ACCA MJ-8 and Window Heat Gain Calculations

Fenestration Cooling Load

There is a big difference between calculating heat gain for generic windows and advanced calculations when windows are rated by the National Fenestration Rating Council

For Generic windows

Tables 3A-1 (vertical glass) and 3C-1 (skylights) are used to locate the HTM.

Looking at Table 3A on page 289 we find all of the needed formulas. Table 3A-1 – Clear Glass is fairly straight forward. To find the HTM, the CTD must be calculated and the exposure known

Just make sure you understand that a Triple Pane window assembly has the same value as a Double Pane Low-e window assembly.

Now let’s look at Table 3D

It’s a whole new ball game, no wonder MJ-8 recommends using software to calculate the window loads. Using this method let’s look at page 303, Table 3D, Default Cooling HTM for NFRC Rated Fenestration Windows, Glass Doors and Skylights. Calculating the fenestration heat gains will require different formulas and more Tables to find formula values such as PSF and CLF (Table 3D).

 

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