Hvac Load Calculations


The accurate calculation of heating and cooling loads is essential to provide a sound bridge between fundamental building design decisions and an operating building. If loads are substantially underestimated, occupants and users will likely be hot or cold. If loads are substantially overestimated, equipment will be oversize (usually wasting money, reducing efficiency, increasing energy consumption, and often imperiling comfort). Accurate load calculations are an important part of the design process. This importance is underscored by the constant evolution of load calculation methodologies—which has steadily made load calculations more complex, less intuitive, and more dependent upon computers.

It is imperative that a beginning HVAC&R engineer have a good grasp of the fundamentals of load calculation concepts. Total reliance upon computer software for load analysis is not wise. The adage “garbage in, garbage out” applies perfectly well to load calculations.

Equipment and systems are sized from “design” loads, which are calculated using statistically significant weather conditions that reflect a building location’s climate. A design heating load represents heat loss from a building under a series of generally agreed upon assumptions. A design cooling load represents heat flow into a building via the building envelope and from internal sources, again under a commonly accepted set of assumptions. The term heat gain is generally used to describe undifferentiated heat flow into a building or space. The term cooling load is used to describe that portion of heat gain that will affect air (as opposed to building material and content) temperature at a given point in time. The vast majority of air-conditioning systems respond directly to cooling loads through thermostatic control (and only indirectly to heat gains).

Loads are sensible (affecting air temperature) or latent (affecting relative humidity) or a combination of sensible and latent. Loads may be external (passing through the building envelope) or internal (originating within the building envelope). Space loads affect a particular portion of a building at some point in time; equipment loads are those seen by equipment at some point in time. Equipment loads for central components may not equal the sum of design space loads due to diversity (non-coincidence) of loads, such as between east-facing and west-facing rooms.

Sensible and latent heating and cooling loads arise from heat transfer through the opaque building envelope; solar heat gain through windows and skylights; infiltration through openings in the building envelope; internal heat gains due to lighting, people, and equipment in the conditioned spaces; and outdoor airflow for ventilation and building pressurization. These loads are described in detail in several chapters in the ASHRAE Handbook—Fundamentals. Typically, design heating load calculations do not include heat gains to the space, since peak losses typically occur during the night (unoccupied hours for most nonresidential buildings). When appropriate, heating credit may be taken for a portion of lighting, occupancy, and equipment gains—but not for solar gains (passive-solar- heated buildings are an exception to this general rule).

Development of a comprehensive building energy analysis requires the HVAC&R designer to consider the many loads, other than just the design load, that occur throughout a typical year in the life of a building. This type of analysis requires year-long hourly weather data (rather than just design conditions) and substantial computation to calculate loads at off-peak conditions and the resulting response of equipment to such loads. Although there are some manual methods that allow an approximation, accurate energy analyses require fairly sophisticated computer simulation capabilities. There are numerous software programs that can provide such analyses, but these are often specialized programs with steep learning curves.

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