Drying Equipment Math

Why this matters

Water damage restoration requires the right number of dehumidifiers and air movers to dry the affected materials in the target time. Too few = drying takes weeks; too many = wasted equipment cost. The calculations aren't complex but they're rarely taught in practical detail. A technician who can size equipment for the affected area produces efficient drying and predictable timelines.

The four classes, and what each one takes

Class here is the S500 class of water, meaning how much water the materials are holding and how hard it will be to get back out. It is a separate question from category, which is how contaminated the water is. Class drives equipment count; category drives containment, PPE and what gets thrown away.

Class 1 (small affected area): air movers + small dehumidifier

For 5 percent or less of the structure affected:

• 2 to 4 air movers
• 1 small refrigerant dehumidifier (50 to 80 pints/day)

Class 2 (medium): standard equipment

For 5 to 40 percent affected:

• 8 to 16 air movers
• 1 to 2 commercial dehumidifiers (100 to 250 pints/day)

Class 3 (major): more equipment

For 40+ percent affected:

• 20+ air movers
• 2 to 4 commercial dehumidifiers
• Sometimes desiccant for high moisture

Class 4 (specialty): different approach

For low-evaporation materials (hardwood, concrete):

• Targeted drying
• Lower air mover count
• Specialized dehumidifiers
• Heat applications

Air mover sizing

Air mover types

Air mover spacing

Standard rule: one air mover per 10 to 16 linear feet of affected wall area.

For floor drying:

• Air mover at the perimeter
• Airflow across the wet area
• Allow circulation through the room

For wall cavity drying:

• Vector airflow into the cavity
• Specific equipment

Dehumidifier sizing

Dehumidifier capacity

Dehumidifier capacity is rated in pints per day:

• 50 pints/day: light use
• 100 pints/day: standard residential
• 150 pints/day: medium commercial
• 200 to 250 pints/day: large commercial
• 300+ pints/day: industrial / large-scale

Calculation method

Total pints of water needing removal divided by pints/day per dehumidifier = number of dehumidifier-days needed.

For a typical residential Cat 1 water damage:

• Affected area: 400 sq ft
• Estimated total water in affected materials: 60 to 150 gallons (480 to 1,200 pints)
• One 100 pints/day dehumidifier = 5 to 12 days drying
• Two 100 pints/day = 2.5 to 6 days

Refine with experience.

Number of dehumidifiers

For typical residential:

• Single room: 1 dehumidifier
• Multi-room: 1 to 2 dehumidifiers
• Whole-floor: 2 to 4 dehumidifiers
• Whole-house: 3 to 6 dehumidifiers

Dehumidifier selection

For residential drying:

• Refrigerant dehumidifier: standard
• Brand: Dri-Eaz, Drymatic, Phoenix
• Sizes: 70, 100, 150, 230, 305 pints/day

For unusual conditions:

• Desiccant: very dry air needed; large volumes
• Heat-drying systems that push conditioned warm air into the affected assembly, paired with a dehumidifier to take the load back out

Air change rate

For the affected area:

ACH = total air-mover CFM x 60, divided by room volume in cubic feet. Run it the other way to get the CFM a target ACH needs: room volume x target ACH, divided by 60.

Example:

• Room: 12 x 12 x 8 ft = 1,152 cubic ft
• Target 6 ACH: 1,152 x 6 / 60 = 115 CFM
• One air mover at 2,500 CFM provides 130 ACH (huge over-circulation; one is enough)

For typical residential, air mover count is often dictated by linear distance more than air changes (with one air mover, you've got way more air changes than needed; the count is for coverage and air agitation).

Combined drying setup

For typical Cat 1 water damage in a 12 x 12 room with damaged carpet and lower drywall:

• 4 to 6 air movers along the perimeter (aimed at the carpet, walls)
• 1 dehumidifier (100 pints/day)
• Run continuously for 3 to 7 days
• Monitor moisture daily
• Reduce equipment as drying progresses

Heat applications

Heat speeds drying by:

• Increasing evaporation rate
• Reducing relative humidity
• Required in cold conditions

When to add heat

• Cold ambient (below 50 F)
• Slow drying despite equipment
• Wood or other slow-drying materials

Heat equipment

• Purpose-built heat-drying systems, which deliver temperature-controlled air to the wet assembly and are designed to run alongside a dehumidifier
• Standalone propane heater (with ventilation)
• Electric heater (for smaller spaces)

Cautions

• Carbon monoxide from propane (need ventilation)
• Fire hazard from electric heater
• Heat damage to materials
• Customer comfort

Energy considerations

Drying equipment uses substantial power:

• One air mover: 4 to 9 kWh per day (roughly 175 to 400 W running continuously)
• One dehumidifier: 5 to 25 kWh per day
• Combined: 10 to 50 kWh per day for typical residential setup
• Operating cost: modest per day

For multi-day drying, energy cost is significant; document for customer awareness.

