Indoor Pool Dehumidifier Sizing Calculator

Calculate

Total water surface area of the indoor pool

Pool water surface temperature

Indoor natatorium air temperature

Indoor relative humidity target (typically 50–60% for natatoriums)

Pool use intensity (0.5 = covered/idle, 1.0 = quiet residential, 1.5 = active recreation)

Sizing margin multiplier (e.g. 1.20 = 20% margin above raw moisture load)

Overview

The Pool Dehumidifier Sizing Calculator estimates how much moisture must be removed from an indoor pool room and converts that load into a recommended dehumidifier capacity. It is intended for preliminary natatorium sizing, where the dominant latent load usually comes from pool-water evaporation. PoolPak notes that pool evaporation is the majority of the total dehumidification load in a natatorium, and that accurate prediction of evaporation is essential.

This calculator uses one fixed model: first it estimates the evaporation-based moisture load from pool area, water temperature, room air temperature, room relative humidity, and activity factor, then it applies a sizing margin to produce the recommended dehumidifier capacity. It is a screening tool, not a full natatorium HVAC design package.

The result helps determine whether the moisture-removal duty is small, moderate, high, or very high, and whether the sizing margin is tight, normal, or robust. Final design still needs full natatorium coordination for air distribution, condensation control, ventilation, heat recovery, and equipment selection. ASHRAE 90.1 also includes indoor-pool-specific dehumidification energy-recovery requirements for many mechanically cooled indoor pool dehumidifier systems.

How to Use This Calculator

  1. Enter the pool water surface area — in ft² (Imperial) or m² (Metric).

  2. Enter the pool water temperature — in °F (Imperial) or °C (Metric).

  3. Enter the room air temperature — in °F (Imperial) or °C (Metric).

  4. Enter the room relative humidity — as a percentage (e.g. 55).

  5. Enter the activity / agitation factor — dimensionless multiplier for pool use intensity (e.g. 1.0 for quiet residential, up to 1.5 for active use).

  6. Enter the safety factor — dimensionless sizing margin multiplier (e.g. 1.20 for 20% margin).

  7. Click "Calculate" — get moisture removal rate, recommended dehumidifier capacity, and safety margin added.

  8. Review the result — use the capacity classification to judge whether the dehumidification duty is small, moderate, high, or very high.

This calculator is a screening tool. Final equipment selection should be confirmed with full natatorium HVAC design review, manufacturer data, and condensation control analysis.

Inputs & Outputs

Inputs

  • •Pool Surface Area (m² / ft²)
  • •Pool Water Temperature (°C / °F)
  • •Room Air Temperature (°C / °F)
  • •Room Relative Humidity (%)
  • •Activity / Agitation Factor
  • •Safety Factor

Outputs

  • •Recommended Dehumidifier Capacity (kg/h / lb/h)
  • •Moisture Removal Rate (kg/h / lb/h)
  • •Safety Margin Added (kg/h / lb/h)

Formula

Fixed Decision Model Used by This Calculator

This calculator uses one fixed evaporation-based sizing model.

1) Saturation Vapor Pressure

The calculator first determines saturation vapor pressure at:

  • pool water temperature
  • room air dew-point-equivalent condition

Using the Magnus formula:

P_sat = 6.112 × exp((17.67 × T) / (T + 243.5))

where T is in °C and P_sat is in hPa (mbar).

2) Evaporation-Based Moisture Load

The moisture load is based on pool evaporation as a function of:

  • pool surface area
  • pool water temperature
  • room air temperature
  • room relative humidity
  • activity factor

Metric:

Moisture Removal Rate (kg/h) = 0.00484 × Area (m²) × (Pw_sat − Pa_actual) (hPa) × Activity Factor

Imperial:

Moisture Removal Rate (lb/h) = Metric Rate (kg/h) × 2.20462

where:

  • Pw_sat = saturation vapor pressure at pool water temperature
  • Pa_actual = actual vapor pressure of room air = Pa_sat × (RH / 100)

3) Recommended Dehumidifier Capacity

Imperial:

Recommended Capacity (lb/h) = Moisture Removal Rate (lb/h) × Safety Factor

Metric:

Recommended Capacity (kg/h) = Moisture Removal Rate (kg/h) × Safety Factor

4) Safety Margin Added

Imperial:

Safety Margin Added (lb/h) = Recommended Capacity − Moisture Removal Rate

Metric:

Safety Margin Added (kg/h) = Recommended Capacity − Moisture Removal Rate

Practical Reference Basis

PoolPak notes that pool evaporation is the majority of the total dehumidification load in a natatorium, and that accurate prediction of evaporation is essential. The five main variables used to calculate pool evaporation are pool surface area, pool water temperature, room air temperature, room relative humidity, and activity factor.

