Scale-Forming Tap Water

December 12, 2019

We recently had to adapt the Orenda Startup for a unique tap water situation.

Most tap water we test is low on the Langelier Saturation Index (LSI). On the Orenda app, it shows up with a red LSI number at the bottom. And by "most tap water", we mean almost all tap water in the country. That is, until the water we tested recently in the Bay Area of Northern California. It had a purple LSI.

The tap water looked like this:


If this kind of water is coming out of the tap, it tells us that carbonate scale is likely, if not imminent. Not only for a swimming pool, but for the water treatment infrastructure too. The pipes themselves could scale, and eventually restrict water flow to a point where a remedy is necessary. It was very surprising to see. But no matter what comes out of the tap, we can adapt.

Related: Protect new plaster with LSI balance from the beginning

Our customer in the area was having issues with the Orenda Startup™ because calcium dust kept falling out of solution.  It looked like plaster dust, but it was what we call "snowing", where an excess of calcium is unable to dissolve because the LSI is too high. Today we learned the tap water was the reason why, so our startup method had to be adapted to this unique situation. Remember, our Orenda Startup™ is about the LSI, not calcium hardness. But for this kind of tap water, rather than needing to raise LSI on fill-up, they actually need to lower the LSI into the green. The trouble is, in a matter of days, they will also need to raise their calcium hardness. So what can be done?

Scale-forming tap water

We talk a lot about carbonate scale, and how it occurs when the LSI is above +0.30. Scale happens because the water is over-saturated with calcium carbonate (CaCO3) and cannot hold any more. In an effort to balance itself, the water drops calcium carbonate out of solution as scale, and does so in the hottest water first. Water temperature is why scale forms first in salt chlorine generators and heat exchangers. But with proactive water chemistry and LSI balance, we can prevent scale in a swimming pool.

calcium startup, too much calcium in swimming pool, high LSI pool

When the tap water itself is above +0.30 on the LSI, that changes the game...especially for swimming pool startups. And while the LSI on our startup should be +0.20 to +0.29 ideally, it can go as high as +0.50 for the initial filling. While it sounds counter-intuitive, it's actually easier for us to add things while the pool fills as opposed to having to use acid.

Too much calcium carbonate saturation may not hurt anything physically, but it can certainly harm a professional reputation with pool owners. If the LSI target on startup is missed on the low end, some etching can occur, which leads to plaster dust and a spiked pH that brings the LSI up to 0.00 at the expense of the surface. If the target is missed on the high end–which is what was happening here, hence me writing this article–calcium 'snows' out (see photo).

Too much calcium carbonate saturation is the issue here, unlike almost everywhere else in the country. Our customer was following our instructions to perfection, but we did not understand how complicated their tap water was. So we need to adapt to what comes out of the tap, and do something different. We need to eventually raise calcium, but we must also lower the pH to pull that off. So we got creative.

A high-LSI tap water startup

Raise calcium a bit (but not to 300 ppm), while also reducing pH just low enough to get to +0.20-0.29.


This requires some muriatic acid and calcium chloride to be added together, and simultaneously added as the pool is filling. The Orenda App called for 61.29 fl.oz. of muriatic acid, which is just less than half a gallon. Rather than adding an entire half gallon of acid to the startup barrel, I advised the customer to add half of it for the first day, since it will take at least a day and a half to fill the pool. Re-test the pool upon returning the next day, and determine how much acid to use the next day. 

Adapt and Adjust

We say it all the time, and I'll say it again. The Orenda Startup™ is a general procedure, and it must be dialed in to each situation. The goal is an LSI target of +0.2 something, and can even go a bit higher than that.  +0.2 is a nice target because the LSI stays green, and dust is unlikely to occur. It is also high enough to prevent the loss of calcium hydroxide, while also giving some cushion from the possibility of going aggressive. How you get to that LSI target is up to you. In this case, they were facing snowing problems on many startups because of over-shooting the target.

Now it's time to adapt and adjust accordingly. If your tap water shows a purple LSI, we hope you found this article valuable. Contact us if you need further advice. 



All information provided is intended for educational purposes and is not implied to replace consultation with a qualified pool professional. It is recommended that all information from this or any other source is to be performed assuming individuals performing these functions will consult local state and federal requirements before you act upon it in any way. While this site attempts to provide information that may be relevant to you, no guarantees are made that some relevant information will not be missed. We recommend you consult a local pool professional before acting. 

Subscribe to Email Updates

Recent Articles