Obtain Building Characteristics
- Construction materials.
- Construction material properties: U-values, R-values, shading coefficients, solar heat gain coefficients.
- Size.
- Color.
- Shape.
- Location.
- Orientation, N, S, E, W, NE, SE, SW, NW, etc.
- External/internal shading.
- Occupancy type and time of day.
Select Outdoor Design Weather Conditions
See https://hvac-eng.com/weather-design-conditions-for-selected-locations/
- Temperature.
- Wind direction and speed.
- Conditions in selecting outdoor design weather conditions:
- Type of structure, heavy, medium, or light.
- Is structure insulated? If the structure is heated or cooled, the structure must be insulated by code.
- Is structure exposed to high winds?
- Infiltration or ventilation load.
- Amount of glass.
- Time of building occupancy.
- Type of building occupancy.
- Length of reduced indoor temperature.
- What is daily temperature range, minimum/maximum?
- Are there significant variations from ASHRAE weather data?
- What type of heating devices will be used?
- Expected cost of fuel.
Select the indoor design temperature to be maintained in each space.
See https://hvac-eng.com/indoor-design-conditions/
- Estimate temperatures in unconditioned spaces.
- Select and/or compute U-values for walls, roof, windows, doors, partitions, etc.
- Determine the area of walls, windows, floors, doors, partitions, etc.
- Compute the conduction heat gains for all walls, windows, floors, doors, partitions, skylights, etc.
- Compute the solar heat gains for all walls, windows, floors, doors, partitions, skylights, etc.
- Infiltration heat gains are generally ignored unless space temperature and humidity tolerance are critical.
- Compute the ventilation heat gain required.
- Compute the internal heat gains from lights, people, and equipment.
- Compute the sum of all heat gains indicated in items G, H, I, J, and K earlier in this list.
- Include morning cool-down for buildings with intermittent use and night setup.
- Take into account excess HVAC system capacity permitted for morning cool-down.
- Consider equipment and materials that will be brought into the building above the inside design temperature.
- Cooling load calculations should be conducted using industry-accepted methods to determine the actual cooling load requirements.
FREQUENTLY ASKED QUESTIONS
Selecting outdoor design weather conditions involves considering factors such as temperature, wind direction and speed, and other conditions specific to the location. The type of structure (heavy, medium, or light), insulation, exposure to high winds, infiltration or ventilation load, amount of glass, time of building occupancy, type of building occupancy, and length of reduced indoor temperature also influence the selection of outdoor design weather conditions. For example, a building in a hot and humid climate will require a higher outdoor design temperature than one in a mild climate.
Infiltration and ventilation load refer to the heat gain and loss through air leakage and intentional ventilation in a building. These loads can significantly impact the cooling load, especially in buildings with high occupancy rates or those located in areas with high outdoor temperatures. Infiltration and ventilation load can be estimated using various methods, including the air change per hour (ACH) method or the infiltration credit method. Accurate estimation of these loads is crucial to ensure that the cooling system is sized correctly.
Determining the indoor design temperature for each space in a building involves considering factors such as the type of occupancy, activity level, and desired comfort level. For example, a office space may require a lower indoor design temperature than a warehouse or storage area. The indoor design temperature should also take into account the outdoor design weather conditions and the building’s construction characteristics. A general rule of thumb is to maintain an indoor design temperature between 22°C to 25°C (72°F to 77°F) for most commercial and residential buildings.
Shading coefficients represent the amount of solar radiation that is blocked by external shading devices such as overhangs, louvers, or blinds. A lower shading coefficient indicates more effective shading, which reduces the cooling load. Shading coefficients can be estimated using various methods, including the ASHRAE Clear Sky Model or the Solar Heat Gain Coefficient (SHGC) method. Accurate estimation of shading coefficients is crucial to ensure that the cooling system is sized correctly and to optimize energy efficiency.
Solar heat gain coefficients (SHGC) represent the amount of solar radiation that enters a building through windows and other fenestration. SHGC values range from 0 to 1, with lower values indicating less solar heat gain. The SHGC value depends on factors such as the type of glazing, frame material, and shading devices. Accurate estimation of SHGC values is crucial to ensure that the cooling system is sized correctly and to optimize energy efficiency. For example, a building with low-E glazing and external shading devices will have a lower SHGC value than one with single-pane glazing and no shading devices.
Occupancy schedules and time of day significantly impact the cooling load, as they influence the amount of heat gain and loss through the building envelope. To account for occupancy schedules and time of day, you can use various methods, including the bin method or the heat gain calculation method. These methods involve estimating the heat gain and loss during different periods of the day and adjusting the cooling load accordingly. For example, a building with a high occupancy rate during the day may require a higher cooling load than one with a low occupancy rate at night.
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