HVAC load calculation is the cornerstone of refrigeration and air conditioning engineering design. An accurate load calculation determines equipment capacity appropriateness, system operating energy efficiency, and the economic performance throughout the entire HVAC system lifecycle. Oversized design values lead to idle equipment, wasted initial investment, and poor part-load efficiency; undersized design values result in indoor environments failing to meet comfort standards, potentially jeopardizing manufacturing processes or equipment safety[1]. This article begins with design day condition selection and systematically explores cooling load components, the evolution of calculation methodologies, and engineering practice considerations from load calculation results to equipment selection.

1. Design Day Condition Selection

The first critical decision in HVAC load calculation is selecting design day outdoor air conditions. A Design Day is a hypothetical day representing extreme climatic conditions at the building's location, serving as the basis for HVAC system capacity design. ASHRAE's Handbook — Fundamentals, Chapter 14, provides statistical design conditions for thousands of weather stations worldwide, classified by exceedance frequency into 0.4%, 1%, and 2% levels[6].

Taking Kaohsiung as an example, ASHRAE weather data shows its cooling design day conditions (0.4% exceedance frequency) as 34.2°C dry-bulb temperature with 27.4°C coincident wet-bulb temperature[6]. This means statistically, only about 35 hours per year will the outdoor dry-bulb temperature exceed 34.2°C. The corresponding values for Taipei are approximately 35.4°C dry-bulb and 27.1°C wet-bulb. Choosing between 0.4% or 1% design conditions depends on the building's purpose and the owner's risk tolerance — hospitals, cleanrooms, and other critical facilities typically use 0.4%, while general commercial offices may consider 1%.

Of particular note is the importance of wet-bulb temperature in cooling design. Wet-bulb temperature directly affects cooling tower heat rejection capacity and chiller condenser-side performance. In Taiwan's high-temperature, high-humidity subtropical climate, the design day wet-bulb temperature is often more critical than the dry-bulb temperature — it determines the minimum cooling tower size and the lowest achievable condenser water temperature[5].

For indoor design conditions, ASHRAE Standard 55 defines the environmental conditions range for human thermal comfort[2]. Typical summer indoor design conditions for commercial spaces are 24–26°C dry-bulb and 50–60% relative humidity. However, indoor condition selection involves more than just comfort — lower indoor temperature setpoints mean greater cooling loads and higher energy consumption. Each 1°C increase in indoor temperature setpoint can reduce annual HVAC energy consumption by approximately 3–5%. Therefore, moderately raising indoor temperature setpoints is the most direct energy-saving measure while meeting operational requirements. Taiwan's Building Technical Regulations also recommend that general office HVAC setpoint temperatures should not be lower than 26°C[7].

2. Cooling Load Components

The Cooling Load is the amount of heat that the HVAC system must remove from the indoor space at a specific time to maintain indoor design conditions. Its components can be divided into external loads and internal loads[1].

External Loads

External loads originate from heat transfer driven by indoor-outdoor temperature differences and solar radiation, including the following items:

  • Solar radiation heat gain through glass: This is typically the single largest component of commercial building cooling loads. Solar radiation passing through glass is partially absorbed by indoor surface objects and released to indoor air through convection. Its magnitude depends on glass area, orientation, shading coefficient (SC) or solar heat gain coefficient (SHGC), and external shading effectiveness. West-facing facades receive the highest solar radiation intensity in the afternoon, particularly significant at Taiwan's latitude[9]
  • Conductive heat gain through exterior walls and roof: Driven by indoor-outdoor temperature differences through conductive heat transfer via the building envelope. Its magnitude depends on envelope area, thermal transmittance (U-value), and equivalent temperature difference. Roofs experience higher equivalent temperature differences due to direct solar radiation exposure
  • Infiltration air load: Sensible and latent heat loads from untreated outdoor air infiltrating through door and window gaps. In modern, well-sealed commercial buildings, this load is typically small, but can be considerable in older buildings or locations with poor access control

Internal Loads

Internal loads originate from various indoor heat sources:

  • Occupant heat dissipation: Human heat dissipation varies by activity level — seated office work approximately 75 W/person (of which sensible heat approximately 75 W, latent heat approximately 55 W), while light factory work can exceed 200 W/person. The latent heat proportion of occupant heat dissipation should not be overlooked, especially in high-occupancy-density spaces such as conference rooms and restaurants[1]
  • Lighting heat dissipation: Lighting fixtures convert electrical energy into light and heat. LED lighting heat generation is approximately 70–80% of rated power, while traditional fluorescent fixtures approach 100% (including ballast heat). Lighting power density (LPD) is typically measured in W/m², with general office spaces at approximately 8–12 W/m²
  • Equipment heat dissipation (Plug Loads): Includes heat from computers, copiers, servers, and other office equipment. Modern open-plan office equipment heat density is approximately 10–20 W/m², while data center server rooms can reach 500–2,000 W/m² or higher. Equipment heat estimation must account for diversity factor — not all equipment operates at full load simultaneously during peak periods

Fresh Outdoor Air Load

To maintain indoor air quality, HVAC systems must introduce a certain volume of fresh outdoor air. ASHRAE Standard 62.1 specifies minimum ventilation requirements for various space types[3]; for office spaces, minimum outdoor air per person is 2.5 L/s plus 0.3 L/s per square meter of floor area. Processing fresh outdoor air (cooling and dehumidification) often accounts for 20–40% of total cooling load; in Taiwan's hot, humid climate, this proportion may be even higher. Outdoor air load calculation must consider both sensible and latent heat — under Kaohsiung's design day conditions, outdoor air humidity ratio is approximately 21–22 g/kg, far higher than indoor design conditions of 10–12 g/kg, making the latent heat load considerable.

