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Blower Door systems typically come with two large Fan Rings ( A and B and a lid cover) but smaller ring inserts (C, D, and E) can be purchased additionally.

It is common to start a blower test with the fan completely blocked, and then removing only the smallest ring. If the target air pressure difference is not reached, the larger rings may be removed to allow for more airflow through the blower fan.

The smallest ring inserts (C, D, and E) are used for buildings with small volumes (tiny house) and very tight air barriers (Net Zero Homes).