Heat Balance Method (HBM) and Radiant Time Series (RTS) method are the two non-residential cooling load calculation methods outlined by ASHRAE in their latest 2021 Handbook of Fundamentals. HBM is which software are normally based on and RTS is what we can perform manually with an excel sheet.

External and internal heat gains. From external heat gains, we need to calculate the sun position to determine the solar irradiance. Then, depending on the type of surfaces (wall, window, etc.), we need to calculate the incidence angle to find out the surface irradiance, including the diffuse and ground reflected irradiance. Together, the sol-air temperature can be calculated and the heat gain of the wall can be determined.

RTS uses the Conduction Time Series (CTS) factor to account for the time delay between the time when the irradiance hits on the surface until the heat gain is realized. Then, RTS applies a split between convective and radiant heat gains. The convective heat gain instantly becomes convective cooling load while the radiant heat gain goes through a time delay known as the RTS value/factor, before becoming radiant cooling load. The addition of the convective and radiant cooling loads give us the total cooling load for equipment sizing.

Use the RTS Cooling Load Calculator that I made to simplify the process, you can easily skip all the complicated formulas and focus on actually calculating the cooling load of your project. You'll learn the rest of the lessons through this calculator.