate.

There are many more days in Vermont that require heat than cool, and vice versa for Florida. The reason for determining the type of system is based on the reasoning that one system cannot effectively supply both heating and cooling through the same ductwork. The demands of certain rooms for heating cfm can vary significantly from that same room's cooling cfm needs even when different blower speeds are factored into the equation. The heat loss for a room peaks at the coldest hour of the day, which is in the early morning darkness, while the heat load for that same room reaches its peak in the mid-afternoon sun. The contribution of solar radiation throws a wrench into the formula, and disconnects the calculations from a pure temperature driven equation.

A window which faces west will gain more heat in the afternoon than a window facing north, but a window facing west will not lose more heat than the same size window facing north. If delivered enough air to that room to meet the peak heat gain requirement during the cooling cycle, the room will receive too much air flow, and therefore too much heat during the heating cycle. This condition is acceptable if the system is primarily a cooling system with only minor time periods spent in the heating mode, but if the system is operating in the heating mode most of the time, the occupants will not be comfortable. This system should be designed on its heat loss figures.