Calculating thermal load is a fundamental skill for HVAC designers and consultants. Consider that space cooling is among the highest energy costs in buildings, especially during the summer. However, to properly size a space cooling system, we must first know how much heat must be removed – this is precisely the purpose of calculating the thermal load.
Heat in buildings can come from internal sources, such as electrical appliances, or external sources, such as the sun. A thermal load calculation considers all sources present and determines their total effect.
Overview of main heat sources
While there are many ways in which heat can be generated, directly or indirectly, the following are some of the main sources in building interiors:
1) Solar heat gain: There are three different ways in which heat from the sun can reach indoor spaces – conduction, convection and radiation. Conduction occurs through walls and roofs as they are exposed to a temperature difference between the interior of the building and the warmer outside environment. Convection refers to the transfer of heat due to the mass movement of hot outdoor air or the movement of indoor air between surfaces at different temperatures. Finally, radiation is a direct form of heat transfer that occurs when sunlight enters buildings through windows or other transparent surfaces. Both radiation and convection can interact with conduction on wall and roof surfaces. For many buildings, the sun is the largest source of heat.
Solar heat gain for a specific room depends largely on its direction or alignment – consider that the sun's position in the sky changes throughout the day. In the morning, east-facing walls and windows are exposed to direct sunlight. South-facing surfaces are exposed to direct sunlight during midday and west-facing surfaces during the afternoon. North-facing walls have the lowest solar heat gain.
Depending on how solar heat gain occurs, its effects may be felt immediately or over a period of time. For example, the entry of solar heat through glass windows (radiation) has an immediate effect. On the other hand, when heat gain occurs through conduction through walls, the walls themselves store heat and this continues to be released inside during the night.
2) Heat from Humans: Occupants are also an important source of heat inside buildings. Consider that a human being consumes hundreds of calories every day in the form of food, and part of this energy is released in the form of heat during metabolic processes. The heat released by humans is even greater during intense physical activities, through perspiration (sweating).
Consider that the warming effect of humans also increases depending on occupant density. As a result, the human contribution to the total thermal load can be especially high in large air-conditioned spaces, such as rooms, auditoriums, theaters, cinemas and airports.
3) Heat from outside air: The warmer air outside conditioned spaces is called outside air or atmospheric air. Due to its higher temperature, outdoor air tends to increase the average ambient temperature when it enters indoor spaces.
While some air exchange is normal when doors and windows are open, outside air can also enter conditioned spaces through leaks around doors, windows and other elements of the building envelope. The heat retained by outside air comes largely from the sun, but it can also come from vehicles or other buildings.
4) Heat from electrical and electronic devices: Interior spaces are full of electrical and electronic devices, such as lamps, televisions, coffee machines, water heaters, etc. These devices consume electricity and release some heat in air-conditioned spaces. Use energy-efficient appliances to minimize their heating effect.
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Thermal load calculation procedure
To calculate the thermal load, it is necessary to carry out a survey of all rooms in the building and identify all heat sources present. Then, based on the calculated thermal load, the HVAC designer recommends the type of air conditioning system suitable for the application and the required capacity. This approach helps homeowners avoid oversized systems that have higher initial and operating costs than necessary, as well as undersized systems that provide insufficient cooling.
Calculating heat load is a highly specialized, time-consuming and complex task that can only be performed by a qualified HVAC professional. It is also a very important step in achieving optimal building performance, providing the basis for selecting an air conditioning system of the appropriate type and capacity for the application in question: residential building, hall, auditorium, theater, cinema, airport, etc.
If you are a property owner, consider that HVAC professionals typically request additional information, such as the building's architectural plans. The calculation procedure begins as soon as all necessary data has been collected. There are two possible methods to perform a thermal load calculation: manually or through software.
Manual process: The data collected in the building inspection and complementary documentation are analyzed using pre-defined equations and tabulated parameters. The exact equations and table values to use are determined based on the building's geometry, construction materials, and the building appliances and systems found inside. Based on these calculations, the HVAC designer recommends an air conditioning system of the appropriate type and tonnage.
Using software: Nowadays, most HVAC designers use software like Trace 700 and HAP (Hourly Analysis Program) to perform heat load calculations. This still requires extensive technical knowledge, but many repetitive and time-consuming tasks are automated. Simply enter the data obtained from the building survey, architectural plans and other relevant documentation collected. Thermal load calculations are made automatically by the software, which also recommends the required capacity of the air conditioning system, making the process easier and faster.
Thermal load calculations are performed for all areas of the building, and the total building load is also determined. Based on these calculations, HVAC designers and consultants can provide technical recommendations to achieve maximum performance.
Final Recommendations
Professional design services may seem like an expense, but they're actually an investment. A well-designed HVAC system meets the cooling load of the building it serves with an optimal cost of ownership. Working with professionals also ensures code compliance and streamlines paperwork, which can be time-consuming in New York.
If your property has a large coverage area, consider solar energy. There are excellent rebate programs in New York, and you can reduce the effect of solar heating while getting a clean source of electricity.
Editor's Note: This post was originally published in 2017 and has been reworked and updated for accuracy and comprehensiveness.
