Calculating the heat load is a fundamental skill for HVAC consultants and designers. Keep in mind that one of the biggest energy consumers in buildings, particularly in the summer, is space cooling. However, in order to properly size a space cooling system, we must first determine the amount of heat that must be removed. Calculating the heat load serves this purpose.
Buildings can generate heat from both internal (from electrical appliances, for example) and external (from the sun) sources. Calculating the heat load takes into account all available sources and calculates their combined impact.
Summary of the Principal Heat Sources
The following are some of the primary sources of heat generation in building interiors, though there are many other direct and indirect methods as well:
- Solar Heat Gain: Heat from the sun can enter interior spaces through conduction, convection, and radiation, among other methods. Due to the temperature difference between building interiors and the warmer outside environment, conduction happens across walls and roofs. Convection is the term for heat transfer caused by hot outdoor air moving in large quantities or by hot indoor air moving over cold surfaces. The last type of heat transfer is radiation, which happens when sunlight enters structures through windows or other transparent surfaces. Conduction can interact with radiation and convection at the surfaces of walls and roofs. The sun is the main source of heat for many structures.
Given that the sun's position in the sky changes throughout the day, the direction or alignment of a room has a significant impact on how much solar heat gain it receives. East-facing windows and walls receive direct sunlight in the morning. In the midday hours, south-facing surfaces and west-facing surfaces are both directly exposed to the sun. The least solar heat gain is experienced by walls that face north.
The effects of solar heat gain may be felt right away or gradually, depending on how it happens. For instance, solar heat (radiation) that enters through glass windows has an immediate impact. On the other hand, when heat is gained through conduction through walls, the walls themselves act as heat storage devices, releasing heat indoors throughout the night.
2) Heat from People: Inside of buildings, people are a major source of heat. Take into account that a person consumes hundreds of calories of food each day, and that some of this energy is converted into heat during metabolic processes. When performing vigorous physical activity, humans produce even more heat through perspiration (sweating).
Take into account the fact that human heat gain varies with occupant density. As a result, in big, air-conditioned spaces like halls, auditoriums, theatres, cinemas, and airports, people can contribute a lot more to the overall heat load.
3) Outdoor Air Heat: Also known as atmospheric air, outdoor air is the warmer air found outside of climate-controlled spaces. The average room temperature tends to rise when outdoor air enters indoor spaces because of its higher temperature.
Outdoor air can also enter conditioned spaces through leaks near doors, windows, and other building envelope components, even though some air exchange occurs naturally when doors and windows are open. Although the sun is a major source of heat in outdoor air, other sources include moving objects and other structures.
4) Heat from Electrical and Electronic Appliances: Electrical and electronic appliances, such as light fixtures, television sets, coffee makers, water heaters, etc., are ubiquitous in indoor spaces. In air-conditioned areas, these appliances use electricity and produce some heat. Reduce the heating effect of your appliances by using energy-efficient models.
Procedure for Calculating Heat Load
A survey of every room in the building is required to determine all the heat sources present in order to calculate the heat load. The HVAC designer then suggests the kind of air conditioner suitable for the application and its necessary capacity based on the heat load calculations. This strategy aids property owners in avoiding undersized systems that provide insufficient cooling as well as oversized systems with higher initial and ongoing costs.
Only a licenced HVAC specialist can perform the highly specialised, time-consuming, and complex task of calculating heat load. It also serves as the foundation for choosing an air conditioning system with the right type and capacity for the application at hand, such as a residential building, hall, auditorium, theatre, cinema, airport, etc., and is crucial to achieving optimal building performance.
If you are a property owner, keep in mind that HVAC contractors frequently need additional information, like the building's architectural plans. When all of the required information has been gathered, the calculation process begins. A heat load calculation can be done manually or with software, which are both viable options.
Manual method: Predefined equations and tabulated parameters are used to analyse the data obtained from the building survey and related documentation. Based on the building's geometry, the materials used in its construction, and the appliances and building systems found inside, the precise equations and table values to use are chosen. The HVAC designer suggests an air conditioning system with the appropriate type and tonnage based on these calculations.
Using software: To calculate heat loads today, the majority of HVAC designers use programmes like Trace 700 and HAP (Hourly Analysis Programme). Although this still requires a substantial amount of technical knowledge, many tedious and repetitive tasks have been automated. The information from the building survey, architectural plans, and other relevant documentation gathered must only be input. The process is made simpler and much quicker by the software's automatic heat load calculations, which also recommend the necessary air-conditioning system capacity.
All areas of the building's heat load are calculated, and the overall building load is also established. HVAC designers and consultants can offer technical recommendations to achieve peak performance based on these calculations.
