How to Size a Commercial Dehumidifier the Right Way

The most common sizing mistake on a dehumidifier is buying based on the saturation rating shown in big print on the box. Saturation ratings (90 percent RH at 90 degrees Fahrenheit) only apply in flood conditions. AHAM ratings (60 percent RH at 80 degrees) apply to standard rooms. Knowing which to use changes the size of the machine you buy by 30 to 50 percent.

AHAM vs Saturation

• AHAM rating. Pints of water removed per day at 60 percent RH and 80 degrees Fahrenheit. The standard for residential basements, warehouses and offices.
• Saturation rating. Pints removed at 90 percent RH and 90 degrees Fahrenheit. Used for restoration and flood drying. Always larger than AHAM for the same machine.

If you compare a 70 pint AHAM dehumidifier with a 100 pint saturation dehumidifier, they may be the same machine. Always check which standard the rating uses.

Sizing Formulas

Residential Basement

Basement sizing depends on square footage and existing humidity:

• Up to 1500 sq ft, dry basement (50 percent RH): 30 pint AHAM
• Up to 1500 sq ft, damp (60 percent RH): 50 pint AHAM
• 1500 to 3000 sq ft, damp: 70 pint AHAM
• 1500 to 3000 sq ft, wet (active seepage): 90 pint AHAM minimum, plus a sump pump

Restoration (Flood Drying)

For water damage drying, IICRC S500 recommends:

• Calculate cubic feet of affected space (length x width x ceiling height)
• Determine class of water loss (Class 1 = minimal, Class 4 = saturation)
• Class 1: 1 dehumidifier per 100 to 150 sq ft (use AHAM rating)
• Class 2: 1 dehumidifier per 75 to 100 sq ft
• Class 3: 1 dehumidifier per 50 to 75 sq ft
• Class 4: pull moisture from porous materials, use 2 to 3 LGR units per 50 sq ft

For class 3 and 4, an LGR (Low Grain Refrigerant) machine is required. Browse LGR commercial dehumidifiers.

Worked Example

A 1500 sq ft basement at 65 percent RH after a sump pump failure. Cubic feet (with 8 ft ceilings) is 12,000. For a class 2 partial wet event, 1500 sq ft / 75 = 20 pints/hour drying capacity needed. That converts to a 70 to 90 pint AHAM machine running 24 hours a day.

Air Movers Multiply the Effect

A dehumidifier alone struggles to dry materials with low surface area exposure. Pair it with air movers to push humid air across drying surfaces. The standard ratio is 1 dehumidifier per 4 to 6 air movers in restoration work.

Ductable Models for Fixed Installs

For warehouses, swimming pools or grow rooms with continuous moisture loads, a ductable LGR dehumidifier ties into the HVAC system. These run unattended, drain to a sewer line and handle 200 to 500 pints per day.

Drainage Setup

Tank emptying is the limiting factor on any dehumidifier. For continuous duty, hose drain to a floor drain, sump pit or condensate pump. A 70 pint dehumidifier fills its tank in 4 to 6 hours of restoration drying. Without a drain hose, you spend more time emptying than drying.

Common Mistakes

• Comparing saturation rating on one machine to AHAM on another
• Using a standard refrigerant unit in a cold space (below 65 F it loses efficiency)
• Skipping the air movers (4x slower drying)
• Running on tank only (operator burns hours emptying)
• Undersizing for the active water load

The right dehumidifier is the one that matches the work load. Pair it with air movers, run it on a hose drain, and the result is fast predictable drying. The wrong one is whichever happens to be in the truck.