Having decided that you want a heater for your swimming pool you then need to decide which type. The main types are gas, solar and electric (mainly heat pumps) but even within these there are a number of choices to be made.
Gas Powered Heaters
These are the most common type and they have the advantage of being able to provide heat faster than other options as they are more powerful. Units designed for above ground pools (such as the Pentair MasterTemp 125) typically supply around 100k BTU, on the other hand a heater designed for an in ground pool (such as the Jandy LXi) can provide up to about 400k BTU. This is particularly useful if you heat the pool intermittently and therefore need to raise the temperature quickly.
Gas pool heaters are available to use natural gas or propane. Clearly natural gas would be the best choice if it is supplied on site or LPG might be a good option if you already store it for home heating. If there is no existing supply of gas it may be best to look at some other form of pool heater.
Gas pool heaters also come in Electronic or Millivolt versions. Electronic versions are like a modern gas boiler and feature electronic spark ignition and micro processor control. Millivolt models use an older technology which uses a permanent pilot light and therefore is slightly less efficient. As well as being used as an ignition source the heat from the pilot light is used to produce a small voltage (where the name comes from) to power the controls. The former is the preferable option but it requires an electricity supply, if this is not possible a Millivolt heater is appropriate.
The major negative with gas heaters, especially those using LPG, is that they can be costly to operate.
You can buy direct electric heaters but these are expensive to run and so are usually only appropriate when heating small above-ground pools.
These days the electric heat pump is becoming an increasing popular choice (such as the Jandy JE Heat Pump). Heat Pumps work by transferring heat from the surrounding air into the pool water, rather than generating heat themselves. This means that in terms of energy used to heat output they can be several hundred per cent efficient. As a result they are cheaper to run than gas heaters and are more environmentally friendly. They are also expected to have a longer lifetime than gas heaters.
Heat pumps do have some disadvantages. They are not as powerful as gas heaters and are best run continuously, to maintain the pool temperature. In addition the air temperature should be at least forty-five degrees if they are to work efficiently.
Solar Pool Heaters
In these systems the pool water is pumped through solar collectors which are designed to trap solar energy. Although each location is different, a good rule of thumb is that the area of solar collectors should be the same as the pool area. Solar heating will probably be more expensive to purchase and install than other types but should have a longer lifetime and be less costly to operate. It is certainly the best option if you want to be environmentally friendly.
There are more things that you need to address with a solar system if it is to work satisfactorily. These include the local climate, finding the best location, heeding local building codes and making sure the system is safe. Solar systems are dependent on the local climate so the most important thing is to ensure that the system will heat the pool for the period you wish to use it.