Solar heating systems consist of a solar collector, a filter, a pump and a flow control valve. The solar collector is actually a heat exchanger that transfers the sun's energy to the pool's water as it circulates through it. They are generally shaped like large panels, although some newer designs vary from this traditional shaped piece of equipment. A filter is installed between the pump and the solar panels to remove any impediments in the water so that the tubing inside of the panels does not become clogged. The pump simply circulates the water through the filter, the solar panels and then returns the warmed water to the pool. Some smaller solar heating systems can sometimes use the existing pool pump if it has sufficient size, but most larger systems require a separate pumping device due to the increased pressure drop across the solar panels and plumbing. The flow control valve, also known as a three way valve, can either be an automatic or manual device to divert the pool water to the solar panels to help regulate the temperature.

Solar heating systems have a slow response time and may need to be run for an extended period of time before the pool water is sufficiently heated, as opposed to gas pool heaters. They are also best suited to applications that do not require unusually high pool water temperatures, but are ideally suited for maintaining water temperatures of 80 to 85°. Although most pool owners that employ the use of solar heating panels use them as the sole source of heat, they are also very well-suited to operate in conjunction with another heating source such as a gas-fired heater or a heat pump.

A typical solar pool heating system will generally have an installed cost of $2-$4000. And although this initial cost is higher than the more conventional systems, solar heating systems typically pay for themselves in a couple of years by eliminating the cost of an external energy source (such as gas or electricity) for heat.

Solar heating systems are dependent on your localized conditions, most notably the local climate and the availability and quality of the solar energy. They need to be cited to have line of sight access to the sun and if this is not possible at a location beside the above ground pool where they are typically installed, they can be located remotely by a longer run of plumbing. In fact, they are often installed on the roofs of nearby structures, for both technical and aesthetic reasons. The amount of solar radiation that is available will depend upon the season, the time of day, the geographic location, the local weather and any nearby buildings or landscape that may obstruct the direct rays of the sun.

The size of the solar heating system will need to be determined by an experienced contractor. Generally speaking, this results in a solar collector that is anywhere between 50% and 100% of the surface area of your swimming pool. The sizing of these panels will also vary depending on the pools swimming season and the localized climatic conditions.

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

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

I have found a used 250,000 BTU Hayward pool heater that I'd like to use to heat our pool. It's a natural gas fired unit, but I'd like to use propane instead. I understand that you can convert natural gas pool heaters to propane, but I don't know what is involved. Do I need an additional part? Any help is appreciated.