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Pool Thermodynamics
Swimming Pool Heat Loss & Evaporation Simulator
Real-time evaporative, convective and radiative heat loss for indoor and outdoor swimming pools using the ASHRAE / Carrier model. Change water temperature, room temperature, humidity, wind speed, pool usage and cover type to see how evaporation rate, total thermal load, daily water loss and annual cost respond.
Parameters
Pool surface area A
m²
Water temperature T_w
°C
Air temperature T_a
°C
Relative humidity RH
%
Wind speed V
m/s
Pool usage
ASHRAE utilization factor auto-set
Activity factor
Surface agitation from swimmers. Still water = 0.5, crowded = 2.0
Pool cover
Cover used outside of swim hours
Results
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Evaporation (kg/h)
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Evaporative loss Q_evap (kW)
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Convective loss Q_conv (kW)
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Radiative loss Q_rad (kW)
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Total heat loss Q_total (kW)
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Daily water loss (L/day)
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Pool cross-section — heat loss paths
Arrows rising from the water surface show the relative strength of evaporative (blue), convective (orange) and radiative (red) flux. Toggle the cover to see how the flow pattern changes.
W: evaporation rate (kg/h), A: water area (m²), V: wind speed (m/s), p_w, p_a: saturation / effective vapor pressures (kPa), h_lat = 2454 kJ/kg, ε = 0.95 (water surface emissivity), σ = 5.67×10⁻⁸ W/(m²K⁴). Evaporative latent heat typically accounts for 70-80% of total pool heat loss.
Swimming Pool Heat Loss & Evaporation — ASHRAE Model
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I've heard indoor pool facilities have huge HVAC bills both summer and winter. What makes them so different from a regular office building?
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The main culprit is "evaporation from the water surface." Every kilogram of water that evaporates carries away 2454 kJ ≈ 0.68 kWh from the pool. Even a modest 200 m² competition pool loses about 14 kg/h at the default settings, which is a 10 kW continuous thermal load. And indoors, you also need to run a refrigeration dehumidifier to keep that vapor from condensing on the walls. So the rule of thumb is that "saving 1 W of surface evaporation saves about 2 W of electricity bill" — that's the basic intuition behind pool design.
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OK… so concretely, how is evaporation calculated? This tool uses W = A · (95 + 0.425·V) · (p_w − p_a). Where does that come from?
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That's the de facto industry standard originally published by Carrier in 1918, and still kept (with tweaks) in the ASHRAE Handbook. It says that the larger the vapor-pressure gap between the water surface (p_w) and the indoor air (p_a = p_sat(T_a)·RH), and the more the wind speed V agitates the surface, the more water molecules are pushed into the air. The 95 is the still-air term, 0.425·V is the wind contribution. Try cranking humidity to 90% in this tool — you'll see (p_w − p_a) collapse and evaporation almost vanish.
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In the "Pool usage" dropdown, competitive is 0.5, recreational 0.7, therapeutic 0.3 and outdoor 1.0. Why is therapeutic the lowest, even though the water is hotter?
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These are the ASHRAE "utilization factors" that correct Carrier's still-water assumption for real usage patterns. Competition pools have relatively calm surfaces (0.5), recreational pools have splashing kids (0.7), therapeutic pools have patients floating quietly (0.3), and outdoor pools experience continuous wind and sun (1.0). Even though the therapeutic water is warmer, the surface stays calm with gentle floating users, so the agitation-driven evaporation term is actually smaller. It's a field-experience-based correction layered on top of the physics.
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I was shocked that pool covers can cut evaporation in half, and a thermal blanket gets 80% reduction. Why do they work so well?
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Three physical effects stack at once. (1) The cover physically blocks the path for water molecules to leave, so evaporation drops directly. (2) The convective coupling between warm water and cool room air is broken. (3) Long-wave radiation to the cool ceiling — or to the cold night sky outdoors — is shielded. A 5 mm bubble cover acts as an insulating air layer, and a multi-cm thermal blanket essentially seals everything. The US ENERGY STAR Pool & Spa program documents about 70% annual heating energy savings from nightly cover use, with payback in 1-2 years for outdoor pools. Just putting the cover on after closing is that powerful.
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Finally, convection and radiation are only about 20% of the total compared to evaporation. Can I ignore them?
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Not always. Indoors with water and air at similar temperatures, convection is indeed small. But for an outdoor pool in winter you might have 27°C water and 5°C air. Q_conv scales with ΔT so it can grow more than 10×. And radiation: on a clear night the effective sky temperature drops to around −20°C, so the Stefan–Boltzmann term T_w⁴ − T_sky⁴ blows up, giving several kW of radiative loss even with no wind. So for year-round outdoor design you must sum all three terms carefully. Try dropping the air temperature to 5°C in this tool — you'll see convection and radiation jump from "negligible" to "leading."
Frequently Asked Questions
It uses the classic empirical correlation adopted by the ASHRAE Handbook and the Carrier System Design Manual: W = A · (95 + 0.425·V) · (p_w − p_a) / Y. Here A is the pool water surface area in m², V is the wind speed just above the water in m/s, p_w and p_a are the saturation and effective vapor pressures of the water and indoor air in kPa, and Y is the latent heat of evaporation (2454 kJ/kg). The coefficients 95 and 0.425 originate from Carrier's 1918 work and include the effect of swimmer activity. ASHRAE applies a utilization factor of about 0.5 for indoor competition pools and 1.0 for outdoor pools.
Typical values are about 15% reduction for a liquid monolayer cover, 50% for a bubble cover and 80% for a thermal blanket. The tool models these as cover factors of 0.85, 0.50 and 0.20 respectively. Because evaporative latent heat is 70-80% of the total pool heat loss, reducing it directly cuts both heater energy and make-up water and chemical costs. Simple payback for indoor pools is typically 3-5 years and 1-2 years for outdoor pools.
