Evaporation and heat-loss

evaporating pool

- outdoor pools gain heat from the sun, absorbing 75%–85% of the solar energy striking the pool surface.  Most swimming pool heat loss is through evaporation, therefore swimming pool heating costs can be significantly reduced by using a pool cover. Even if the pool has no heating it will still get warm quicker in the Spring and remain warm longer in the Autumn.

Though a pool-cover slightly reduces the total amount of solar heat absorbed by the pool (Liquid Pool Covers don't reduce the amount of sun hitting the pool), the cover eliminates heat loss due to evaporation and reduces heat loss at night through its insulating properties.

Use of a pool cover also can help reduce the amount of pool chemicals (Chlorine, etc.) needed to keep the pool sanitized and healthy. (However this small economy will not be enough to 'pay for your pool cover' - despite what a less-honest pool-cover salesman might tell you!)

To keep the pool-water warm in the colder months of the year it is necessary to add heat to the water - and prevent all that costly heat from leaving the pool.AstralPool Heat-pump

Indoor pools are usually heated also, and the resulting evaporation is a big headache for the operators. Dehumidifying equipment must be installed to dry the air, with associated power cost. Reducing evaporation in indoor pools can save a lot of money.


A Heat-Pump is the most efficient, economic and cost-effective  way of heating the pool and this is even more true when a Heat-pump is backed up by an Electric Heater for night-time use at Off-peak rates.

Heat-pumps work on the same principles as a refrigerator; heat is taken from one place and ‘pumped’ elsewhere.  A refrigerator takes heat from inside itself and radiates it to the atmosphere. A pool heat-pump takes ambient heat from the outside air and transfers it to your pool-water via a heat exchanger.

Even on an overcast or rainy day a heat-pump will heat your pool .  Most modern units will operate down to ambient temperatures of 7° or so. You should also fit a pool-cover to minimise evaporation and reduce heat-losses.

Heat is lost from a pool in a variety of ways.  Some radiates away via the surface, some is conducted through the sides and bottom and some is lost during a filter backwash; when you pump warm water away to the drains.

None of these losses are the real villain, though.  Over 90% of heat lost from a pool is by evaporation of the water from the surface.  Heat is measured in BTUs (British Thermal Units).  One BTU is ‘the amount of heat needed to heat 1 pound of water by 1 degree Fahrenheit’

However, evaporation of 1 pound of water takes 1048 BTU’s, all of which comes from your pool-water, thus making it much cooler.

Evaporated water is then replaced by cold mains-water, further reducing the temperature.

Evaporation also has another nasty side-effect - it contributes to increases in TDS and Calcium Hardness. This is because when water evaporates ONLY the water is lost; the minerals dissolved in the water remain in the pool and when you top the pool up more minerals enter the pool. Eventually, dissolved minerals build up to a point where some or all of the water has to be changed.

Note: - Please contact our recommended pool professionals for supply and installation of all pool-related services: - 

Costa del Sol - This email address is being protected from spambots. You need JavaScript enabled to view it.

Nerja area - This email address is being protected from spambots. You need JavaScript enabled to view it.

For free and unbiased advice, a site visit or no-obligation Quotation*, please make initial enquiries by email. Tell us a little about your pool problem and include a daytime phone number. Thanks, we look forward to meeting you soon

NOTE: - On the Costa del Sol please contact PoolSchool for supply and installation of all pool-related services and equipment including Jolly Gel

*We only offer Quotations: - a fixed, agreed price that cannot, and will not, be increased. We do not give 'estimates', as an estimate offers the client no guarantee of final price

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Ken Walker - MyPoolGuru©