Calculator Variables

Variable Meaning Units
poolArea Pool water surface area ft² / m²
waterTemp Pool water temperature °F / °C
roomTemp Room air temperature °F / °C
roomRH Room relative humidity %
activityFactor Pool use intensity multiplier —
safetyFactor Sizing margin multiplier —
moistureRemovalRate Calculated evaporation-based moisture load lb/h / kg/h
recommendedCapacity Moisture removal rate × safety factor lb/h / kg/h
safetyMarginAdded Recommended capacity − moisture removal rate lb/h / kg/h

What is Pool Dehumidifier Sizing?

Pool dehumidifier sizing is the process of selecting enough moisture-removal capacity to control humidity created by evaporation from an indoor pool. In a natatorium, this is one of the most important HVAC tasks because uncontrolled humidity causes condensation, corrosion, poor comfort, and building-durability problems. PoolPak notes that indoor pools produce large quantities of water vapor through evaporation, and that this moisture must be removed to prevent damaging condensation.

This calculator uses one fixed evaporation-based model. It derives the moisture load from pool area, water temperature, room air temperature, room relative humidity, and activity factor using the Magnus formula for saturation vapor pressure. A safety factor converts the raw moisture load into a recommended dehumidifier capacity. The result classifies the dehumidification duty as low, moderate, high, or very high.

Why Natatorium Dehumidification Matters

Pool evaporation is the dominant internal dehumidification load in most natatoriums. Unlike standard comfort-cooling spaces, an indoor pool has a continuous latent source at the water surface regardless of occupancy level. Insufficient moisture removal leads to condensation on walls, ceilings, and structural elements, accelerating corrosion and shortening building life.

Room conditions affect evaporation directly. Lower relative humidity increases the vapor-pressure difference between pool water and room air, raising both dehumidification load and pool-water heating demand. Maintaining room air temperature about 2–4°F above pool water temperature reduces evaporation. ASHRAE 90.1 requires many mechanically cooled indoor pool dehumidifier systems serving pools larger than 400 ft² with water at 94°F or less to be capable of using condenser heat for pool-water or space heating before other heating is used.

When to Use This Calculator

Use this calculator for preliminary sizing screening across many indoor pool types: residential pools, hotel pools, school natatoriums, therapy pools, and competition pools. The result is a first-pass capacity estimate for early project decisions, not a final equipment specification.

This calculator does not model full outdoor-air ventilation load, spectator latent load, corrosion risk on specific surfaces, part-load behavior, or annual operating schedules. Final equipment selection requires full natatorium HVAC coordination, manufacturer selection software, and condensation control analysis.

Key Facts

  • Pool evaporation is usually the dominant internal dehumidification load in an indoor natatorium. PoolPak states that it represents the majority of the total dehumidification load.
  • Lower relative humidity increases evaporation, which increases both dehumidification load and pool-water heating requirement. PoolPak explicitly notes this relationship.
  • Maintaining room air temperature about 2–4°F above pool water temperature helps reduce evaporation, according to PoolPak's design guidance.
  • ASHRAE 90.1 requires many mechanically cooled indoor pool dehumidifier systems serving indoor pools larger than 400 ft² with water at 94°F or less to be capable of using condenser heat for pool-water heating or natatorium space heating before other heating is used.
  • High-capacity natatorium systems usually require integrated attention to dehumidification, ventilation, air distribution, and corrosion control, not just moisture-removal tonnage.

Applications

  • Indoor community pools
  • Hotel pools
  • School and university natatoriums
  • Therapy pools
  • Competition pools
  • Preliminary screening for natatorium dehumidifier selection

Example Calculation

Imperial Example

Given:

  • Pool Surface Area = 1,200 ft²
  • Pool Water Temperature = 82°F
  • Room Air Temperature = 84°F
  • Room Relative Humidity = 55%
  • Activity Factor = 1.0
  • Safety Factor = 1.20

Step 1 — Moisture Removal Rate

Convert to metric for evaporation calculation:

  • Area = 1,200 × 0.0929 = 111.5 m²
  • Tw = (82 − 32) × 5/9 = 27.8°C
  • Tr = (84 − 32) × 5/9 = 28.9°C

Saturation vapor pressures:

  • Pw_sat = 6.112 × exp(17.67 × 27.8 / (27.8 + 243.5)) = 37.0 hPa
  • Pa_sat = 6.112 × exp(17.67 × 28.9 / (28.9 + 243.5)) = 39.6 hPa
  • Pa_actual = 39.6 × 0.55 = 21.8 hPa

Evaporation:

  • Rate (kg/h) = 0.00484 × 111.5 × (37.0 − 21.8) × 1.0 = 0.00484 × 111.5 × 15.2 = 8.20 kg/h
  • Rate (lb/h) = 8.20 × 2.20462 = 18.08 ≈ 18.1 lb/h
Moisture Removal Rate = 18.1 lb/h

Step 2 — Recommended Dehumidifier Capacity

Recommended Capacity = 18.1 × 1.20
Recommended Capacity = 21.7 lb/h

Step 3 — Safety Margin Added

Safety Margin Added = 21.7 − 18.1
Safety Margin Added = 3.6 lb/h

Interpretation: This is a moderate dehumidification load, typical of many small to mid-size indoor pool applications. The sizing includes a practical buffer above the raw evaporation load.

