Pool Evaporation Rate (ASHRAE / Carrier)
How fast a pool loses water to evaporation from the water temperature, the air temperature and humidity.
Example
You enter
- Pool surface area (ft²) 1000
- Water temperature (°F) 82
- Air temperature (°F) 84
- Relative humidity (%) 50
- Setting indoor
- Activity factor (indoor) 0.8
- Wind over the water (mph, outdoor) 0
You get
- Evaporation (lb/hr) 41.06
- Evaporation (gal/day) 118.2
- Evaporation (in/day) 0.1896
Checked against published worked examples from: ASHRAE Handbook -- HVAC Applications, Ch. 6 (Carrier equation); Carrier outdoor pool evaporation (as given in ASHRAE chapter presentations).
Details, formula, and sources
ScopeIndoors by the ASHRAE natatorium equation W = 0.1 A (pw - pa) Fa with an activity factor, outdoors by the Carrier form with wind. A 1,000 sq ft hotel pool at 82 F under 84 F, 50% air evaporates about 41 lb/h, 118 gal a day, a 43,000 BTU/h latent load. Gives the in/day rate the cover-savings calculator asks for. A design estimate.
Formula or tableIndoor: W = 0.1 A (pw - pa) Fa lb/h; outdoor: W = A (pw - pa)(95 + 0.425 V)/Y lb/h, V = 88 x mph (fpm), Y = 1,046 BTU/lb; pw = e_s(T_water), pa = RH x e_s(T_air) in in. Hg (Magnus e_s in hPa / 33.86389); gal/day = 24 W/8.34; in/day = gal/day / 7.481 / A x 12; latent = W x 1,046 BTU/h.
Source and editionASHRAE Handbook -- HVAC Applications, Chapter 6 (Places of Assembly, Natatoriums), the Carrier pool-evaporation equation and its activity factors, cited by name.
Free accessThe equation and activity factors are reproduced in free ASHRAE chapter presentations and design guides (for example the Rocky Mountain ASHRAE natatorium presentation, which gives the Carrier wind form); the temperatures, humidity and area are the user's inputs.
What governsEstimate. AHJ and licensed professional govern.
API field namessurface_area_ft2, water_temp_f, air_temp_f, rh_pct, setting, activity_factor, wind_mph, evaporation_lb_hr, evaporation_gal_day, evaporation_in_day
- Vapor pressures Magnus saturation curve (catalog pure-math), within about 0.5% of steam tables at pool temperaturespure-math.js
- Condensing air vapor pressure at or above the water's: no evaporation, reported as 0physics