Natatorium Evaporation Rate Calculator

Calculate

Total water surface area of the indoor pool

Pool water surface temperature

Indoor air dew point — reflects the actual moisture content of the room air

Pool agitation and use intensity (0.5 = calm/covered, 0.65 = typical leisure, 1.0–1.5 = active or competitive use)

Overview

The Natatorium Evaporation Rate Calculator estimates the indoor pool evaporation rate and resulting latent moisture load for natatorium HVAC design. The final result is shown in lb/h in Imperial mode and kg/h in Metric mode.

This calculator uses a fixed screening model based on pool water surface area, pool water temperature, room dew point, and activity factor. It is a preliminary natatorium screening tool, not a full dehumidifier selection, full envelope analysis, or condensation-risk model.

In real projects, natatorium design also has to consider outdoor air requirements, supply air delivery to exterior surfaces, glazing protection, and room pressure control.

In practical natatorium screening, the activity factor often ranges from about 0.5 for relatively calm water to around 1.0–1.5 or higher for more active swimming, splashing, or recreational water use. That is why two pools with similar area and temperature can produce very different evaporation rates. This calculator is intended as a first-pass tool for engineers screening natatorium latent load, comparing pool conditions, and assessing early dehumidification capacity requirements.

How to Use This Calculator

  1. Select Imperial or Metric units — use the toggle at the top of the calculator.

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

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

  4. Enter the room dew point — in °F (Imperial) or °C (Metric).

  5. Enter the activity factor — dimensionless multiplier for pool agitation and use intensity (e.g. 0.5 = calm, 0.65 = typical, 1.0–1.5 = active).

  6. Click "Calculate" — get the evaporation rate in lb/h or kg/h.

  7. Review the result — check the status badge and interpretation for dehumidification guidance.

  8. Use the result as a preliminary basis for dehumidification sizing, latent load review, supply-air distribution review, and condensation-control review.

This is a preliminary screening tool. Final natatorium design requires full dehumidification sizing, outdoor-air analysis, condensation control review, and envelope coordination.

Inputs & Outputs

Inputs

  • •Pool Water Surface Area (m² / ft²)
  • •Pool Water Temperature (°C / °F)
  • •Room Dew Point (°C / °F)
  • •Activity Factor

Outputs

  • •Evaporation Rate (kg/h / lb/h)

Formula

Fixed Decision Model Used by This Calculator

This calculator uses a fixed ASHRAE-style natatorium evaporation model.

1) Saturation Vapor Pressure

The calculator determines saturation vapor pressure at:

  • pool water temperature (Pw)
  • room dew point (Pa)

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. Because the input is room dew point (not dry-bulb temperature), Pa is calculated directly as the saturation vapor pressure at the dew point temperature — no relative humidity conversion is required.

2) Imperial Formula

Step 1 — Vapor-Pressure Difference (in. Hg)

Convert temperatures to °C, compute Magnus saturation pressures in hPa, then convert to in. Hg (1 hPa = 0.02953 in. Hg):

ΔP = (Pw_hPa − Pa_hPa) × 0.02953 [in. Hg]

Step 2 — Evaporation Rate

W = 0.1 × A × ΔP × AF

Where:

  • W = evaporation rate, lb/h
  • A = pool water surface area, ft²
  • ΔP = vapor-pressure difference, in. Hg
  • AF = activity factor

3) Metric Formula

Step 1 — Vapor-Pressure Difference (kPa)

Compute Magnus saturation pressures in hPa, then convert to kPa (1 hPa = 0.1 kPa):

ΔP = (Pw_hPa − Pa_hPa) × 0.1 [kPa]

Step 2 — Evaporation Rate

W = 0.0488 × A × ΔP × AF

Where:

  • W = evaporation rate, kg/h
  • A = pool water surface area, m²
  • ΔP = vapor-pressure difference, kPa
  • AF = activity factor

Empirical Coefficients

The coefficients 0.1 (Imperial) and 0.0488 (Metric) are standard empirical screening constants commonly used in natatorium evaporation calculations. They represent the practical relationship between pool surface area, vapor-pressure difference, and evaporation rate in customary and SI unit forms.

Calculator Variables

Variable Meaning Units
poolArea Pool water surface area ft² / m²
waterTemp Pool water temperature °F / °C
dewPoint Room dew point °F / °C
activityFactor Pool agitation / use intensity multiplier —
Pw Saturation vapor pressure at water temp in. Hg / kPa
Pa Saturation vapor pressure at room dew point in. Hg / kPa
evapRate Final pool evaporation rate lb/h / kg/h

What is Natatorium Evaporation Rate?

