Real-time cooling and heating load breakdown: envelope, window, solar gain, occupancy, lighting, equipment, and ventilation. Computes SHR, recommended AC capacity, and annual energy use.
What exactly is a "thermal load" in HVAC? Is it just how much heat is inside a building?
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Basically, it's the amount of heating or cooling energy you need to add or remove to keep a space at your desired temperature. It's a balance between heat gains and heat losses. In practice, for cooling, we calculate all the heat sneaking in through walls, windows, from the sun, and even from people and computers. Try moving the "Outdoor Temp" slider in the simulator above—you'll see the load change instantly as the temperature difference ($\Delta T$) between inside and outside changes.
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Wait, really? People and computers count as a heat source? And what's this "SHGC" parameter for the windows?
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Absolutely! A person at rest gives off about 100 Watts of sensible heat. SHGC, or Solar Heat Gain Coefficient, is crucial. It tells you what fraction of the sun's solar radiation passes through the glass and becomes heat inside. A common case is a west-facing office with high SHGC windows—it can get very hot in the afternoon. In the simulator, if you increase the SHGC or the Window-to-Wall Ratio, you'll see the "solar gain" part of the load shoot up.
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So the calculator gives me one number for cooling. But I've heard about "sensible" vs "latent" load and something called SHR. What's that about?
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Great question! Sensible load ($Q_{sens}$) changes the air temperature—what a thermometer feels. Latent load ($Q_{lat}$) is the energy needed to remove moisture from the air, like from people breathing or cooking. The SHR, or Sensible Heat Ratio, is $Q_{sens}/ (Q_{sens}+Q_{lat})$. For instance, a typical office has a high SHR (~0.85), meaning mostly temperature control. A humid swimming pool area has a low SHR (~0.5), needing a system that can dehumidify. The simulator calculates both and shows you the SHR, which is key for selecting the right HVAC equipment.
Physical Model & Key Equations
The total sensible cooling load is the sum of heat transfer through the building envelope (transmission), solar radiation through windows, and internal heat from occupants, lights, and equipment.
Where $U$ is the U-value (W/m²K) of walls ($w$), roof ($r$), and glass ($g$); $A$ is the corresponding area; $\Delta T = T_o - T_i$ is the outdoor-indoor temperature difference; $I$ is the solar irradiance (W/m²) based on location and orientation; and $SHGC$ is the Solar Heat Gain Coefficient of the glazing.
The Sensible Heat Ratio (SHR) is a critical design parameter that determines the type of cooling and dehumidification equipment needed. It is the ratio of sensible load to the total (sensible + latent) cooling load.
$$SHR = \frac{Q_{sens}}{Q_{sens}+ Q_{lat}}$$
Here, $Q_{lat}$ is the latent load, primarily from occupant moisture generation and outdoor air ventilation. A low SHR (e.g., < 0.7) indicates a high latent load, requiring specialized HVAC systems with strong dehumidification capability.
Frequently Asked Questions
It may be because JavaScript is not enabled in your browser. Also, pressing the Enter key after entering a value or moving focus away will trigger recalculation. If you are using a slider, it updates automatically while dragging.
If the SHR is below 0.7, it indicates a large latent heat load (dehumidification). General air conditioners are designed with an SHR of around 0.7 to 0.8, so if it is low, consider using a model with higher dehumidification capacity or a separate dehumidifier.
Singapore has a hot and humid climate with a high cooling load throughout the year, especially from solar radiation and outdoor air intake, resulting in higher sensible and latent heat loads than Tokyo. This tool calculates based on the design outdoor air conditions (temperature and humidity) for each region, so the capacity changes.
The SHGC depends on the window glass specifications. Typical double-glazed glass is around 0.6, Low-E double-glazed glass is 0.3 to 0.4, and glass with solar control film is 0.2 to 0.3. If the manufacturer's specifications are unknown, use a general value (e.g., 0.5) as an initial setting.
Real-World Applications
Preliminary Building Design & Equipment Sizing: Before running complex hourly energy simulations, architects and engineers use calculations like this to estimate the required capacity of chillers, air handlers, and ducts. For instance, setting the "Location Preset" to Phoenix vs. Seattle will drastically change the solar and temperature loads, impacting the tonnage of the AC unit needed.
Code Compliance and Green Building Certification: Tools like this support quick checks against standards like ASHRAE 90.1 or for LEED certification. By adjusting parameters like Wall U-value and Window SHGC, a designer can see if their proposed building envelope meets the maximum allowed thermal load targets.
Retrofit and Renovation Analysis: When upgrading an existing building, engineers can model the impact of improvements. A common case is replacing old single-pane windows (high U-value, high SHGC) with modern double-pane low-E windows. Sliding those values in the simulator shows the direct reduction in both transmission and solar gain loads.
Specialized Space Design: The SHR output is vital for spaces with unusual moisture loads. Designing a HVAC system for a gym, restaurant kitchen, or museum archive requires knowing if a standard AC unit (high SHR) or a dedicated dehumidifier (low SHR) is needed to protect the space and ensure comfort.
Common Misunderstandings and Points to Note
First, understand that "maximum load does not equal constant load". This tool calculates the "peak load" during the hottest (or coldest) period. For example, the cooling load in Tokyo during summer peaks around 2 PM when solar radiation is strongest. However, it's much smaller in the morning or at night, right? While your air conditioner needs the capacity to handle this peak, the actual annual energy consumption is often determined by "part-load" conditions—much smaller loads sustained over longer periods. Therefore, when considering energy savings, you need to look not just at the peak value but also at efficiency during part-load operation (e.g., IPLV).
Next, do not blindly accept the "default values" for parameters. For instance, if the default for "lighting heat gain" is 10 W/m², but your room in question uses all LEDs at 5 W/m², you must correct this value to avoid overestimating the load. Conversely, in a room with unusually high equipment heat gain like a server room, failing to review that value carefully could result in insufficient cooling capacity. The tool is convenient, but remember: you are responsible for the input values.
Finally, note that the effects of "air infiltration" or "partitions" are not included in the calculation. This calculator assumes a single space facing the outside air. In practice, however, heat transfer from adjacent hot rooms or air inflow from door openings become factors. Heat exchange with neighboring rooms through ceiling voids or underfloor spaces is particularly easy to overlook and can change the actual load by 10-20%. After completing your estimate, get into the habit of questioning: "Is this room truly isolated?"