BTU's required = Gallons of water X 8.5 X Desired change of water temperature in Fahrenheit.
I use 8.5 instead of 8.33 because swimming pool water at 1500 parts per million of Total Dissolved Solids weighs more than pure water at zero parts per million of total dissolved solids.
What should the temperature of a swimming pool be?
The decision on how warm to keep the pool is up to the individual owner. The pool temperature recommended by the American Red Cross for competitive swimming is 78° F. However, this may be too cool for young children and the elderly who may require 80° F or higher. The typical range is 78°- 82° F.
(Keep in mind that the energy consumption for each degree rise in warmth will cost from 10 to 30% more in energy costs, depending on your location. In warmer climates the percentage is higher due to the relatively low cost of heating to 78° F. For more about the cost of heating and heaters, click here.)
Why should a pool used for competitive swimming be kept cooler?
If you are vigorously training, your body generates an increased amount of heat energy. In air, this heat is dissipated by sweating. The evaporation of the sweat is the cooling mechanism to help control the body temperature at 98.6° F. In water, your body generates the same amount of heat, but there is no evaporation to provide cooling. Cooling is provided by conduction/convection of heat from your warm body to the cool water. This is roughly proportional to the temperature difference between your body and the water. The warmer the water the less heat is dissipated.
The pool temperature information above was edited by Chris Whitten based on combined source material from the U.S. Department of Energy and the Division of Educational Programs of Argonne National Laboratory.
Heat pump pool heaters
An emerging method of heating pools is the heat pump pool heater. A heat pump's efficiency is dependent on the outside temperature and since pools are usually used during warm and mild weather, heat pumps are a very efficient method of heating pools.
Heat pumps do not generate heat, they simply capture it and move it from one place to another. Below is a diagram that shows how heat pumps operate.
How efficient is a heat pump? That's not an easy question to answer. Heat pumps don't have a simple efficiency number to work with. Their efficiency is measured by Coefficient of Performance (COP).
Just like other heating options, heat pumps can have low efficiencies or high efficiencies. Their COP can range from 3.0-7.0. The higher the number the more efficient the heat pump. What this means is that for every unit of electricity that you put in to run the compressor, you get 3-7 units of heat out of the heat pump. These COPs are usually determined by testing the unit with an outdoor air temperature of 80°F and a water temperature of 80°F. But there is no standard test, so you should be aware of this. The higher efficiency units usually use scroll compressors versus the reciprocal compressors of the standard units.