Dehumidifier Calculations

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When performing remediation getting the room dry fast is the job at hand. Knowing how much equipment you need is the key. Fortunately, it’s easy to calculate how many commercial dehumidifiers you need for any job. The math is simple for dehumidifier calculations.

First, let’s clarify a few moisture-removal and dehumidifier terms.

Class of Water

There are four water classes professionals use when calculating dehumidification. Each class refers to the amount of water to be removed.

  • Class 1 – Minimal amount of water, absorption and evaporation.
  • Class 2 – Large amount of water, absorption and evaporation (ex: in carpet, furniture cushions and/or along the base of walls).
  • Class 3 – Greatest amount of water, absorption and evaporation (ex: in the ceiling, walls, insulation and/or flooring).
  • Class 4 – Specialty drying situations (ex: in hardwood floors, saturated concrete, etc.).

Type of Dehumidifier

There are three types of dehumidifiers. We need to know the type being for our dehumidifier calculations as we use slightly different values for each. The three types of dehumidifiers are:

  • Conventional – Conventional dehumidifiers are also called refrigerant-based dehumidifiers. Conventional dehumidifiers rely on cooling coils to induce condensation.
  • LGR – LGR stands for Low-Grain Refrigerants. These dehumidifiers can dry a space to a lower humidity than conventional dehumidifiers.
  • Desiccant – Desiccant dehumidifiers work on a different process entirely. As humid air passes through them, moisture is removed by a chemical attraction process.

Pints-per-Day (PPD)

Pints-per-day, or PPD, is the amount of moisture a dehumidifier can remove in one day. When comparing PPD efficiency of dehumidifiers you need to know if the PPD rating is for AHAM or Saturation conditions.

AHAM and Saturation

When researching professional dehumidifiers you may have come across these terms. AHAM and saturation describe the dehumidifier testing conditions. AHAM stands for American Home and Appliance Manufacturers. The acronym is widely used because the group recommends specific testing conditions: 60 percent humidity and 80 degrees Fahrenheit.

Why 60% humidity and 80 degrees? According to AHAM they best represent the actual conditions dehumidifiers will be used in.

Saturation is more extreme at 90 percent humidity and 90 degrees Fahrenheit. Most dehumidifiers will never face these conditions, however. The purpose of this testing is to determine the maximum amount of moisture that could be extracted in one day.

Size of the Area to being Dehumidified in Cubic Feet

If you’ve read our calculations for commercial air scrubbers, you’ll remember this calculation. To calculate the cubic footage of a room or any other area use the following calculation:

cubic feet = length * width * height

Tip: Make sure you use cubic feet as opposed to square feet. The wrong number will impact your calculations.

Initial Dehumidification Recommendations

Use the following table for initial dehumidification recommendations.

DehumidifierClass 1Class 2Class 3Class 4
Desiccant1 ACH (/60)2 ACH (/30)3 ACH (/20)2 ACH (/30)

Calculate the Required Number of Dehumidifiers

As mentioned earlier, the dehumidifier calculations for any restoration job are easy. Use the following steps:

  1. First, calculate room size in cubic feet. We just covered that equation. Multiply the room’s length, width and height.
  2. Next, calculate AHAM. To do so, take the room size in cubic feet and divide it by the appropriate number based upon dehumidifier type and class.
  3. Finally, divide the result by the dehumidifier’s rating to arrive at the number of dehumidifiers required.

Example Dehumidifier Calculations

Example: Initial number of dehumidifiers calculation

For this example we’ll work with the following scenario:

  • A room that is 18′ x 26′ with a 14′ ceiling
  • The situation requires a Class 2 cleanup
  • We are using commercial conventional dehumidifiers rated at 110 PPD

To calculate the number of dehumidifiers we use the steps from above:

  1. Room Size: 18′ x 26′ x 14′ = 6,552 cubic feet
  2. AHAM: 6,552 cubic feet / 40* = 163.8 pints
  3. Number of Dehumidifiers: 163.8 / 110 (the unit’s AHAM PPD rating) = 1.49, or at least 2 dehumidifiers

* The dehumidifier is a conventional one and this is a class 2 cleanup. Therefore, referencing the table above, use 40 for this calculation.