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.

These data are simply calculated values, so you will need to make considerations with your eventual pool heater selection, since they are commonly sized in 50,000 BTU/Hr increments. Also, this data is based on the heating units operating at an 80% efficiency. Gas pool heaters are available that exceed this rating, so an appropriately smaller sized unit could be substituted.

Gas pool heaters are rated based on the BTU input of the heater. Natural gas has a heating value of 1,000 BTU/cubic foot, so a 100,000 BTU/Hr heater that operates at 80% efficiency would transfer 80,000 BTU/Hr of heat to the pool water. A heater with an efficiency of 90% would transfer 90,000 BTU/Hr to the water and result in a fuel savings of approximately 13%. This will have a greater impact on your fuel budget the more that you use the heater, so the greater expenditure for a higher efficiency system may justify the higher capital cost.

A pool heater that can raise the water temperature by 30 degrees is needed if you plan to use your pool in early April through October (and you live in the NE US) when the air temperature is 50 deg, but if you only want to extend your swimming season by a month on each end then a smaller sized unit would suffice. A smaller unit would also work out well for you if you can run it for several hours to warm the water or only need to supply the water temperature of the daytime with some additional heat at night or on overcast days.

Again, these tables are presented as a guide. Discuss your specific requirements with your pool equipment supplier. However, for most homeowners with above ground swimming pools, a heater that is rated at 100,000-200,000 BTU/Hr will handle many situations. But if you are to err on one side or the other, you will never be disappointed by choosing a larger unit over a smaller one.

Table 1: 1 deg F Rise/Hr

	10 deg F rise	20 deg F rise	30 deg F rise
Round Pools
15	26494	52988	79481
18	38151	76302	114453
21	51928	103856	155783
24	67824	135648	203472
27	85840	171680	257519
30	105975	211950	317925
Oval Pools
15 x 30	60244	120488	180731
18 x 33	78651	157302	235953
18 x 40	97551	195102	292653

Table 2: 2 deg F Rise/Hr

	10 deg F rise	20 deg F rise	30 deg F rise
Round Pools
15	52988	105975	158963
18	76302	152604	228906
21	103856	207711	311567
24	135648	271296	406944
27	171680	343359	515039
30	211950	423900	635850
Oval Pools
15 x 30	120488	240975	361463
18 x 33	157302	314604	471906
18 x 40	195102	390204	585306