Why must you derate a 500 CFM air scrubber by 15%–25% when calculating ACH?

Manufacturer nameplate CFM (e.g., 500 CFM on a Dri-Eaz DefendAir HEPA 500) is measured with clean filters and zero exhaust ducting. Once a two-stage pre-filter loads with drywall/mold dust and 25 feet of lay-flat exhaust ducting is attached, effective airflow drops to ~375–400 CFM.

What is the Xactimate code difference between WTR NAFAN and WTR NAFA>?

WTR NAFAN covers standard portable 500–700 CFM HEPA air scrubbers, whereas WTR NAFA> covers Large negative air machines (typically 1,000 to 2,000 CFM cabinet/upright units). Always photograph the machine data plate and CFM rating.

IICRC S520 Negative Air CFM & Air Changes Per Hour (ACH) HEPA Scrubber Calculator

Engineering Formula (ANSI/IICRC S520 §10.4 & EPA Mold Remediation Guidelines)

Required CFM = (Containment Cubic Feet [L × W × H] × Target ACH [4 to 6+]) ÷ 60 minutes. Number of HEPA AFDs = Required CFM ÷ Effective Loaded Filter CFM per Machine (always round UP).

ANSI/IICRC S520 §10.4 and OSHA indoor air quality protocols require maintaining continuous negative air pressure (-0.02 in. w.c. / -5 Pa) and a minimum of 4 to 6 Air Changes Per Hour (ACH) inside mold, sewage, and hazardous dust containment zones using HEPA Air Filtration Devices (Xactimate WTR NAFAN for standard 500 CFM units, or WTR NAFA> for large 1,000–2,000 CFM units). Desk adjusters often reduce multiple HEPA air scrubbers down to a single machine or question why a Large AFD (WTR NAFA>) was billed in a high-ceiling room. By calculating the exact Required CFM from room dimensions and factoring in pre-filter/ducting static pressure drop (20% derating), this calculator outputs a bulletproof F9 justification for your WTR NAFAN count.

Reference Table

Containment Chamber Size (L × W × H)	Required CFM @ 4 ACH vs. 6 ACH	Xactimate HEPA Air Scrubber Selection
15 ft × 15 ft × 8 ft (1,800 CF Bedroom/Bath)	120 CFM (4 ACH) — 180 CFM (6 ACH)	1 × WTR NAFAN (500 CFM HEPA Air Scrubber)
25 ft × 20 ft × 10 ft (5,000 CF Living/Kitchen)	334 CFM (4 ACH) — 500 CFM (6 ACH)	1 × WTR NAFA> (Large AFD) or 2 × WTR NAFAN (with duct static loss)
40 ft × 30 ft × 10 ft (12,000 CF Basement)	800 CFM (4 ACH) — 1,200 CFM (6 ACH)	2 × WTR NAFA> (Large AFD) or 3 × WTR NAFAN (500 CFM units)
60 ft × 50 ft × 12 ft (36,000 CF Commercial)	2,400 CFM (4 ACH) — 3,600 CFM (6 ACH)	3 × WTR NAFA> (1,400–2,000 CFM Negative Air Machines)

Copy-Paste Xactimate F9 Note

F9 NOTE (IICRC S520 NEGATIVE AIR ACH CALCULATION — WTR NAFAN / WTR NAFA>): HEPA Air Filtration Device capacity was sized per ANSI/IICRC S520 §10.4 using the standard engineering formula: Required CFM = (Room Volume CF × Target 6 ACH) ÷ 60 minutes, accounting for 20% static pressure loss across primary HEPA media and exhaust lay-flat ducting to maintain >= -0.02 in. w.c. negative pressure.