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 Heaters Comparison Summary

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.