A heat load is a calculated thermal quantity used to establish the size of an air conditioner necessary to effectively control the temperature in a building or space. Essentially the term heat load refers to the amount of heat that must be removed from any space in a given period to meet user requirements. This thermal characteristic of buildings or individual rooms is the product of numerous structural, environmental, and usage factors which all need to be considered. Very few individual spaces will exhibit identical thermal properties, so heat load calculations should ideally be made for each individual air conditioned area. Thermal load calculations may be done manually using tables of known values or by using heat load calculator software.
The thermal characteristics of any space are a complex combination of many factors. The location of the space, its use, the number of people who use it, and the number and type of appliances within the area all play a role. Seldom do two identical spaces, even if directly adjacent to each other, result in the same load. For instance, a bedroom housing three children each with their own computer will have a different heat load than the identical bedroom of a single young adult right next door. This would be particularly noticeable if one of the rooms received direct sunlight for most of the day.
The materials used in the construction of the space, the number of windows and doors in the space, and the prevailing weather conditions play a significant role in defining heat loads. The single largest contributor to the heat gain of any area is solar radiation. Whether the area receives direct sunlight and for how long each day are important variables in heat load calculations. The number or external doors and windows also plays a large role. The materials used in the construction and the type of insulation are also critical considerations.
Heat loads are also affected by the number of people who regularly inhabit a room. The number and type of electrical appliances in operation on a regular basis in the space are also essential parts of the calculation. If the space is used as a storage area, the type and quantity of materials stored and the materials they are stored in need to be included in any estimates of heat gain. This is particularly important when calculating heat load for freezers and cold storage rooms.
Heat load calculations may be carried out manually or with software applications. Manual estimates require tables which list known average values for the thermal latency of commonly encountered variables. Heat loads are expressed as a British thermal unit (BTU) value, and each item on the chart has its own BTU signature. These are added up to arrive at a total BTU rating for the space and an air conditioner with a suitable rating chosen. This allows heating and air conditioning units to be closely matched to the requirements of the application, thereby preventing over- or underrated systems.