Dead Zones: How They Happen and How to Avoid Them

Scott Webb Headshot
rendering of a vinyl liner pool

Historians tell us that Genghis Khan worshipped freely coursing spring water and could become violent if it were allowed to become sluggish or fouled. So we must infer the Great Khan was a pool man before all the plundering started.

The ruthless leader of the Mongol Horde knew — as does anyone that takes a moment to observe — that water on the move tends to remain fresher and more wholesome while stagnant water tends to become putrid.

This holds true for swimming pools as well. Areas reached by the circulation current, where chemicals mix freely, tend to remain clean and algae free. But where water is stagnant — in a pool’s dead zone — algae is encouraged to bloom and grow, bacteria get a reprieve and debris has the opportunity to drift to the bottom instead of making its way toward the skimmer and filter where it can be removed.

photo of pump installation in a pool
“With a main drain, two skimmers and maybe three or four returns in the walls, we separate the plumbing all the way back to the pump.” — Dan Lenz

photo of pump installation in a pool

photo of pump installation in a pool

Thus it falls to the builder and service technician to think of pool systems, not just in terms of flow through pipes, but also in terms of flow within the pool. In healthy vessels it remains active throughout the entire volume, in sickly pools one finds large, abiding, sinister dead zones.

Making It Easy

We should care about these lethargic zones of circulation because they impact maintenance, and therefore the customer experience, says pool designer Dan Lenz of All Seasons Pools & Spas in Orland Park, Ill.

“We want our pools to be as little work for the homeowner as possible. Certainly eliminating dead zones is a big key to that,” he says. “In a pool’s dead zone, you’re going to have trapped debris, you’re going to have algae growth and higher concentrations of bacteria, and things like that which are going to make the pool more work for the homeowner.”

According to Lenz, surveys show that one of the three biggest reasons people don’t buy pools is they believe them to be a lot of work. So in reality, his company is selling against perceived hassle and effort. All Seasons confronts these worries head-on.

“Alleviating that concern is huge, so we bring it right into the conversation with prospective clients. We tell them, ‘We know it’s a concern, and we design these pools to be as maintenance free as possible. If you allow us to do it, we’ll make it really low maintenance so you’re not out there fighting battles. You’ll be out there relaxing and enjoying your pool.”

Introductions All Around

Intra-pool circulation strategy begins with the way water is introduced to the vessel after it passes through the filter and (perhaps) the heater. The goal is even distribution, both in terms of flow through each return outlet and flow through the pool.

The dynamics of the pool surface take top priority as the pool is laid out, Lenz says, and that plan has to start with an examination of prevailing and local wind patterns. “The surface of the pool is much more affected by natural forces (such as wind) than anything we can do in the pool, and we try to work with those forces in our pools as much as possible. In the summertime here in the Midwest, the wind comes mostly from the Southwest (unless you’re out by Lake Michigan, where obviously the winds are coming off the lake during the day.)

photo of an infinity-edge pool
All Seasons Pools & Spas built this 16-by-32 gunite infinity-edge pool with an accompanying gunite spa in the spring of 2013. The pool is located on the eastern edge of a water-ski lake, so the lake-effect winds combine with the prevailing Southwesterly breeze, driving debris toward the skimmer at right.

“We try to understand the environment and place the skimmers so that we can work with — and not against — the natural wind current. So in regard to eliminating dead zones and getting good circulation, the first thing I ask myself is, ‘Where am I going to put my skimmers?’”

With the skimmers placed to accept the gifts of the prevailing winds, Lenz plans a return-jet scheme to contribute to the effort. “We place them to work together with the wind to move water and debris to the skimmer. We don’t want a jet that is next to the skimmer and working against the natural current. We put the jets across from the skimmer or in patterns that provide a circular motion through the pool. If you’re going to put 2, 3 or 4 returns on a pool, the worst thing you can do is have them shooting at each other.”

Many contractors plan their returns to create a swirl effect so that projecting currents will align and work together instead of in conflict. Lenz adds a twist to that convention, setting directional eyeballs to the side to create swirl, and also downward to reach the bottom of the pool.

“The surface already gets quite a bit of circulation because of the wind currents, and of course your skimmers are up there drawing everything in. By directing your eyeballs on your returns down, you’re helping to get the water penetrating deeper into the pool toward the main drain to get a better distribution of heat and chemicals.”

Getting There And Back

photo of an infinity edge pool at night
Front view of infinity edge at night. Super-tight tolerances allow for steady uniform flow over this edge at very low rpm.

Nearly as important as even circulation within the pool is balanced plumbing, or equal flow distribution through multiple suction and return lines.

A common problem here is that if the pool has multiple returns, all being fed off one main return line, the ones farthest from the pool naturally get less flow, which unbalances the circulation scheme.

All Seasons separates the plumbing for each inlet and outlet (abundantly shown in photo, p. 50), giving each pipe its own valve. This plan allows precise manual control in each line, regardless of its distance from the pump.

Without separate valving — with all the returns coming off one line from the pump — Mike Fowler, commercial marketing and sales manager for Pentair Aquatic Systems in Sanford, N.C., suggests capping the end of the return line plumbing to improve flow in those returns that are furthest away.

“If you’re capping the end, what you’re really doing is forcing the water all the way to the end, and then it’s working its way backward. That helps to ensure you get as much flow as possible to your farthest return,” he says.

Dead 12 Hours

Perhaps the most important factor in a pool’s circulation and mixing scheme is going to be the size and projection of the return current, and that will depend on the output of the pump. In the heyday of the large, single-speed pump, this projecting current was fixed and of short duration.

But since the coming of variable speed pump, the situation has dramatically changed. In fact, the advent of the variable speed pump has done more to eliminate dead zones than any other strategy or innovation in history, because it has allowed longer circulation cycles. Before pools began the advanced regimen of 24-hour, low-flow pumping, the entire pool was a dead zone for half of the day.

In theory, the high-horsepower, single speed pumps of the old days of pool building (many such pools remain today, of course) could be used for long circulation cycles, but the cost would be prohibitive. Today’s variable speed pumps allow pool flow to remain active at a utility cost lower than even the short cycles of the old single speed pumps, due to the exponential savings of the affinity law.

“We just program the pumps to run 24 hours a day at low rpm to provide really great circulation that is never off; because of that, your dead zones are improved drastically,” Lenz says. “During that 24-hour period, we’ll ramp up to provide the flow needed for the cleaning system (see sidebar), and then ramp back down for filtration.”

“Even without a cleaning system, just returns, it still makes sense to ramp up the flow for a short time just to move more water to the skimmer. Skimming action is so necessary to keep the pool clean and easy to care for, which is the goal.”

Fundamental to that aim is an effective overall pool circulation strategy, one that includes variable speed pumps that run longer and slower, carefully planned skimmer and return jet locations, as well as balanced plumbing.

Up From The Bottom

One of the best ways to alleviate dead zones and thoroughly circulate pool water is with an in-floor cleaning system. Such systems are ideal circulators, as they bring water into the pool through a series of heads along the floor that pop up in sequence, sending currents sweeping down the floor toward the main drain. There’s nothing like it for deep-end mixing.

Of course, with heated water added to the bottom of the pool, you get a rising mixing current along with the projections along the floor — an excellent mixing strategy and a good way to improve heating efficiency, where cost is not an obstacle.

Comments or thoughts on this article? Please e-mail [email protected].

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