Sizing Residential Heating and Air Conditioning Systems
The size of the heating system is directly related to the amount of heat lost from the house or building. All structures lose heat to the outdoors or to adjacent unheated or partially heated spaces when the temperatures of the outdoor air or adjacent spaces are colder than those inside the structure. The heat within the building is normally lost by transmission through the building materials and by infiltration around doors and windows.
The loss of heat from a structure must be replaced at the same rate that it is lost. Consequently, determining the correct size of the heat system and the rated capacity of the heating plant required by the steam are very important. Unfortunately, many heating and/or cooling systems are either undersized or oversized, with the latter being the most common mistake. Undersizing means that the heating and/or cooling equipment does not have the capacity (output) to meet the heating and cooling requirements of the structure. Oversizing means that the heating and/or cooling equipment has more capacity than required. Both undersizing and oversizing are caused by using guestimates or rule-of-thumb sizing calculation methods.
Oversized heating equipment is more expensive to install, operates inefficiently, uses more energy resulting in higher fuel bills, creates uncomfortable indoor temperatures by providing more heat than the structure requires, and produces wide temperature swings. Oversized heating equipment also breaks down more often. Oversized units require larger air flow, resulting in noisier operation.
Undersized heating equipment lacks the capacity to provide sufficient heat, especially during extreme cold spells. Oversized air conditioners and heat pumps create higher than normal humidity levels indoors because they do not run often enough to dehumidfy the air. This is known as short-cycling. The dampness in the air can also result in unhealthy mold growth indoors.
This chapter describes several methods for calculating heat loss, ranging from rule-of-thumb methods to the more precise method of using overall coefficients of heat transmission (U-values) computed for the various construction materials and combinations of construction materials through which heat is commonly transmitted.