There are three common technologies that are available in above ground pool filters. They are the diatomaceous earth filter, the sand filter and the cartridge filter. The design and manufacturer of these filters will generally have a similar appearance, but they are distinguished from each other by the internal media that actually performs the job of filtering the water.

Diatomaceous earth filters, or DE filters, utilize a set of filter elements that are coated with diatomaceous earth. The grids are the supporting structures that hold the media that actually performs the filtering process. Diatomaceous earth particles are extremely small and have the ability to remove microscopic impurities from the pool water. These types of filters are cleaned by either removing the filter elements and removing the impurities or by reversing the water flow through them and forcing the dirty particles off, in a process called back washing. These filtering units are sized by the area of the grids.

As the name implies, sand filters use sand as the filtering media. Sand filters are typically installed in a round housing, and are sized by both the surface area of the sand bed and the engineered flow rate of water that they can handle. Sand filters are an old and trusted technology and are found in many water and wastewater treatment plants. Water is introduced into the top of the sand filter and is sprayed over the sand bed where the cleaning process takes place. The top layer of the sand and the trapped impurities perform the bulk of the filtering process while the level of sand below the top performs the secondary and polishing stages of cleaning. Sand filters are usually clean by back washing them.

Cartridge filters use filter elements to clean the pool water in the same manner that your heating or air conditioning system uses filters to clean the air within your house. These units contain filtering elements that are cylindrical, and they may contain one or more elements within their housing. They are sized by the surface area of the filter cartridge, and since they are pleated they will provide a much greater surface area than the actual diameter of the cartridge would imply.

In order for swimming pool filters to effectively clean the pool water, they will require a turnover ratio of 6 to 8 hours. All pool filters will need to be cleaned periodically, and this frequency can be lessened by purchasing a filter with a higher rating. Diatomaceous earth filters will remove the finest particles from the pool water, followed by cartridge filters and then lastly by sand filters. However, the finer filtering media will offer more resistance (indicated by the pressure drop across the filtering unit) and will require more frequent cleaning to maintain their operation or a higher rated pump. Most owners select a sand filter as their above groun pool filter since they do not require the extra fine removal capabilites of DE and cartride filters, because they have a lower pressure drop across them and because sand is the most economical filtering media.

There are two types of pool covers that are most commonly used today. These are the vinyl cover solar the solar cover. Vinyl covers are made from thick plastic sheeting and generally have a long life. They are beneficial in that they prevent evaporation of the pool's water, but they do not add any heat to the pool from the sun's radiant energy. They will help retain the pool's heat, but the insulated vinyl covers will do a better job of heat retention than the single ply vinyl sheet covers.

On the other hand, solar covers both prevent evaporative heat loss and allow the radiant energy of the sun to pass through the cover and heat the pool. Solar covers resemble the bubble wrap that is used in packaging, but are made from a thicker plastic material and have additives to prevent degradation from the ultraviolet light of the sun. Outdoor swimming pools will generally collect 80% of the sun's energy that hits its surface, and a transparent solar cover will allow 90% of this energy through it.

Solar pool covers are estimated to heat the pools water by 10 to 15 degrees F. This will allow the above ground pool heater to use less energy to heat the water and will also bring the water temperature up to the desired level quicker since there is less of a temperature differential to overcome. By employing the use of a swimming pool cover to greatly reduce the pool's evaporative loss, fewer chemicals will need to be added to the pool. Consequently, this will minimize the increase in the pool water's hardness, calcium and total dissolved solids levels and will require less make-up water. This will help prevent scaling in the above ground pool heaters, pumps and filters.

During the swimming season, pool covers should be kept in place while the pool is not being used. When used during the daytime, it is best to remove the cover right before you swim in the pool and replace it as soon as you are done. However, during the warm and humid summer months, it may be more beneficial to leave the cover off since evaporation is minimized anyway. If this is the case, and your pool is located in a windy area, the installation of windbreaks in the form of bushes and shrubs will help further prevent evaporation loss.

Solar pool covers will save money and energy by requiring less heat input from above ground pool heaters, In addition, a solar pool cover will reduce the make-up water required by your pool, lessen the amount of pool chemicals that need to be added to maintain your water balance and reduce your cleaning time since it will help keep dirt and other foreign material out of the pool.

Table 1
Above Ground Pool Water Values

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Chlorine is used to sanitize your pool and protect your health. Residual chlorine is the amount of chlorine that is left in the pool water after its reaction process has taken place. The chlorine that is available for pool water treatment can come in either a liquid or solid form. Liquid chlorine is available as sodium hypochlorite. It is preferred by some because it is easy to use and already in a soluble form so you just need to pour it into the pool water.

There are two types of dry chlorine that are commonly used in swimming pools. The first type is a chemical called calcium hypochlorite and it is available as either a tablet or a granular powder. This is favored by many due to its low cost and because it is easy to use. However, this solid product is also slow to dissolve and leaves a sediment in the pool. It is also a fairly unstable form of the chemical. Calcium hypochlorite is more typically used as a pool shocking agent when an immediate and wholesale disinfection process is needed.

