What should the temperature of a swimming pool be?
Obviously the point of installing a pool heater is to make the pool comfortable for users, and operators of commercial pools generally find that the warmer it is the more people want to use it. Having said that individual preferences vary significantly, so there is no right answer as to the best temperature.
For residential pool owners cost will be a major factor, as energy costs quickly mount as you increase pool temperature. If you use a pool cover to minimize heat loss, just increasing the pool temperature from 78° to 82°F will more than double the heating bill in most locations (see http://www.energysavers.gov). In fact, depending on your location, for each 1°F rise in water temperature your energy costs will increase between 10% and 30%.
It is also worth remembering that if a pool is kept very warm there is a danger of bacteria and other micro-organism multiplication becoming a problem. Also chlorine-based disinfectants become less effective as the temperature raises.
If you are active in the pool 78°F may be OK as this is the temperature recommended for competitive swimming to prevent swimmers from overheating. For general use 80° – 82°F should be acceptable but you may need to go as high as 86°F for babies or disabled people.
If you use your pool most days it will be worthwhile maintaining the temperature but if the pool will not be used for several days it is best to turn off the heater. You will use less energy in reheating the pool than in maintaining the temperature.