Pool water is moved by means of the pump from the pool, through the filtering system and then through the heater where the warm water is discharged back to the pool. The water is heated when it is passed through the unit's integrated heat exchanger where the heat from the combustion of the fuel is transferred to the pool water.

Gas pool heaters generally have the highest operating costs due to the higher pricing of fossil fuels and because of their relatively low operating efficiencies. In the recent past, gas pool heaters typically had efficiencies of 60% to 70%, but today standard gas pool heaters normally operate at an 80% efficiency. However, efficiencies approaching 95% are available in these units but have not gained popular use due to their high cost.

The most efficient way to operate a gas fired pool heater is to run it for a short period of time to bring the temperature of the pool water to the desired setting. For this reason, they are ideal for pools that aren't being continuously used. Although solar pool heaters and heat pumps require certain ambient conditions for their optimum performance, gas pool heaters can be used to raise the pool water temperature without regard to the weather or climate. The only requirement for gas pool heaters is the ability to connect them to a natural gas line or the siting requirements for an LP tank.

The Department of Energy provides a handy formula to size an above ground gas pool heater. It is expressed as: Pool Area X Termperature Rise X 12. The pool area is simply computed from the dimensions of the above ground pool and the temperature rise is determined by obtaining the normal ambient temperature of the coldest month that you will use the pool and subtracting this from your desired pool water temperature. It is based on a temperature rise of about 1 degree per hour and a wind speed of 3-4 mph. Refer to the associated Sizing Chart for Above Ground Gas Pool Heaters.

There are three main pool heater technologies that are in common use today: the gas pool heater, the electric heat pump and the solar pool heater. Electric pool heaters that you used a resistance heating element and fuel oil fired pool heaters were also fairly common in the past, but due to the high operating cost associated with their energy sources they have fallen out of favor with the public.

Gas powered pool heaters utilize either natural gas or liquid propane as a fuel source. They are relatively simple pieces of mechanical equipment and consequently have the lowest capital cost associated with them. However, although natural gas and liquid propane prices have stabilized recently, gas pool heaters still have the highest operating costs relative to the other technologies. But at the same time, newer gas heaters can operate at 80% or greater efficiency and that leads to an offset of the energy cost. Another benefit of gas heaters is their ability to raise the water temperature in a short period of time.

Electric heat pumps are finding more use today since there have been advances in the efficiencies and technologies associated with them. They are very similar to the heat pumps used in home heating systems and work in much the same way as a central air-conditioning unit does, only in reverse. The systems use a chlorofluorocarbon like Freon and utilize the thermodynamic properties of the expansion and compression cycles of gasses and fluids. The low pressure side of the system is a gas and this gaseous fluid is forced through a nozzle into the high pressure side of the system where heat is released. This heat of compression is used to heat the pool water. Although electrical energy is not used to heat the water directly, electricity is required to operate the pump that moves the fluid through the system.

Solar pool heaters collect the radiant heat of the sun through their solar panels. This radiant energy is transferred to the water through the high surface area of the panels. This is analogous to the solar energy of the sun hitting the pavement or the beach sand. The pool water is circulated through the solar panels by a pump that is operated by a control panel. The pool thermometer sends a signal to the control panel that instructs the pump to keep circulating water to the solar panels until the water temperature reaches its designated setting.

Obviously, solar panels have the lowest energy cost associated with the operations. However, this is somewhat offset by their high initial cost due to their plumbing and installation. Additionally, solar panels also require a significant area for installation and access to the sun's rays so there may be siting constraints involved. Therefore, the use of solar panels is somewhat restricted by the available area to place the panels and a geographical area that is subject to strong sunshine.

The choice of the correct pool water heating system for your specific application depends on several factors, which should be discussed with your pool equipment supplier. The major factors that need consideration are:

• The water temperature desired
• The rate or time that you prefer to raise the pool water temperature
• The months that you need to heat your pool
• Your climatic location
• Availability of direct sunshine on the pool
• The availability of a fuel source
• Your budget for capital costs (equipment and installation)
• Your operating budget (electricity and fuel costs)
• The expected life of the pool heater

Your pool water is circulated through the heater by the pool pump and gathers the generated heat from the heater in the unit's heat exchanger. A fluid inside of the heat pump is in a closed system that is not exposed to atmosphere. A refrigerant such as Freon or a similar chlorofluoro hydrocarbon is circulated through this loop. This fluid is compressed by the compressor and when this warm gas is compressed it concentrates that heat of the fluid. At this point that heat is transferred from the compressed fluid to the water via a heat exchanger, which is essentially a set of tubes with the hot fluid on the inside and the cool water on the outside. At this point, the two fluids try to reach a temperature equilibrium where the pool water is heated and the heat exchanger's fluid is cooled. Upon exiting the heat exchanger, the refrigerated passes through an expansion valve and returns to the low pressure side of the system. While this fluid is under low pressure, it passes through the evaporator coils and cooling fans of the system to remove the cold air via a fan. At this point the refrigerant again starts to cycle through the system and gain heat through the compression process.

Heat pump pool heaters use either scroll compressors or reciprocal compressors. The scroll compressors are more efficient in their operation and are found in higher efficiency heat pumps.

Heat pump pool heaters use more energy in their operation when the ambient temperature is lower. However, when the outside air temperature is 50° or higher they will be able to operate in an efficient manner. This is generally not a consideration for most pool owners since they only utilize their swimming pools during the warmer weather.

Although heat pump pool heaters for above ground swimming pools have a higher initial cost than gas pool heaters, their operating costs are much lower due to their increased efficiency. Generally, they will produce five dollars worth of heat for each dollar of electricity used. Additionally, heat pump pool heaters generally last 10 years or more and have a usable lifetime that is about two times greater than the life of the average gas fired pool heating unit.

I am going to get a pool heater for my above ground pool. Although I have an above ground pool, I'd like to know if I can install an inground pool heater. My pool is 15 feet by 30 feet and I'd like to install a 200,000 BTU/Hr heater but I can only find inground pool heaters in that size. Is there a difference in these two heater types that would preclude me using an inground heater on my above ground pool?