Diversity Factor

In practical calculations, the diversity factor is a correction that cannot be overlooked. Different building zones do not reach peak load simultaneously — east-facing rooms peak in the morning, west-facing rooms peak in the afternoon; conference rooms are not full all day. Properly applying diversity factors prevents system oversizing. The typical whole-building diversity factor for general commercial office buildings is approximately 0.7–0.85[4], meaning the simple sum of zone peak loads should be multiplied by this coefficient to derive the whole-building system design load.

3. Calculation Methodologies

The evolution of HVAC load calculation methods reflects the engineering community's deepening understanding of building thermal physics and advances in computational tools[8].

CLTD/SCL/CLF Method

The CLTD (Cooling Load Temperature Difference)/SCL (Solar Cooling Load)/CLF (Cooling Load Factor) method was the earliest widely used manual calculation method, developed by ASHRAE in the 1970s. This method uses pre-calculated correction factor tables to convert instantaneous heat gain into cooling load, accounting for building thermal mass storage effects. CLTD values represent equivalent temperature differences corrected for solar radiation and thermal storage, used for calculating conductive loads through exterior walls and roofs. The method's advantage is operational simplicity, suitable for preliminary estimates and manual verification, but its tabulated coefficients are based on specific building type assumptions, limiting applicability[4].

Transfer Function Method (TFM)

TFM was introduced by ASHRAE in the mid-1970s as the first dynamic load calculation method based on rigorous mathematical models. It uses discrete-time Z-transfer functions to simulate the transient heat transfer behavior of building envelopes, converting time-series environmental excitations (such as 24-hour temperature and solar radiation variations) into indoor cooling load responses. TFM requires hourly calculations and pioneered computerized load calculation, though its transfer function coefficient generation process is relatively complex[8].

Radiant Time Series (RTS) Method

The RTS method is currently recommended by ASHRAE for nonresidential building cooling load calculations[1]. Its core concept separates heat gain into convective and radiant portions: convective heat gain immediately becomes cooling load, while radiant heat gain is distributed over time through "Radiant Time Factors" to convert into hourly cooling loads. Compared to TFM, the RTS method is more transparent and intuitive — engineers can clearly see how each heat gain item contributes to cooling load at each hour after thermal storage delay. The radiant time factors are determined by building construction thermal mass characteristics; lightweight construction buildings have less delay effect, while heavy construction (such as thick concrete) has greater delay effect.

Software Tools

In modern engineering practice, load calculations almost entirely rely on professional software. Mainstream commercial tools include Carrier's HAP (Hourly Analysis Program), Trane's TRACE 700/3D Plus, and the ASHRAE-supported open-source engine EnergyPlus. HAP and TRACE integrate load calculation, equipment selection, and annual energy analysis functions, suitable for engineering design practice. EnergyPlus, with its highly flexible modeling capability and open-source architecture, has become the preferred choice for academic research and green building certification simulation[5]. Regardless of the tool used, the engineer's judgment on input parameters and ability to validate results remain the key to ensuring calculation quality. Software is just a tool — professional judgment cannot be replaced.

4. From Load to Equipment Selection

The ultimate purpose of load calculation is to provide the basis for equipment selection. However, between the calculated peak load and actual equipment capacity, several important engineering considerations remain[5].

Safety Factor Considerations

In engineering practice, adding a safety factor to the calculated load is common practice to cover uncertainties in calculation assumptions, flexibility for future space use changes, and performance degradation as equipment ages. A generally recommended safety factor is 10–15%[4]. However, the safety factor should not be excessive — an oversized safety factor is equivalent to intentional equipment oversizing, causing chillers to operate at low loads long-term, wasting initial investment and reducing system operating efficiency. Experienced engineers should use precise load calculations instead of oversized safety factors.

Chiller Selection

Chiller selection is based on the whole-building peak cooling load (including safety factor). Key design decisions include: number of chillers (typically 2–4 units, balancing redundancy and part-load efficiency), machine type selection (centrifugal, screw, or scroll, depending on capacity range), and variable-speed vs. fixed-speed options. In Taiwan's climate conditions, building HVAC systems operate at 50–75% part-load approximately 70% or more of the time, making IPLV (Integrated Part Load Value) a more representative indicator of actual annual energy performance than full-load COP[5].