For continuous pool sanitization, cyanurates are the better option because they contain a stabilizing agent that helps to prevent the breakdown of the chlorine. Cyanurates are available in two forms, dichlor (sodium dichloro-s-triazinetrione) and trichlor (trichloro-triazinetrione). These products are preferred for routine chlorination because they are long-lasting and have few byproducts.

There are other methods for disinfection, but these are not typically used in private swimming pools. Bromine has similar chemical characteristics and disinfection properties as chlorine but it is difficult to stabilize and relatively expensive. More elaborate systems make use of either ultraviolet light or ozone for disinfection but they are not favored by homeowners due to their complexity and expense.

The scientific definition of pH is that it is the inverse log of the hydrogen ion concentration. However, since we are not in chemistry class we will use this scale to simply indicate whether the water conditions are acidic are alkaline (basic). In absolute terms, 7.0 is considered neutral and anything below that is acidic and anything above that is alkaline. Each unit below 7.0 that is measured is indicative of an increase in the acidity of 10 times. And the alkalinity of a solution increases by 10 times for each pH unit above 7.0.

For our swimming pool environment we will consider 7.5 to be the neutral target point as indicated in the table. Acidity in the water acts as a corrosive and the corrsoivity of the water increases the further the pH level deviates from neutral. Besides negative health effects, this will also degrade pool pump, filter and above ground pool heater. When the pool water becomes too alkaline, it can lead to the formation of calcium deposits that clog the plumbing and pool equipment, especially the heat exchanger plumbing in the above ground pool heater. This is also called scaling. Proper pH management is also essential to allow the oxidizing chemicals, such as chlorine and bromine, to effectively disinfect the pool water.

Acid, in the form of a powder or as a solution of Hydrochloric Acid (HCl), is used when the pH level of the pool water is too high in order to bring down the pH level. Soda ash or some proprietary pool chemicals are added to the water to raise the pH level. Although pH meters that are most typically used in a laboratory environment, the acidity and alkalinity of your pool water can be checked with either test strips or an indicator dye called phenyl red.

Total alkalinity (TA) in the swimming pool is an indicator of the alkaline nature of the pool water and helps act as a buffer to changes in acidity and alkalinity. The normal TA level for your above ground pool water should be between 80 and 120 ppm. This means that it will help maintain a stable pH level by resisting extreme and rapid changes to it. Sodium bicarbonate can be added to the pool water if the alkalinity is too low and hydrochloric acid or sodium disulfate can be added if the total alkalinity is too high. The pool water's total alkalinity must be adjusted before any pH corrections are made.

Hardness is a measurement of the calcium and magnesium content in the pool water, but is directly measured as calcium carbonate and most commonly referred to as calcium carbonate when associated with pool water quality. It is problematic because too much calcium will precipitate out of the water and form deposits on the pool wall and in the above ground pool's heater, filter and pump. This is a process known as scaling and will usually occur when the hardness level exceeds 600 ppm. High hardness levels can be indicated by a white chalky residue around the sides of the pool above the water line. Although you can treat high calcium levels with a water softener, this is not a very practical solution for application in above ground swimming pools. Therefore, the best way to reduce the calcium hardness level in swimming pools is to partially drain and refill them with fresh water.

Total dissolved solids (TDS) is the parameter that quantifies all of the minerals that are in your swimming pool water. It is a residue that is left behind from the addition of the pool chemicals, the inherent dissolved solids that are in normal tap water and any extraneous materials that may have entered the pool and not been filtered out. The total dissolved solids in the pool water will increase over time due to evaporation from both the sun and the increased water temperatures that result from the use on an above ground pool heater. It is important to control the TDS level in the water because it will interfere with the other chemicals added to maintain a good water chemistry. The TDS of the swimming pool water should be below 2000 ppm, and when this level is exceeded, its water should be replaced. This can be somewhat mitigated if you annually drain some of the pool water for its winter closure. The only way to measure the total dissolved solids content in your pool water is by a gravimetric analysis. Since this analytical procedure requires a sensitive and expensive laboratory balance, it is best done on an outsourced basis.

Cyanuric acid is important to your pool's water health because it stabilizes the chlorine that is used for sanitation. It can more or less be thought of as a conditioning agent for this process. By adding it to your pool water once or twice a year, you will lower your chlorine cost and consequently lessen than the generation of the total dissolved solids. It should be maintained at a level between 30 and 80 ppm. Test strips have recently become available to the homeowner to check the cyanuric acid levels in their pools.

The tap water that you receive from your water utility company is first treated in a water treatment plant. The chemistry levels are determined by analyzing the water either manually or with instrumentation. The results of these tests let the plant operators know what chemicals need to be added to the water so that it is both aesthetically pleasing and safe to drink. You will need to follow a similar process so that you have a safe and pleasant environment in which to swim. You can check the various chemistry levels with either a pool test kit or by taking a water sample to your pool center where they will do the analysis for you. If any of the parameters are not within the specified region, you will need to take appropriate measures to bring them in line.

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

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.