If your house doesn't already have natural gas lines running to it, you will most likely want to go with a propane heater. It would not be practical financially to install natural gas lines solely for the purpose of running a pool heater. Propane comes in tanks that can be refilled, making propane heaters ideal for houses without natural gas lines.
The exact prices of natural gas and propane will depend on the area in which you live. In general, natural gas is a bit cheaper than propane, though. Before deciding on one type of heater, ensure that you compare the prices between the two types of gases. Remember that natural gas will be billed monthly, while you will only need to pay for propane when you refill the tank. The price of the heater itself depends largely on the size of your pool. Never buy a pool heater that is not suited for your pool size.
Natural gas burns cleaner than propane, so it is better for the environment to use a natural gas pool heater instead of a propane pool heater. Unfortunately, it is not always practical for someone to use a natural gas heater. Luckily, low-emission heaters of both types are available that cut the amount of pollutants released into the environment.
The output of gas pool heaters is measured in British thermal units. A BTU is defined as the amount of energy required to raise 1 lb. of water one degree Fahrenheit. When equal volumes of the two gases are present, propane provides 2 1/2 times as many BTUs as natural gas. Because of this, a propane heater will warm up a pool faster than a natural gas heater. As long as the heaters are kept in proper working condition, explosion is extremely unlikely.