D1 The free areas in Section 1 of this approved document are geometric open areas that assume a clear sharp-edged orifice with a 0.62 coefficient of discharge (Cd). Different opening types will reduce the amount of air flow by both affecting the way air flows and reducing the physical area. Accounting for these factors gives the equivalent area.

D2 The equivalent area of a window can be calculated using one of the following.

a. The discharge coefficient calculator, available online at: https://www.gov.uk/government/ publications/classvent-and-classcool-school-ventilation-design-tool.

b. Tables D1 to D9.

NOTE: As stated in paragraph 1.12, measurement of equivalent area to BS EN 13141-1 is also appropriate.
Measurement of equivalent area is more accurate than calculation and is therefore preferable.

D3 Using Tables D1 to D9 to calculate equivalent area is appropriate for the following types of window.

a. Side, top or bottom hung windows. The opening hinge length is referenced as ‘h’ in the tables – Diagram D1 shows this for a side hung window. To use the tables for bottom and top hung windows, the ‘h’ and ‘w’ should be reversed.

b. Centre pivot windows. Each of the two sections of the centrally hinged window should be assessed as a single side, bottom or top hung window.

c. Sash windows. The opening should be assessed using Table D9, assuming an opening angle of 90 degrees.

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Diagram D1 Side hung window illustration with table references

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Table D1 Equivalent area of a window with an opening angle of α = 10°

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Table D2 Equivalent area of a window with an opening angle of α = 20°

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Table D3 Equivalent area of a window with an opening angle of α = 30°

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Table D4 Equivalent area of a window with an opening angle of α = 40°

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Table D5 Equivalent area of a window with an opening angle of α = 50°

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Table D6 Equivalent area of a window with an opening angle of α = 60°

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Table D7 Equivalent area of a window with an opening angle of α = 70°

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Table D8 Equivalent area of a window with an opening angle of α = 80°

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Table D9 Equivalent area of a window with an opening angle of α = 90°

Appendix E: Standards referred to

BS EN 13141-1 Ventilation for buildings. Performance testing of components/products for residential ventilation – Externally and internally mounted air transfer devices [2019]

Appendix D: Calculating equivalent area