Whole-house ventilation infiltration credit — total minus infiltration equals mechanical fan flow

The 2019 credit subtracts blower-door infiltration by superposition (Eq 4-2), not the deprecated 2010 "half the excess" rule.

A three-box equation: the total required rate (90 CFM by ASHRAE) minus the natural infiltration estimated from a blower-door Q50 (about 20 CFM) equals the mechanical fan flow to add (about 70 CFM). Below, a green bar marks the correct 2019 method (superposition with a measured Q50, Equations 4-2 and 4-3) and a red bar marks the deprecated 2010 shortcut of reducing by half the excess above 2 CFM per 100 square feet. — Source: ASHRAE 62.2-2019 §4.1.2 (Eq 4-2/4-3)

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A three-box equation: the total required rate (90 CFM by ASHRAE) minus the natural infiltration estimated from a blower-door Q50 (about 20 CFM) equals the mechanical fan flow to add (about 70 CFM). Below, a green bar marks the correct 2019 method (superposition with a measured Q50, Equations 4-2 and 4-3) and a red bar marks the deprecated 2010 shortcut of reducing by half the excess above 2 CFM per 100 square feet.

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