Some Notes On Heating Swimming Pools

A few references:

  1. The classic reference: Morrison, G.L. & Donnelly, E.W., Thermal Performance of Solar Energy Systems for Swimming Pools, UNSW Report No. 1980/FMT/2 May, 1980 ISSN 0157-5104

  2. 1999 ASHRAE Applications Handbook

  3. RetScreen Simulation Program

The US red Cross suggests temperatures be maintained at 78°F – 82°F.

Swimming pools gain and lose heat by conduction, convection, radiation and evaporation.

Conduction: Usually assumed to be relatively insignificant.

Convection: (Morrison & Donnelly):

q = (3.1 + 4.1 ) v * (Tw – Ta)

q = Heat loss in Watts/m2

v = Air velocity over pool surface m/sec

Tw = Water temperature in Degrees Kelvin

Ta = Air temperature in Degrees Kelvin

Radiation: (Morrison & Donnelly):

q = σ (Tw4 -Ts4)/(1 + 1/Єw)

σ = Stefan-Boltzmann Constant (5.67E-8 W/m2K4)

Єw = Water Emittance (0.95)

Tw = Water temperature in Deg K

Ts = Apparent radiation temperature of environment

Radiation will be most significant for uncovered pools on cloudless nights, and sunlit pools during the day.

Evaporation (1999 ASHRAE Applications Handbook):

wp = 0.1 A (pw – pa) Fa

wp = Evaporation rate of water in pounds/Hour.

Note: Evaporative heat loss is approximately 1000 Btu/lb.

A = Area in square feet

pw = Water saturation vapor pressure taken at the surface water temperature in Inches of Mercury

pa = Saturation pressure at room air dew point in Inches of Mercury

Note: Vapor pressure (In.. Hg) vs. Temperature (Deg. F)

Fa = Activity Factor:

Type of Pool

Typical Fa









Public School


Whirlpool or Spa


Wavepool or Water Slide

1.5 or Greater

Evaporation will generally be significant and will increase with water temperature, air flow, pool activities, and low air humidity. A pool cover may reduce evaporation significantly. There are “liquid pool covers.”