Natatorium evaporation rate is the rate at which water evaporates from an indoor pool surface into the surrounding air. In natatorium HVAC design, this evaporation is the dominant latent moisture load that the dehumidification system must control. Pool area, water temperature, room moisture condition, and pool activity level are the main factors that drive evaporation rate.

Higher evaporation generally means greater dehumidification demand, increased condensation risk on glazing and cold surfaces, and tighter requirements on air distribution and humidity control. Evaporation from the pool surface is distinct from spectator loads, wet deck loads, and outdoor air latent load — all of which also contribute to total natatorium HVAC demand.

How This Calculator Works

This calculator uses a fixed ASHRAE-style natatorium evaporation model. The user enters pool area, water temperature, room dew point, and activity factor. The calculator returns the evaporation rate in lb/h (Imperial) or kg/h (Metric) and classifies the result as LOW, NORMAL, HIGH, or VERY HIGH based on standard natatorium screening thresholds.

Room dew point is used rather than dry-bulb temperature because evaporation is driven by vapor-pressure difference. Dew point directly reflects the air’s moisture content without requiring a relative-humidity conversion. Activity factor accounts for pool agitation and use intensity — two pools with the same area and temperature can produce very different evaporation rates depending on whether the pool is used for calm lap swimming or active recreational use.

Typical natatorium evaporation rates often fall in the range of roughly 50–500 lb/h (22.7–226.8 kg/h), depending on pool area, activity level, water temperature, and room moisture conditions. Smaller or less active pools can fall below this range; larger or more active facilities may exceed it.

Why Evaporation Matters

Natatorium HVAC design must manage both the evaporation load from the pool and its effects on the building envelope. Cold glazing and exterior surfaces can condense moisture even when average room humidity seems acceptable, because insufficient supply-air delivery leaves cold surfaces unprotected. Dehumidification capacity alone does not solve poor air distribution.

Full natatorium design also requires outdoor-air sensible and latent load analysis, spectator ventilation, wet deck evaporation, room pressure control, and building-envelope condensation review. Use this calculator as a preliminary screening estimate for pool surface evaporation — the starting point for natatorium HVAC decisions, not a replacement for full dehumidifier selection or project-specific psychrometric analysis. For dehumidifier capacity sizing with safety factor, see the Indoor Pool Dehumidifier Sizing Calculator.

Key Facts

  • Pool evaporation is driven mainly by vapor-pressure difference between pool water surface and room air — higher water temperature and lower room dew point both increase evaporation.
  • Activity factor accounts for splashing and agitation above calm-water conditions. Two pools with the same area and temperature can produce very different evaporation rates depending on pool use.
  • Room dew point, not dry-bulb temperature, is the correct humidity parameter to use in evaporation calculations — it directly reflects the air's moisture content.
  • Exterior glazing and cold surfaces in a natatorium can condense moisture even when average room humidity seems acceptable, because insufficient supply-air delivery leaves cold surfaces unprotected.
  • ASHRAE Handbook – HVAC Applications provides natatorium design guidance covering pool evaporation, supply-air delivery to exterior surfaces, condensation control, and outdoor-air requirements.
  • Full natatorium design also addresses outdoor-air sensible and latent load, spectator ventilation, deck evaporation, glazing condensation risk, room pressure control, and building envelope protection.

Applications

  • Indoor pool latent load screening
  • Natatorium dehumidification planning
  • Comparing occupied versus quieter pool activity conditions
  • Reviewing the effect of water temperature and dew point on moisture load
  • Early condensation-control assessment
  • Preliminary HVAC load discussions for natatorium retrofits or new design

Example Calculation

Imperial Example

Given:

  • Pool area = 2,000 ft²
  • Water temperature = 82°F
  • Room dew point = 60°F
  • Activity factor = 0.65

Step 1 — Temperatures to °C

Tw = (82 − 32) × 5/9 = 27.8°C
Tdp = (60 − 32) × 5/9 = 15.6°C

Step 2 — Saturation Vapor Pressures (Magnus formula)

Pw = 6.112 × exp(17.67 × 27.8 / (27.8 + 243.5)) ≈ 37.3 hPa
Pa = 6.112 × exp(17.67 × 15.6 / (15.6 + 243.5)) ≈ 17.7 hPa

Step 3 — Vapor-Pressure Difference (in. Hg)

ΔP = (37.3 − 17.7) × 0.02953 ≈ 0.579 in. Hg

Step 4 — Evaporation Rate

W = 0.1 × 2000 × 0.579 × 0.65 ≈ 75.3 lb/h

Interpretation: This result falls in the NORMAL range, indicating a moderate natatorium latent moisture load under these conditions.