The water vapor that leaves the pool cannot stay in the room. To keep relative humidity at 50-60% (avoiding condensation, corrosion and mold on walls and ceiling), the air must be dehumidified by either outdoor air with reheat or a dedicated dehumidification unit (Bock, Dectron, etc.). The refrigeration cycle consumes roughly the same energy as the latent evaporation load, so saving 10 kW of evaporation also saves about 10 kW on the refrigeration side. That is why pool-cover energy savings are often cited as roughly twice the raw heat loss.
Wind speed dominates. Going from 0.5 m/s to 3 m/s raises the wind term (95 + 0.425V) by about 1.4× and convective loss in proportion. Effective measures are (1) windbreaks or planting to lower surface wind, (2) a thermal blanket at night that suppresses both evaporation and radiation, and (3) solar pool heating (USD 200-400 / m²) to bank daytime heat for night make-up. Adjust the wind-speed slider here to see the direct sensitivity to annual energy cost.
Real-world applications
Public pools and sports clubs: There are roughly 25,000 public pools in the United States alone. A typical indoor 25 m competition pool (≈ 312 m²) consumes more than 100,000 kWh per year. Designers start with the ASHRAE evaporation equation as in this tool, allocate 50-60% of the load to a dehumidification unit and the rest to a boiler or heat pump. Combining a pool heat pump (COP 4-6) with nightly covers routinely cuts operating cost by two-thirds.
Hotel, spa and therapy facilities: Therapy pools at 30-35°C can evaporate more than double the rate of a competition pool of the same area. Operators typically combine off-hours cover use, variable-speed circulation pumps and heat recovery (using the dehumidifier's condenser heat to reheat pool water). Comparing "therapy + no cover" against "therapy + thermal blanket" in this tool reveals annual savings often in the tens of thousands of USD per pool.
Residential outdoor pools and supplemental heating: Most of the roughly 5 million US residential pools are outdoor, used either seasonally or year-round with solar covers and heat pumps. Enter outdoor air 15°C, wind 2 m/s and water 28°C here and total loss climbs sharply, providing a realistic basis for heater sizing. The reduction in evaporation also lets you estimate savings on make-up water, water bills and chlorine chemicals.
HVAC engineers — preliminary sizing and education: Before stepping into detailed CFD or vendor-specific tools (Dectron, Bock, etc.), it is standard practice to size loads with a hand calculation like the one in this tool. If the split between evaporation, convection and radiation looks off, you have likely mis-entered indoor boundary conditions. The tool also works as a teaching aid for students reading ASHRAE Handbook Chapter 6 (Indoor Swimming Pools).
Common misconceptions and pitfalls
The biggest misconception is the simplistic "just drop the water temperature by 1°C to save energy." Yes, the driving force (p_w − p_a) depends strongly on water temperature: going from 27°C to 26°C cuts evaporation by about 7%. But for therapy pools, infant or elderly pools the water temperature is fixed by comfort, and competition pools are bound by FINA / world swimming standards (25-28°C). In practice you cannot freely change the water temperature — the realistic levers are surface coverage and dehumidification efficiency. The priority order is cover > heat recovery > water-temperature trim.
Next, "the Carrier equation is too crude" is a common bias. It is a 1918 empirical formula, and newer Smith-Lof or VDI 2089 expressions add detail. But ASHRAE and Carrier keep updating the coefficients, and the field accuracy is ±15%, which is good enough for most design work. The bigger source of error is the input data — particularly indoor relative humidity. People often size for a 60% design value while the real building runs at 70%, leading to 50% lower actual evaporation. Always sanity-check against measured humidity sensor data.
Finally, "buy a pool cover and you'll save energy" is not automatic. A thermal blanket can deliver 80% reduction, but only if it is actually deployed every night. US surveys report that roughly 40% of installed manual covers sit unused at the side of the pool because staff find them too heavy to handle daily. Automatic motorized covers cost USD 5,000-20,000 but are reliably used, which is why their measured savings are much higher. Always evaluate "theoretical savings × realistic deployment rate" — the field rule.
How to Use
Enter pool surface area in square meters (typical range 20–200 m² for residential to commercial pools).
Input water temperature in Celsius and ambient air temperature; the simulator calculates temperature differential driving evaporative and convective losses.
Specify relative humidity percentage (0–100%); lower humidity dramatically increases evaporation rates according to psychrometric principles.
Click Calculate to compute instantaneous evaporative, convective, and radiative heat loss components in kW, plus cumulative daily water loss in liters.
Worked Example
Olympic-sized pool: 2500 m² surface, water at 28°C, air at 20°C, 60% relative humidity. Evaporation rate ≈ 8.5 kg/h → 29.8 kW evaporative loss. Convective loss ≈ 12.4 kW (wind-dependent). Radiative loss ≈ 3.2 kW (clear sky). Total daily loss: 1040 kWh, equivalent to 18,500 L water replacement. Lowering humidity to 40% increases evaporative loss to 42.1 kW and daily water loss to 28,400 L.
Practical Notes
Covered indoor pools (95% humidity, 25°C air) lose 60–70% less water than outdoor pools; use covers during non-peak hours to reduce evaporative loss by 50–80%.
Radiative loss dominates at night with clear skies; install radiant barriers or thermal blankets on outdoor pools to recover 8–15 kW on larger facilities.
Wind speed (included in convective term) increases heat loss non-linearly; windbreaks reduce Q_conv by 30–45% for pools in exposed locations.
AS/NZS 3626 correlations assume still-water surfaces; turbulence from fountains or splash features can increase evaporation 2–3×.