Heat load calculation is a critical task and foundational skill that HVAC consultants and designers must master to be successful.  When you take into consideration that heating and cooling a commercial building is one of the highest energy expenses in building management, it becomes very clear how important this calculation is.  In order to accurately determine the proper size cooling system for your project, you must first know the volume of heat that needs to be transferred out of the building. This, by definition, is the exact purpose of heat load calculation.

There are many sources of heat in a building; some originate internally, such as appliances, and some originate from external sources, such as the sun.  A proper heat load calculation factors in all of the sources at the time of measurement and determines the total effect of their heat.

Most Common Heat Sources

Heat can be generated many ways both directly and indirectly— and some are from very unlikely or often overlooked sources. That being said, there is a commonality in all buildings of heat sources that tend to remain constant no matter what building you are studying.

Solar Heat

While solar energy is a great way to conserve energy and generate power, it also can cause heating and cooling issues depending on the building design. There are three different types of solar heat that can affect the heat load calculation of a building—convection, radiation, and conduction.

Conduction is when the outdoor barriers of a building, such as walls or the roof, are exposed to a difference in temperature between interiors and the warmer outdoor environment.

Convection is a term used to describe heat transfer due to movement of large pockets of warmer outdoor air or indoor air from rooms that are at a warmer temperature.

Radiation is a direct heat transfer that occurs when unfiltered sunlight penetrates buildings through windows or other clear surfaces.

For most buildings, the sun is that largest source of heat and therefore proper design to mitigate the effects of solar heat in a design is extremely important.

Solar heat gain in a specific room is heavily dependent on its alignment with the sun at the time of calculation. For example, in the morning hours eastward walls will be exposed to direct sunlight, whereas southward walls and surfaces get direct sunshine around noon, and walls facing north experience the lowest solar heat gain.

Solar heat gains effects can be felt differently depending on how it occurs. For example solar heat coming from glass windows exposed to sunlight (radiation) will be felt immediately and raise temperatures quickly. Conversely, heat gain from conduction through walls will have a long lasting effect as they store heat and release it over a long period of time.

Heat From People

Human beings are a major source of heat within building structures. Humans consume on average at least 2,000 calories through food and drink. The metabolic process that converts that to energy is a thermogenic process that generates heat. Even more heat is generated when humans are engaged in strenuous physical activity, such as working out or brisk walking. Occupant density magnifies the heat gain from humans and this means the human contribution to total heat load can be quite significant.

Atmospheric Air Heat

The air outside that is warmer is referred to as outdoor or atmospheric air. Since its temperature is typically warmer, this air raises the indoor temperature when it enters interior corridors and spaces.

Some air transfer from outdoor to indoor spaces is normal when doors and windows are ajar, but it can also be a result of leaks and other elements. The heat outdoors mainly comes from the sun, although some of it can come from vehicles or heat from other structures. This is very common in congested downtown areas, such as NYC or Chicago.

Heat from Electrical and Electronic Appliances

Interior spaces are filled with appliances, such as lights, A/V equipment, coffee machines, water heaters, and computers.  These devices consume electricity, warm up and must expel heat so they don’t overheat. This heat affects your heat load calculation more than one might realize. To mitigate this heat, use energy efficient appliances that are designed to be more efficient in both energy use and heat dissipation.

Heat Load Calculation Process

For a heat load calculation, it is imperative to take survey of all the spaces in the building, notate the heat sources in each space and tally their heat production levels. Once the heat load is calculated, an HVAC designer will suggest the best HVAC system that will sufficiently cool the building and the size of unit needed.

This approach helps prevent unnecessary spending on units that go over the suggested specifications which cost more to run and maintain. On the other end of the spectrum, this calculation also helps avoid purchasing undersized units that will struggle to properly cool the building, resulting in an overstressed unit which will shorten its lifecycle.

MEP Delta Design Recommendations

Heat load calculation is a highly specialized and tedious task that is complex in nature and should only be handled by a qualified HVAC professional. It is one of the key steps in achieving optimal cooling performance by providing the basis for choosing an HVAC system that is appropriate for the application.

If you are a property owner, consider that HVAC designers typically need complementary information, such as the building plans and purpose of the space, and other similar information to account for the calculation.

While professional HVAC consulting and design services may seem like an expense to your project, they are actually an investment. When you hire MEP Delta Design to handle your project, we deliver with an HVAC system recommendation that meets the building needs dead-on—no more, no less.

This gives you the proper cooling without overspending unnecessarily or from not buying a proper system and paying more in the long run for correction.  If your project needs MEP consulting you can trust, contact us today to schedule an initial consultation regarding your project.