The cold air is also trying to sneak into the building through every little crack in every nook and cranny. This is known as infiltration. Your heat, on the other hand, is trying to escape through every nook and cranny. This is known as exhilation. It's as if the house was breathing. Breathing both air and temperature in and out.
The total of all this leaking and losing at a specific low temperature for your region, is known as the heat loss. This total will be calculated in btu's per hour, and the heating system will need to produce and distribute this same amount of btu's per hour to maintaint your 68° F room temperature. As most rooms differ from one another, each room's heat loss must be determined. The total loss of all rooms added together will determine the size and design of the heating system.
In simple structures, the mere replacement of this lost heat is sufficient; but in complex houses with open floor plans and multiple levels, the flow of heat within the building becomes a factor. Heat rising from the first floor to the second, increases the demand on the first floor while decreasing the demand on the second.
The formula used here is a combination of three ingredients developed to reflect the internal conditions of a modern structure. It combines industry accepted standards of heat transfer with the old fahioned tin knocker "cfm method" of computation; blended together by 25 years experience designing and installing heating and cooling systems. The result is an estimate of comfort.
In the heat loss calculation, all windows are created equal, no matter which direction they face. Disallowing for wind factors, similar types of glazings lose heat at the same rate. On the other hand, when calculating heat gain, windows facing east and west gain more heat that those facing north and south. This results in larger quantities of air being distributed to rooms with east and west facing windows. This air is necessary for cooling but not for heating. In the more northern climates, where heating is a priority, enter all window areas as east and west shaded, regardless of which direction they face. This will restore the emphasis on a balanced distribution system rather than one weighted toward solar radiation.