Daily monitoring

Each day during drying:

Step 1: Visual inspection

• Verify all equipment running
• Check for any new water issues
• Note any visible changes

Step 2: Moisture readings

For each affected material:

• Floor (carpet, hardwood, subfloor)
• Drywall (multiple heights)
• Framing (if accessible)
• Other materials

Document readings.

Step 3: Equipment adjustment

Based on readings:

• Add or remove air movers
• Add or remove dehumidifiers
• Change location of equipment
• Adjust dehumidifier setpoint

Step 4: Track progress

Daily log:

• Date
• Moisture readings
• Equipment count
• Equipment changes made
• Estimated days remaining

When drying is complete

Drying is complete when:

• All materials at target moisture (varies by material)
• Multiple consecutive days of similar readings (no further drying)
• Visual inspection of all areas
• No suspicious wet spots

Acceptance is set against a dry standard, not against a universal number. Take a reading on unaffected material of the SAME type in the SAME building, with the SAME meter, and dry the wet material back to that. A number carried over from another job or another meter is not a dry standard.

• Wood and wood framing: the dry standard usually lands somewhere in the low teens percent moisture content in most of the country, and the seasonal and regional swing is exactly why you read your own dry standard instead of quoting a figure.
• Drywall: read it as a comparison to the dry standard, not as a percentage. Gypsum holds very little water by weight when dry, so a meter's percent scale on drywall is a relative indicator.
• Concrete and other slabs: per the flooring manufacturer's spec, and measured the way that spec calls for, which is normally an in-situ relative humidity probe or a calcium chloride test rather than a surface meter.

Common drying mistakes

Insufficient air movers

• Drying takes much longer
• Mold risk if drying time extends past 48 to 72 hours of wetness

Too few dehumidifiers

• Moisture rises in the air; not removed
• Drying slows

Too many dehumidifiers / air movers

• Wasted equipment cost
• Higher energy bill

Wrong location of equipment

• Air movers not aimed at wet areas
• Dehumidifiers not in optimal position
• Reduced effectiveness

Not monitoring daily

• Drying progress unknown
• Equipment runs longer than needed
• Or removed too early; materials still wet

Failing to adjust as drying progresses

• Initial high count of equipment maintained throughout
• Cost continues
• Should reduce as drying nears completion

Equipment management

For an active restoration business:

Owning vs renting

The math is utilization, not sticker price. Restoration equipment bills to the insurance claim per unit per day at a published line rate, so a machine pays for itself in a countable number of billed days. Air movers typically return their purchase price inside a single-digit number of billed job-weeks; LGR dehumidifiers take longer, usually a few dozen billed days depending on size. Divide the purchase price by the daily line rate, then ask honestly how many days a year that unit will actually be on a floor.

Own your base load. That is the equipment count you deploy on a typical week with no catastrophe running: enough air movers and LGRs to cover two or three concurrent residential jobs, plus meters, extraction, and containment. This gear runs constantly, so utilization is high and rental would be pure margin loss.

Rent the peak. Freeze events, hurricanes, and multi-unit commercial losses spike your demand for a week or two. Buying to peak means owning idle inventory eleven months of the year, plus the storage, the maintenance, and the capital. Establish the rental account and the after-hours contact before storm season, not during it.

Rent specialty gear until it proves itself: desiccants, trailer-mounted units, injection drying systems, hardwood floor mat systems, large air scrubbers. Track how many times you rent each one. When the same item shows up on three or four jobs a quarter, the rental spend has started to exceed ownership and it is time to buy.

Factor the carrying costs owners forget: filters and pads, wheel and hose replacement, annual amp-draw and coil checks on the LGRs, truck space, and the shop labor to clean and stage after every job. A dehu that skips maintenance loses capacity quietly, and you will pay for that in extra drying days on the next claim.

References

• IICRC S500 (Standard for Professional Water Damage Restoration).
• IICRC ASD (Applied Structural Drying) certification.
• ASHRAE Handbook, HVAC Applications, Moisture Control and Drying chapter.
• Manufacturer literature for the specific units you run, for rated water removal at the AHAM condition and at the low-grain condition you will actually be working in.
• Manuall internal: Water Restoration IICRC S500 Standards Summary.