Confined Space Ventilation Purge Calculator

Scheduling arithmetic for forced-air ventilation of manholes, wet wells, and vaults: purge time from space volume, required air changes, and blower output derated for duct losses. Runs entirely offline; inputs persist in this browser via localStorage.

This tool does not clear a space for entry. Ever. Only calibrated atmospheric testing (oxygen, LEL, H₂S, CO at minimum — top, middle, and bottom of the space) by the qualified person, under the entry permit, clears a space — and continuous monitoring keeps it cleared. A calculated purge time tells you when it's worth testing again, nothing more. Sewer-connected spaces can re-contaminate in minutes from upstream slugs regardless of how long the blower ran. The permit, the attendant, the retrieval equipment, and the competent/qualified person requirements are all upstream of this arithmetic.

Space Volume

Shape

Inside diameter (ft)

Depth (ft)

Ventilation

Air changes for initial purge

Common practice is on the order of 7–10 complete air changes before the pre-entry test (or blower manufacturer's guidance) — your program's number governs.

Blower rated output (CFM)

Effective delivery (% of rated, after duct losses)

Flexible duct is brutal on axial blowers — every bend and every extra section costs real flow. The blower manufacturer's duct-loss chart beats these rules of thumb.

Results

Space volume	—
Effective airflow	—
Time per air change	—
Initial purge time ( changes)	—

After the purge: test the atmosphere per the permit before anyone enters, and keep the blower running continuously during occupancy — purging is a precondition for testing, not a substitute for it.

How the Numbers Are Derived

The math

t = V × N / Q_eff — volume (cf) times required air changes divided by effective delivered airflow (CFM), giving minutes. Effective airflow is the rated CFM times the duct-loss factor. That's the whole calculation; everything hard about confined spaces is in the parts that aren't math.

Placement matters as much as time

Duct the supply to within a couple of feet of the bottom of the space (or the far end of a horizontal one) so fresh air displaces from the bottom up. A blower parked at the rim churns the top and leaves the bottom — where H₂S sits, heavier than air — undisturbed.
Position the blower intake upwind and away from generator exhaust, truck exhaust, and the space's own vent discharge, or you're pumping CO in with great efficiency.
Sewer-connected structures: plug or block channels where the entry procedure allows, and expect the atmosphere to change when upstream flow changes — which is what the continuous monitor is for.
If the pre-entry test fails after a full calculated purge, the space has a supply problem (off-gassing, connected atmosphere), not a time problem — more minutes on the blower isn't the fix; the entry supervisor's re-evaluation is.

References

OSHA 29 CFR 1910.146 — Permit-Required Confined Spaces (general industry) and 29 CFR 1926 Subpart AA (construction). https://www.osha.gov/

The employer's written confined space program and the entry permit — governing procedures.

Blower manufacturer duct-loss data — actual delivered CFM for the duct setup in use.