Dehumidifier Selection Guide — LGR vs Desiccant, Capacity Calculations & Placement for Structural Drying
Types of Dehumidifiers in Restoration
Three types of dehumidifiers are used in structural drying: conventional refrigerant, low-grain refrigerant (LGR), and desiccant. Selecting the correct type for the conditions is as important as having the right number of units. ### Conventional Refrigerant Dehumidifiers Conventional refrigerant dehumidifiers work by drawing moist air across a cold evaporator coil, causing water vapour to condense. They are effective in warm, humid conditions but performance drops sharply below 20°C inlet temperature. Standard refrigerant units should generally not be relied upon in unheated structures during Australian winter conditions in southern states. Rated capacity is typically expressed in litres per day (L/day) at AHAM conditions (26.7°C, 60% RH). Performance in actual job conditions will almost always be lower than the AHAM rating. ### Low-Grain Refrigerant (LGR) Dehumidifiers LGR dehumidifiers use a pre-cooling stage that cools the inlet air before it reaches the main evaporator coil. This pre-cooling condenses moisture at a higher inlet temperature and lowers the grain content of the air entering the main coil, improving water removal efficiency at lower ambient temperatures and humidity levels. LGR units are the standard for structural drying in the Australian restoration industry. They outperform conventional refrigerant units at the lower grains per pound conditions typical of mid-to-late drying phases. Per IICRC S500:2021 Appendix B, one LGR unit serves approximately 50–60 m² of affected area for a Class 2 loss. ### Desiccant Dehumidifiers Desiccant dehumidifiers pass air through a rotating wheel impregnated with a hygroscopic material (typically silica gel or lithium chloride) that adsorbs water vapour from the air stream. The loaded desiccant is continuously regenerated by a hot air stream that drives the moisture out and exhausts it outside the structure. Desiccant units are effective at temperatures as low as 1°C and perform consistently across a wide humidity range. They are the preferred choice for: - Unheated structures in winter in southern Australian states (Victoria, Tasmania, southern NSW) - Class 4 drying of dense materials requiring very low ambient humidity - Cold storage facilities or large concrete structures Desiccant units require an exhaust duct to remove the regeneration air stream (which carries the extracted moisture) from the drying space.
Capacity Calculations
Capacity selection should be based on the IICRC S500:2021 Appendix B tables, which provide litre-per-day removal requirements based on: 1. Damage class (1–4) 2. Affected floor area (m²) 3. Volume of wet materials For a Class 2 loss affecting 60 m²: Appendix B indicates approximately 100–120 L/day of total removal capacity required. Two LGR units each rated at 70 L/day (AHAM) would provide adequate capacity, noting actual performance will be somewhat below AHAM rating in real conditions.
Placement Principles
- Position dehumidifiers centrally in the affected area, not in corners - Ensure the inlet is not in the direct exhaust stream of air movers — receiving pre-humidified air increases the moisture load but does not proportionally increase removal capacity - On multi-room jobs, one unit per room (or zone) outperforms a single large unit in the largest room - Leave clearance around the unit for adequate airflow — minimum 600 mm on all sides where possible
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