Metric Example

Given:

  • Pool area = 186 m²
  • Water temperature = 28°C
  • Room dew point = 16°C
  • Activity factor = 0.65

Step 1 — Saturation Vapor Pressures (Magnus formula)

Pw = 6.112 × exp(17.67 × 28 / (28 + 243.5)) ≈ 37.8 hPa → 3.78 kPa
Pa = 6.112 × exp(17.67 × 16 / (16 + 243.5)) ≈ 18.2 hPa → 1.82 kPa

Step 2 — Vapor-Pressure Difference

ΔP = 3.78 − 1.82 = 1.96 kPa

Step 3 — Evaporation Rate

W = 0.0488 × 186 × 1.96 × 0.65 ≈ 11.6 kg/h

Interpretation: This result falls in the LOW range, indicating a relatively light latent load under the selected assumptions.

Standards & References

  • ASHRAE Handbook — HVAC Applications — natatorium / indoor pool design guidance (pool evaporation, supply-air delivery, condensation control, outdoor-air requirements).
  • ASHRAE-style natatorium design practice — fixed screening model based on pool area, vapor-pressure difference, and activity factor.
  • ASHRAE 62.1 — ventilation rate requirements commonly referenced in natatorium outdoor-air design discussions.
  • Natatorium manufacturer design guides — practical guidance emphasizing evaporation load, humidity control, condensation protection, and air distribution.

Limitations

  • This is a preliminary screening tool, not a full natatorium HVAC design tool.
  • It uses a fixed ASHRAE-style evaporation model — results are approximate and depend on the accuracy of entered inputs.
  • It does not calculate outdoor-air sensible load, spectator ventilation load, space heating load, glass surface temperature, or full condensation analysis.
  • It does not account for evaporation from wet decks, water attractions, or spectator-related latent and ventilation effects.
  • It does not replace full natatorium dehumidifier selection, envelope review, or project-specific psychrometric analysis.
  • Actual natatorium performance depends on occupancy pattern, spectator load, wet deck area, air distribution, room pressure, and envelope details.

Common Mistakes to Avoid

  • Treating pool evaporation as the only natatorium load — spectators, wet deck, outdoor air, and sensible loads must also be considered.
  • Using calm-water activity factors for active pools with splashing and recreational water use.
  • Confusing room dry-bulb temperature with room dew point when estimating vapor-pressure difference.
  • Forgetting that colder glazing can still condense even when average room humidity seems acceptable.
  • Assuming dehumidification capacity alone solves poor air distribution to cold surfaces.
  • Ignoring outdoor-air requirements and room pressure control in the natatorium design.
  • Using unrealistic off-peak assumptions for occupied design conditions.
  • Treating this as a full equipment-selection tool rather than a preliminary screening calculation.

Frequently Asked Questions

What does this calculator estimate?
It estimates the pool evaporation rate and resulting latent moisture load basis for natatorium HVAC screening. The result is shown in lb/h (Imperial) or kg/h (Metric).
Why does activity factor matter?
Because water agitation and splashing increase evaporation above baseline calm-water conditions. Two pools with the same area and temperature can produce very different evaporation rates depending on how the pool is used.
Why does room dew point matter instead of dry-bulb temperature?
Because evaporation is driven by vapor-pressure difference, and room dew point directly reflects the air's moisture condition. Dry-bulb temperature alone does not capture the moisture content of room air.
What does a HIGH evaporation result mean?
It means the natatorium has a stronger latent moisture load and likely needs careful review of dehumidification capacity, supply-air distribution, and condensation protection on glazing and cold exterior surfaces.
Is this enough to size the full natatorium HVAC system?
No. Full natatorium design also needs outdoor air, sensible load, envelope, condensation-control, and distribution review. This calculator provides only a preliminary evaporation load estimate.
Does this include spectators and wet deck load?
No. This calculator focuses on pool evaporation rate only. Spectators, wet deck, and other loads must be added separately in a full natatorium HVAC analysis.
What outdoor air rate is commonly referenced for natatoriums?
A commonly referenced screening value is about 0.48 CFM/ft² of pool water and wet deck area in natatorium design discussions, though project requirements may vary based on occupancy, code, and ASHRAE 62.1 provisions.
How often should natatorium room dew point be monitored?
In natatorium systems with active dehumidification, continuous monitoring is typically preferred because room moisture conditions can shift during occupancy, activity changes, and outdoor-air variation. Spot checks alone are usually not enough for stable humidity control.

Frequently Used Together

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

Dehumidifier Sizing For Pools

Latent Heat Load Calculator

Psychrometric Calculator

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