Air Handling Unit (AHU) Coil Selection

Cooling coil selection for AHUs must simultaneously satisfy sensible and latent heat processing requirements. The Sensible Heat Ratio (SHR) is the critical parameter — SHR is defined as the proportion of sensible heat load to total load. Typical office spaces have an SHR of approximately 0.7–0.8, while high-occupancy-density or high-outdoor-air-ratio spaces (such as conference rooms, restaurants) may have SHRs as low as 0.5–0.6, meaning more than half the load is latent heat (dehumidification). Low-SHR spaces require lower coil leaving air temperatures to achieve sufficient dehumidification, directly impacting chilled water temperature and coil row design.

Pump and Cooling Tower Selection

Chilled water pump flow is determined by chiller cooling capacity and chilled water supply-return temperature differential (ΔT). Using a larger temperature differential design (such as 7°C instead of the traditional 5°C) can effectively reduce water flow and pump energy consumption. Cooling tower selection depends on condenser-side heat rejection (approximately 1.2–1.3 times the cooling capacity) and design day wet-bulb temperature. Cooling tower approach temperature is typically designed at 3–5°C; under Kaohsiung's design wet-bulb of 27.4°C, condenser water leaving temperature is approximately 30.4–32.4°C[6].

Part-Load Considerations

Excellent HVAC system design focuses not only on meeting design day peak loads but also on operating efficiency under various part-load conditions throughout the year. Staged chiller configurations, variable frequency drives (VFDs), dynamic chilled water temperature adjustment, and cooling tower free cooling strategies are all critical means of improving part-load performance. The IPLV weighting formula defined by AHRI Standard 550/590 (100% load at 1%, 75% at 42%, 50% at 45%, 25% at 12%) clearly reflects the dominance of part-load operation.

5. Special Considerations for Taiwan's Subtropical Climate

Taiwan is located in a subtropical island climate zone at 22–25 degrees north latitude, with several special considerations for HVAC load calculation and equipment design that differ from temperate regions[9].

High Humidity and Dominant Latent Heat Load

Taiwan's average summer relative humidity reaches 75–85%, with outdoor air humidity ratios frequently exceeding 20 g/kg. Compared to North American or European continental climates, Taiwan's HVAC systems must handle far greater latent heat loads (dehumidification demand). This means cooling coil design leaving air temperatures must be sufficiently low (typically 12–14°C) for effective dehumidification, chilled water supply temperatures should not be too high (generally maintained at 7°C), and energy recovery ventilators are particularly effective for reducing outdoor air processing loads. In certain high-outdoor-air applications (such as hospital operating rooms, cleanroom vestibules), Dedicated Outdoor Air Systems (DOAS) paired with reheat systems have become the preferred solution for indoor humidity control[3].

Plum Rain and Typhoon Season Impacts

Taiwan's annual plum rain season (May–June) and typhoon season (July–September) bring extreme humidity conditions. During the plum rain season, weeks of high-humidity, low-temperature outdoor air (e.g., 25°C, 95% RH) severely test the HVAC system's dehumidification capability — sensible heat load is not large, but latent heat load is extremely high, and systems relying solely on temperature control may lose indoor humidity control. Engineering design should consider independent humidity control loops or AHUs with reheat capability. Low atmospheric pressure and strong wind effects during typhoon approaches also increase building infiltration loads[9].

West-Facing Solar Radiation

At Taiwan's latitude (approximately 23–25°N), summer west-facing facades receive solar radiation intensity exceeding 700–800 W/m² during afternoon hours. For commercial buildings with large-area glass curtain walls, west-facing solar heat gain may account for over 40% of that zone's total cooling load. HVAC load calculations must accurately model hourly solar heat gain through west-facing glass, and equipment selection must ensure adequate cooling capacity during afternoon peak periods. If west-facing window area can be effectively controlled or high-performance shading systems employed during the architectural design phase, HVAC system design capacity and operating energy consumption will be significantly reduced[7].

Urban Heat Island Effect

The Urban Heat Island Effect in metropolitan areas like Kaohsiung and Taipei can cause city center temperatures to be 2–4°C higher than surrounding suburban areas. ASHRAE design day weather data is typically based on airport weather station observations; buildings in urban areas may face actual outdoor air temperatures higher than these design values. In HVAC load calculations for urban core areas, engineers should evaluate whether design day temperatures need heat island effect corrections. Additionally, dense urban environments reduce natural ventilation potential and increase long-wave radiation exchange effects between adjacent buildings[9].

Conclusion

HVAC load calculation may appear to be a purely numerical exercise, but it actually embodies the engineer's comprehensive understanding of building physics, climate characteristics, occupant behavior, and equipment characteristics. As calculation tools become increasingly advanced, software can handle complex mathematical models, but the reasonable assumptions for input parameters, engineering judgment of calculation results, and the various trade-offs in the process from load to equipment selection remain the irreplaceable core value of professional engineers. Particularly under Taiwan's unique subtropical high-humidity climate conditions, directly adopting design practices from temperate countries is often inappropriate — only through deep understanding of local climate characteristics combined with rich engineering experience can truly rational designs be achieved.