Metric Example

Given:

  • Pool Surface Area = 111.5 m²
  • Pool Water Temperature = 27.8°C
  • Room Air Temperature = 28.9°C
  • Room Relative Humidity = 55%
  • Activity Factor = 1.0
  • Safety Factor = 1.20

Step 1 — Moisture Removal Rate

Saturation vapor pressures:

  • Pw_sat = 6.112 × exp(17.67 × 27.8 / (27.8 + 243.5)) = 37.0 hPa
  • Pa_sat = 6.112 × exp(17.67 × 28.9 / (28.9 + 243.5)) = 39.6 hPa
  • Pa_actual = 39.6 × 0.55 = 21.8 hPa

Evaporation:

Rate (kg/h) = 0.00484 × 111.5 × (37.0 − 21.8) × 1.0 = 0.00484 × 111.5 × 15.2 = 8.20 kg/h

Moisture Removal Rate = 8.20 kg/h

Step 2 — Recommended Dehumidifier Capacity

Recommended Capacity = 8.20 × 1.20
Recommended Capacity = 9.84 kg/h

Step 3 — Safety Margin Added

Safety Margin Added = 9.84 − 8.20
Safety Margin Added = 1.64 kg/h

Interpretation: This metric result also indicates a moderate dehumidification duty with a normal sizing margin for practical humidity control.

Standards & References

  • ASHRAE 90.1 — includes indoor-pool-specific dehumidification energy-recovery requirements for many mechanically cooled natatorium dehumidifier systems.
  • PoolPak Natatorium Design Guide — practical natatorium guidance on RH, evaporation, and operating conditions.
  • PoolPak Humidity Control Calculations — identifies the five main variables used to calculate pool evaporation.
  • ASHRAE Handbook — HVAC Applications, Ch. 5 (Places of Assembly; natatorium design) — design guidance for indoor pool environments.

Limitations

  • This calculator is a screening tool, not a full natatorium HVAC design tool.
  • It assumes moisture load is dominated by pool evaporation.
  • It does not calculate full outdoor-air ventilation load, spectator latent load, detailed corrosion risk, or complete psychrometric process selection.
  • It does not replace manufacturer selection software or project-specific natatorium design review.
  • It does not model annual operating schedules, part-load behavior, or detailed control sequences.
  • It does not by itself prove that condensation will be prevented on all building surfaces.

Common Mistakes to Avoid

  • Setting indoor relative humidity too low, which increases evaporation and energy use.
  • Treating indoor pools like normal comfort-cooling spaces.
  • Ignoring pool-water temperature when estimating evaporation.
  • Ignoring activity factor and surface agitation.
  • Using a dehumidifier size with no safety margin.
  • Forgetting that air distribution affects condensation control.
  • Ignoring condenser heat recovery opportunities.
  • Assuming dehumidifier tonnage alone solves natatorium humidity problems.

Frequently Asked Questions

What does this calculator actually size?
It sizes the moisture-removal capacity needed to control evaporation from an indoor pool room.
Why is pool dehumidification different from normal HVAC dehumidification?
Because natatoriums have a continuous evaporation source from the pool surface, which creates a persistent latent load and condensation risk.
Does lower room humidity always help?
Not automatically. Lower RH increases evaporation, which raises both dehumidification load and pool-water heating demand.
Why does room air temperature matter?
Because room-air temperature affects evaporation rate. Keeping room air slightly warmer than pool water generally reduces evaporation.
Does this calculator include ventilation and outdoor air?
No. It focuses on the dehumidification capacity required for evaporation control, not full natatorium ventilation design.
Can I use this for hotel pools and school natatoriums?
Yes, as a first-pass sizing tool for many indoor pool types.
Does ASHRAE require heat recovery for natatorium dehumidifiers?
ASHRAE 90.1 requires many mechanically cooled indoor pool dehumidifiers serving pools larger than 400 ft² with water at 94°F or less to be capable of using condenser heat for pool-water heating or natatorium space heating before other heating is used.
Is this enough for final equipment selection?
No. Final selection still needs full natatorium HVAC coordination, including air distribution, condensation control, ventilation, and equipment performance review.

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

Pool Heating Load

Condensate Pump Sizing

Coil Capacity Calculator

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