Above ground pool heaters are a great way to warm your pool water to the perfect temperature during those chilly days, nights, or even whole seasons—when ordinarily you’d be a fish out of water. A pool heater for an above ground pool is essential to extend your swim season and control the exact temperature of your water. With a heater in place, you’re always prepared for an unexpected chill.To choose a heater that’s best for your above ground swimming pool, determine the number of BTUs (British Thermal Units) you need. A BTU is the amount of energy needed to heat one pound of water to one degree Fahrenheit. Above ground pool owners typically choose heaters that have a lower BTU rating because of the small, compact size. Most heaters for above ground swimming pools are gas or propane. These are the most popular option of above ground heating systems since they generally heat the pool quickly and maintain a consistent temperature. Keep in mind that these heaters must be connected to a gas or propane supply.