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The Role of Skimmers In Pool Systems – Why They Need Careful Consideration

There’s a yin and a yang to the art of building and maintaining pools and spas. On one hand, the water chemistry can be fairly complex. I realize that there are many people who make the case that water quality maintenance is not rocket science, which of course is true, but that should not dismiss the fact that there is a lot to it. Water chemistry is constantly shifting, and all the parts work together. It’s the kind of discipline that you can spend a career studying and refining but never completely perfecting.

On the other hand, many aspects of pool and spa “science” are extremely simple in concept and application. One of the most important, and also the least considered by many people, is the square openings on the sides of underground pools, aka skimmers. Fact is, basically all man-made bodies of water have some form of skimming mechanism. From man-made lakes, to ponds, to commercial pools, to fountains and the smallest spa, they all need skimmers. Pool skimmers are so common that even some manufacturers who sell them barely say much or promote them at all.

That lack of attention is even more evident on the builder end of the equation where many, if not a majority of pools, are built with just one or two skimmers, often with imbalanced or inadequate flow rates. In those systems, it can be extremely difficult to maintain water quality because the top half-inch of water, which is where most of the contamination and debris exists within any pool, is not being removed.

What I find odd about that disconnect is that it flies in the face of just how important skimmers are to the function of pools and maintaining water quality. It’s really just common sense; basically, all the stuff that mucks up water enters through the surface, whether it’s leaves, dust, bugs, bird crap, or suntan lotion on the bodies of the people entering the pool. Most of that stuff will float long enough so that when you do have adequate skimming action, it can be removed before it sinks. The big stuff gets caught in the skimmer basket, the smaller particles in the filter, and all the organic compounds and microorganisms and algae are chemically treated. All those fundamental functions start with the skimmer.

That’s why if you’re paying attention, it’s pretty easy to tell when a pool is lacking adequate skimming action. There will almost inevitably be an oily scum line around the pool. It’s easy to ignore if you don’t look closely at the surface, but you can feel the oily texture when you put your hand in the water. If you were to somehow compare pools side-by-side, one with proper skimming and one without, you’d notice a dramatic difference in the appearance of the water. But in practice, people become accustomed to the way their pool looks and it becomes one of those out of sight, out of mind things.

Making it Work

There are a handful of key skimmer considerations that need to be calculated in order to make sure the surface is being removed along with all its contamination. First, you have to have enough skimmers. I see many 20-by-40 pools with two or even just one skimmer. That’s not enough. Some might say it’s overkill, but I wouldn’t hesitate to put in six skimmers in that size pool.

There are standards for numbers of skimmers in the Model Aquatic Health Code, one per 500 square feet in a commercial pool, but I’ve always thought those minimum standards are usually not enough, depending on certain variables such as bather load and environmental factors.

But there’s more to it than just the number. Skimmers also need to be arranged properly to avoid dead spots. That can have a lot to do with the shape of the pool and the location of the returns. For example, when I run skimmers down opposite sides of a rectangular pool, I’ll stagger them so that each one is essentially skimming a lane or corridor of water across the surface of the pool. For pools with irregular shapes, I’ll be sure to locate skimmers and returns in areas that will have spots where debris can gather.

Finally, having the specified flow rate through each skimmer is huge. That range is typically anywhere from 30 to 50 gpm. That’s simple enough to divide the flow over the number of skimmers to achieve a desired turnover rate for the entire system. The problem is that many pools are plumbed incorrectly and the flow is not evenly distributed among the skimmers. That’s especially true when you see skimmers plumbed in sequence, where the first one in line pulls too much water and the subsequent skimmers don’t move enough. The right way to plumb skimmers is using a looped trunk line where each one has a balanced flow.

Being Equalized

Another common mistake I see is the absence of an equalizer line and/or an auto fill system. Obviously, skimmers require a given water level to function properly. We also know that there will be times when the water level drops, often by simple splash out when a bunch of people jump in the water at the same time. Water loss can also happen by way of excessive evaporation in extremely hot and dry climates or by leaks.

Whatever the cause, when the water drops, skimmers need equalizer lines to continue to pull water into the system. Otherwise, the pump starts to suck air through the dry skimmer, which causes all sorts of problems and damage to equipment. It also helps to have a float valve that closes when the water level drops to further protect against air entering the system via the empty skimmer.

With an auto-fill system, the level will return to normal, but that doesn’t happen instantly, so you definitely need the equalizer line to protect the system when the water level is low.

When skimmers are properly incorporated into the original design and construction, all of these issues are easily accommodated at a relatively low cost. When the number, layout, and plumbing of skimmers are not carefully considered, odds are the pool will never function properly, and setting the system straight after the fact as part of a remodel is an extremely expensive proposition.

Yes, skimmers may be extremely commonplace, even visually unattractive and certainly easy to take for granted, but understanding why they are so important and how they should be used is one of those fundamentals all aquatic professionals should have dialed in from the start.

the Water Geek


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A blog about all things water, written by SRK's founder Steve Kenny.