Radon Fan Static Pressure and Pipe Velocity

What a radon mitigation fan is actually working against, which is almost entirely the soil and not the pipe.

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Pipe velocity and friction are computed here, by Darcy-Weisbach with a smooth-pipe friction factor appropriate to PVC, so the comparison can be made explicitly rather than assumed. On a short 4 in residential stack the pipe is on the order of a tenth of the resistance the fan develops -- small, and not the negligible fraction it is often described as. THAT COMPARISON IS THE POINT, because the intuitive fix for a struggling system is to upsize the pipe, and it changes almost nothing. The resistance lives under the slab, in the aggregate or the fines or the disturbed soil the suction has to travel through, and no amount of duct improves it. THE DIAGNOSTIC THAT DOES HELP IS VACUUM AGAINST FLOW, read at the fan, and it separates the two failure modes cleanly. HIGH VACUUM AT LOW FLOW means the sub-slab is tight and the suction field is not extending: the fan is doing all it can against a small communicating area, and the fix is MORE SUCTION POINTS rather than a bigger fan, which would only pull harder on the same patch. LOW VACUUM AT HIGH FLOW means the opposite -- the fan is finding an easy path to outdoor air through a crack, an open sump pit, or an untrapped floor drain, and is moving air that never passed under the slab. The fix there is sealing, and a bigger fan makes it worse by moving more outdoor air and, on a tight house, by depressurising the building enough to interfere with combustion appliances. A RADON FAN IS SELECTED FROM ITS CURVE RATHER THAN ITS RATING, because the operating point is where the fan curve crosses a system curve that the soil defines and nobody knows until the system runs. That is why the post-installation measurement matters more than the selection did, and why a fan chosen on a nameplate flow is chosen on a number it will never see. And the measurement that decides whether the system WORKS is neither of these: it is the suction field under the slab, read at test holes across the floor area. A system can move plenty of air and develop good vacuum at the fan while leaving part of the slab at no measurable pressure difference, and radon enters through exactly that part. This computes pipe velocity, friction and their share of an entered fan static. It does not select a fan or read a fan curve, predict the sub-slab resistance or the flow a given fan will achieve (which depends on the aggregate, the fines, the slab's cracks and the soil, and is not calculable in advance), size or locate suction points, measure or predict the suction field, evaluate radon concentration or a test result, address the electrical, condensate, discharge location and freeze protection a system needs, or determine what any standard requires. ANSI/AARST RMS-LB and SGM-SF, a post-mitigation radon measurement, and the certified mitigation professional govern.

pipe velocity = flow / area; friction by Darcy-Weisbach with the Blasius smooth-pipe factor f = 0.316 / Re^0.25, converted from feet of air to inches of water column; the share divides that loss by the fan's entered static.

Duct friction by Darcy-Weisbach on a smooth pipe, with the sub-slab resistance ENTERED as a measured fan static rather than predicted, because it depends on the aggregate, the fines and the soil and is not calculable in advance.

Standard duct friction relations and two unit conversions.

Estimate. AHJ and licensed professional govern.

Field names used by the API: flow_cfm, pipe_diameter_in, pipe_length_ft, fan_static_in_wc, measured_vacuum_in_wc, alt_pipe_diameter_in, area_ft2, velocity_fpm, pipe_loss_in_wc, pipe_share_pct, alt_velocity_fpm, alt_pipe_loss_in_wc

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