Heating the pool is not a requirement of pool operations...but heated water sure feels good early in the spring, during unusually cool summer mornings or evenings, and into the fall. While gas-powered heaters are the most popular types of heaters - because they do the best job of purely heating the water (although at the highest energy-consumption cost) - you do have other options to heat your pool water.
Here is a list of the various options to heat your pool:
Gas-Powered Heaters:
There are two types of gas powered heaters - natural gas and propane (which is also a gas). A gas-powered heater uses gas (natural gas or propane) to heat your pool. Without getting too technical about the process of converting gas into heated pool water, just know that a heat exchanger (internal part) transfers the heat that is generated inside the heater to the water that is circulating through the heater. As the pump circulates water to and from the pool and through the equipment, it will ultimately circulate that water into the (natural gas or propane) heater and its heat exchanger. A heated pool is a wonderful pool...but an expensive pool each month in gas consumption. While some pool equipment is expensive upfront yet saves you money each month, a gas-powered heater is fairly expensive upfront and can be very expensive each month. The monthly fee really is worth it (in terms of the comfort and warmth of the water), but let it be clear, it will be expensive to maintain that comfort and warmth each month.
Heat Pump:
A heat pump is an energy-efficient alternative to the gas-powered heaters. A heat pump captures the outside air, raises the heat of that outside, and transfers that heat to the pool water as it passes through the heat pump. A heat pump is a glorified solar heater. While a heat pump is expensive upfront, the monthly fee is extremely inexpensive; heat pumps are very energy-efficient. The heat pump, however, is not meant for capturing heat quickly, especially on cool and cloudy days. If you want to heat your pool, if you want to save significant money each month, if you are a patient person, and if you realize you cannot generate quick heat (i.e. for a schedule pool party during an unseasonably cool and overcast day or stretch of days), then a heat pump is a viable pool heating option.
Solar Heaters:
A solar heater captures heat from the sun and transfers that heat to the pool water as it passes through heat collector tubes or panels on its way to the pool. While a worthwhile solar heating system is somewhat expensive upfront, the monthly fee is free; whereas all other pool heaters and heating systems require an energy source with a monthly consumption cost, solar heaters use the sun to heat your pool water, which is free to all of us each month. Of course, on a cloudy or overcast day, the sun is not readily available to help heat your pool. If you are considering a solar heater, you must pay close attention to the location of the collector tubes or panels and the size of your pool pump (especially if the collector tubes or panels are located rooftop, which typically provides a clear path to the sun). A solar heater is certainly not for every pool, but if duration of sunlight, direct sunlight (to the solar heater), and your backyard and overall neighborhood setting allow for a solar heater, it certainly is a viable heating option; and it is a heating option with no monthly consumption cost for energy.
Electric Heaters:
An electric heater directly uses electricity to heat your pool. Without getting too technical about the process of heating the water, just know that a heating element (internal part) transfers the heat that is generated inside the heater to the water that is circulating through the heater. As the pump circulates water to and from the pool and through the equipment, it will ultimately circulate that water into the electric heater and its heating element. A heated pool is a wonderful pool...but an expensive pool each month in electricity consumption. While some pool equipment is expensive upfront yet saves you money each month, an electric heater is pretty expensive upfront and can be very expensive each month. In fact, the monthly energy (electricity) consumption is the most expensive of any type of heater, even the gas-powered heater options.
Heating the pool is a luxury...a luxury you will pay for; but, the warmth and comfort of the water is worth the monthly fee.
You can read the Pool Manual for additional information about the various pool heating options.