Over the past 30 years, there have been hundreds of articles written on vanishing-edge pools. Now you can add a few more to the list due to the growing popularity of Lautner or rim-flow pools, or a combination of rim-flow and vanishing-edge incorporated together. There is no shortage of articles telling us how beautiful, sophisticated and peaceful they are — and they are correct.
Over many decades as a builder, I have contributed to the rapidly increasing number of these pools. Looking back today, it seems silly to think that these installations are that complex — or you would think so until you get into the service and repair business. After nearly three decades building pools, that’s what I did, when I started a service operation.
High Consumption Pools
By nature, vanishing-edge pools are high-consumption pools. Conservatively, the evaporation rate of what I’ll shorthand as an “edge pool” is two to three times that of a conventional installation.
It wasn’t until I built my first small Lautner edge pool that I discovered just how much evaporation loss there is with an edge pool compared to a conventional design. On this job during the landscape irrigation stage, the water to the autofill had been shut off for just about a week, causing the level in the surge tank to drop enough for the waterfeature pump to lose prime. The client said that the pool was leaking. During our investigation, we discovered that the water supply to the autofill had been shut off.
This discovery wasn’t enough to convince the homeowner there was no leak, so we performed a leak detection, which yielded no leakage. Our conclusion was evaporation.
So, we tested our theory. We refilled the pool and surge tank to its static level and made sure the autofill was kept off during the test. The pool was turned on daily at the same time, and we returned to measure the water loss over seven days. During mid-June in San Diego with limited direct sunlight, we discovered our seven-day evaporation rate was 2 inches of surface area.
I love learning moments, so I quickly calculated the water replacement rate to be 2.16 times per year from evaporation. This is a conservative calculation of how many times in a year we would have to replace the entire volume of the pool due to evaporation. Since then, we have tested several other edge pools. One is a vanishing-edge/rim-flow combination in coastal North San Diego County with a 6-foot-high fieldstone edge wall. We calculated this pool at more than 3 inches of evaporative loss per week.
At that point, I started to realize just how much more chemical consumption there was as well. We install a lot of automated chemical control systems, so we started tracking some chemistry issues inherent to edge pools.
One is high calcium: As water evaporates, calcium remains; then as makeup water is introduced, the calcium level rises. This could be an issue if you reside in areas with high calcium in the tap water. Until late 2018 here in San Diego, it was fairly common to have 350 ppm calcium from the tap. Edge pools filled at 350 ppm calcium then refilled two to three times in a year — all the while compounding the calcium — usually spells disaster.
We would regularly see edge pools filled before the end of 2018 with calcium levels above 1,300 to 1,500 ppm inside a year.
The salt connection
Have you noticed that edge pools with salt chlorine generators consume significantly more acid than conventional pools with salt chlorine generators?
By tracking and comparing chemical consumption across a significant number of pools, we found that our edge pools require as much as 30% more chlorination and acid to maintain the same volume of water as a conventional pool.
Why is this? As water moves across the edge surface, it is both oxidizing chlorine and off-gassing carbon dioxide. This means less chlorine and a higher pH in the trough. The chemically deficient water is then circulated back into the pool by the edge system, depleting what chlorine is in the pool as well as raising the pH.
It seems that if one were not using some form of chemical automation, this type of pool would be nearly impossible to maintain on a weekly basis. I have been told the secret to solving this issue is to use more chlorine and throw a tab floater in the trough. Problem solved, they say. I like to think we can do a better job than that.
I firmly believe that a salt chlorine generator shouldn’t be installed without acid injection. It should go without saying that no edge pool ever be designed using only a salt chlorine generator as the sole means of chemical automation. It’s a recipe for disaster. Salt chlorine generators use electrolysis to produce the chlorine gas. When sodium in the salt combines with hydrogen and oxygen in the water, it also forms sodium hydroxide. As a strong base, it neutralizes the acid.
There also are issues associated with precipitating calcium compounds, including the formation of scale on any surface directly in contact with high pH water for extended periods. There will also likely be precipitation in the form of calcium fallout and or calcium flakes, both being the product of high temperatures produced by thermal sources. Edge pool surfaces are especially affected as a result of this natural condition.
