IICRC Equipment Placement Ratios — Air Movers, Dehumidifiers & Air Scrubbers for Structural Drying
Why Equipment Ratios Matter
The quantity of drying equipment placed on a water damage job is not a matter of guesswork or convenience — it is a technical calculation governed by the physics of evaporation and the guidelines in IICRC S500:2021. Under-equipping a job prolongs drying time and increases the risk of secondary mould growth. Over-equipping without justification can attract scrutiny from insurance adjusters and claims of padding. Correctly calculated and documented equipment ratios are your defence on both fronts.
Air Mover Ratios
The standard IICRC S500 starting ratio for air movers is one unit per 15–18 m² of affected floor area for a Class 2 loss with carpet and cushion affected. This ratio is sometimes expressed as one air mover per room perimeter wall section (approximately 3–4 linear metres of baseboard). For Class 3 losses where walls above 60 cm are affected, additional air movers are required for wall-directed drying — typically one additional unit per 9–10 m² of wall cavity area. For Class 4 losses involving dense materials, directed placement at the material surface is required and the ratio may increase further. Air movers must be positioned in a consistent pattern — offset serpentine along perimeter walls, angled at 45° to direct airflow under carpet or along the wall surface. Random placement significantly reduces drying efficiency even with the correct number of units.
Dehumidifier Ratios
The IICRC S500:2021 Appendix B guideline for dehumidifiers is one standard refrigerant unit per 40 m² of affected area for a Class 2 loss, or one LGR unit per 50–60 m² due to the higher removal capacity of LGR equipment. These ratios assume ambient temperatures above 16°C. In Australian winter conditions — particularly in Victoria, Tasmania, and southern New South Wales — ambient temperatures may fall below the operating range of refrigerant dehumidifiers (typically 7–13°C minimum). In these conditions, desiccant dehumidifiers are required, as they operate efficiently at temperatures as low as 1°C. Dehumidifier placement should be central to the affected area, away from air mover exhaust streams that carry high-humidity air directly past the inlet. For large areas, multiple units distributed across the space outperform a single large unit in one corner.
Air Scrubber Ratios
Air scrubbers are required on Category 2 and Category 3 jobs, and on any job where mould growth is present or suspected. The IICRC S500 and S520 guideline is a minimum air change rate of 4 air changes per hour (ACH) for the affected space. Calculation: Room volume (m³) ÷ scrubber capacity (m³/h) × 60 = minutes per air change. Target: ≤ 15 minutes per air change (4 ACH). For a 30 m² room with a 2.7 m ceiling, volume = 81 m³. A scrubber rated at 600 m³/h achieves approximately 7.4 ACH — well above the minimum. One unit is sufficient. For larger or more contaminated spaces, additional units are placed to maintain 4 ACH throughout.
Electrical Load Compliance
Under AS/NZS 3012:2019, the total electrical load of drying equipment must not exceed 80% of the circuit capacity. Before deploying equipment, calculate the total ampere draw of all units and confirm it does not exceed 80% of the rated circuit breaker amperage. Distributing equipment across multiple circuits is standard practice on larger jobs.
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