Anticipating cooling and heating loads is an essential stage in HVAC design. Underestimate and your system may not be able to maintain a comfortable environment. Overestimate and you could waste money on an oversized unit. Below are some of the most impactful factors to be aware of.
As you may expect, the climate a building or home is subject to is most often the dominating factor. The larger the temperature difference between the outside and inside on any given day, the greater the HVAC load of the building. This heat transfer is generally conduction through the walls and roof of a building. However, climate also affects the amount of latent heat your HVAC system must respond to. Latent heat is the energy required to change the phase of any substance at a constant temperature. Removing humidity from the outside air is a sizable source of latent heat that your HVAC system must have the cooling capacity to handle. Humid, warm climates require the largest cooling capabilities.
Energy losses or gains through the exterior of a house are expedited by the presence of windows. This is because windows are subject to radiation heat transfer far more than walls and roofs.
While conduction and convection heat transfer are still present, radiation dominates energy losses and provide another challenge for buildings with a large window to wall ratio. Radiation through windows can be reduced by increasing the number of glass panes or adding low emission or “Low-E” coatings to the glass. Outside overhangs and interior shading can also help mitigate the impact windows have on cooling loads.
Heat gain from people inside the building is a very real consideration for HVAC design. The human body releases both sensible and latent heat through the metabolic process. In areas where the density of people is especially large (gyms, auditoriums, classrooms, airports, etc.), the human body can be the largest source of heat gain.
The type of activity people undertake while inside the building is also important to consider. Physical activity increases the amount of heat one gives off. Gyms, yoga studios, indoor athletic complexes, and more must take into account the physical activity of their occupants when designing their HVAC systems.
Lighting and Miscellaneous Equipment
As electricity is consumed by lights and electronics, waste energy is expelled as heat.
When the building is in heating mode, these influences can help the conditioning process. However, in warmer climates, internal gains from lights and electronic equipment represent another major cooling load to consider. In order to combat wasteful energy consumption, the International Energy Conservation Code has developed restrictions on watts per square feet for different building types. Thanks to the emergence of more efficient LED lights, satisfying code requirements is generally obtainable through reasonable design.
Modeling and Conclusion
It is important to note that these are not the only factors affecting HVAC loads, just some of the most common. HVAC engineers model these considerations and more (Size, Orientation, Construction, ETC) in specialized HVAC software to provide a comfortable living or work space.