Is there a big difference between filters for aboveground pools and inground pools? They both filter water right? Besides, the manufacturer's specifications tell me that this filter can filter a swimming pool up to 20,000 gallons. So why should I have to pay more than double the price? The short answer is yes, but let's not be penny wise and pound foolish.
Those are typical, reasonable questions that many pool owners ask. Let's look at some of the major differences between filters for aboveground pools and inground pools. But first we'll look at the similarities.
The similarities between inground and aboveground pool filters are pretty simple. The first similarity is that there are 3 types of pool filters – Sand, Diatomaceous Earth (DE), and Cartridge. The second similarity is that they work essentially the same – a pump draws water from the pool through the skimmer, then the water is pumped through the filter tank and its media (sand, DE or cartridge) and returns to the swimming pool clean and filtered.
The differences deal with pool plumbing or piping, necessary flow and finally size. Let's discuss them in order.
Pool Plumbing. The biggest difference may seem like pool size and gallonage. But in reality, it's more of a function of pool plumbing. Most aboveground pools have their filter system right next to the swimming pool. Plus, the filter and pump & motor are generally located below the water level. In other words, the filter system is usually on the ground with the hose from the skimmer feeding directly down into the pump and the filtered water returning up to the pool's return jet – gravity feed. Most aboveground pools have no more than 6 to 9 feet (about 2 to 3 meters) of hose on each side – a total of 12 to 18 feet.
Inground pools on the other hand can and do have many more feet of piping and plumbing. On top of that, there are usually multiple skimmers or other suction lines – sometimes working in tandem, sometimes not. Then there are multiple return fittings or “eyeballs” or jets. Here's an example, my swimming pool (with only about 16,000 gallons) has a skimmer, lower suction, and 2 return fittings. Each wall fitting has its own pluming line. Even though the filter sits just 15 feet from the pool, let's look at how many feet of pipe there are for each line. The skimmer and lower suction are relatively close to each other, so they each use about the same amount of piping – roughly 40 feet each line or a total of 80 feet going into the pump. The return fittings are on opposite sides of the pool. One return line is about 30 feet from the filter, the other is about 50 feet – another 80 feet. My pump & motor has to pull and push water a total of more than 160 feet or TEN times the distance of an average aboveground pool. Plus we haven't mentioned heater, valves, elbows and other fittings used in inground pools.
Necessary Flow. Understanding flow is the “great mystery” of the pool industry. Very few people practice it well. Simply stated, FLOW is putting the correct amount of water through the system to adequately filter the water. It's not all about Horsepower. In fact, most pool systems operate with too much horsepower! And that's a waste of energy and money. Think of it this way: all pipes or hoses (depending on their size or diameter) can only handle a certain amount of water flow – let's say it's 100 gallons per minute (gpm) for example. You or pool guy decide that a new pump is needed. “Let's use something a little bigger that will give you ‘better' suction,” he says. Now a pump is installed that pumps 125 gpm – great! Not so fast! Your pipes can only handle 100 gpm. Do you have better suction? Maybe. Are you using more energy to run the larger pump? Definitely. Roughly 25% more energy. Not only that, but the filter doesn't seem to filter as well as before. That's because the water is being pushed too quickly through the media (especially true of sand filters) – the filter system has to run longer because the dirt can't be as easily trapped.
But that's just one part of the flow discussion. The other part is something called “head.” Head is how high the pump has to pull or push water up. Most inground pools have their filters above the water level. Sometimes within a foot or two (less than 1 meter). Sometimes, it can be 10 to 15 feet. The pump has to draw all of that pool water 15 feet up to the filter. Think of this way: is it easier to hold a glass of water over your head and pour it into your mouth or to use a straw to sip (suction) it? Plus then there are solar heating panels (often installed on the roof of the house or pool shed) and fountains or other water features.
Size. Finally we come to size. Pool size. Filter size. It's all important. Can an aboveground filter system filter 20,000 gallons of pool water? Yes. But is an aboveground filter system able to properly push water through 100 plus feet of piping and create enough circulation in the pool to push water down 6 or 8 feet in depth and back up again? Or how about doing all of that plus getting the water 25 to 30 feet in various directions? Inground pumps are made to do all of that pushing and pulling of water. Aboveground pumps can't.
How about filter size? Inground pools typically have 20,000 to 40,000 gallons of water. All of that water needs to be filtered 2 to 3 times daily for best performance (see our ezine article on Proper Circulation: “Circulation – First Key to Good Pool Care” for more information). A larger filter is needed for the larger volume of water.
As a necessary aside, larger aboveground pools ( 27 ft round and larger, and 18 x 33 oval and larger ) should consider utilizing a properly sized inground filter system. After the first season, many large aboveground pools experience water quality problems due to inadequate filter systems. Unfortunately, these pools are “hampered” by the problem of having only one skimmer and one return cut-out. Ask your local pool professional to configure your system correctly. Will it cost more initially? Yes, but you will be much more satisfied in years to come.