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Dehumidifier sizing calculator (water restoration)
Sizing the dehumidification for a water loss is what keeps the structure drying fast enough to beat mold, and the conventional IICRC S500 method is simple: divide the affected volume in cubic feet by a class factor to get the AHAM pints per day of dehumidification needed. Enter the room length, width, and ceiling height, and the class factor. The factor gets smaller as the water class gets wetter, so a wetter room calls for more dehumidification: for a low-grain-refrigerant (LGR) dehumidifier the factors run roughly 100 for class 1, 50 for class 2, 40 for class 3, and about 45 for class 4, but the exact factor depends on the dehumidifier type, standard refrigerant versus LGR versus desiccant, and the version of the method you follow. Take the pints-per-day result and divide it by the AHAM rating of the dehumidifier you are placing to get the number of units, then pair them with enough air movers to keep evaporation feeding the dehumidifiers. Treat this as a starting estimate, confirm the class and the factor against the IICRC S500 and the manufacturer data, and monitor the drying daily to a documented dry standard rather than to the equipment count.
Result
Dehumidifier sizing (water restoration): the conventional IICRC S500 method divides the affected volume in cubic feet by a class factor to get the AHAM pints per day of dehumidification needed. Enter the room length, width, and ceiling height, and the class factor. The factor gets smaller as the water class gets wetter, so more dehumidification is required: for a low-grain-refrigerant (LGR) dehumidifier the factors run roughly 100 for class 1, 50 for class 2, 40 for class 3, and about 45 for class 4, but the exact factor depends on the dehumidifier type (standard refrigerant, LGR, or desiccant) and the version of the method you follow. Take the pints-per-day result and divide by the AHAM rating of the dehumidifier you are placing to get the number of units, then pair it with enough air movers to keep the evaporation feeding the dehumidifiers. Treat this as a starting estimate and confirm the class, the factor, and the final equipment plan against the IICRC S500 and the manufacturer data, and monitor the drying to a dry standard.
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Dehumidifier sizing FAQ
What is water damage restoration?
Water damage restoration is drying a wet building out and saving what can be saved after a leak or flood, then rebuilding what had to come out. The mitigation half stops the loss and dries the structure; the restoration half rebuilds. Done to IICRC S500, it runs from inspection through extraction, drying, and a proven dry standard.
What are the water categories?
IICRC S500 defines three. Category 1 is clean water from a sanitary source, like a broken supply line. Category 2 is gray water with significant contamination, like appliance discharge. Category 3 is black water, grossly contaminated by sewage or flooding. The category sets your protective equipment and what can be saved versus removed.
How long before mold grows after water damage?
Mold can begin growing in porous materials in roughly 24 to 48 hours under favorable temperature and humidity, a window cited in EPA guidance. That is why water restoration is a race. Treat the clock as already running and start extraction and drying immediately, since the actual rate depends on temperature, humidity, and what got wet.
What is a dry standard in water restoration?
The dry standard is the moisture content of a material when it is dry, measured by metering the same material in an unaffected area of the building. It is the documented target you dry the wet material back to. You meter daily against it, and you do not pull equipment until the affected material reaches it.
Why extract the standing water before drying?
Extraction removes far more water than evaporation ever will, so every gallon you vacuum out is a gallon the dehumidifiers do not have to chase from the air over days. Pulling the bulk water first shortens the drying time, cuts equipment cost, and narrows the mold window. Build the drying environment around what is left.