SWG with liner type indoor pool.

[chem geek]

Having a pool saturated with Calcium Carbonate clearly is of benefit when there is plaster/gunite/grout exposed to water as it prevents corrosion (dissolving) of such surfaces. There is also supposed to be benefit in preventing metal corrosion by having a thin film of Calcium Carbonate protecting metal surfaces or perhaps otherwise interfering with corrosion processes. This kind of protection is usually mentioned as protecting copper in heat exchangers. However, what I do not know is if it will protect stainless steel and if it still works in a saltier environment.

First, test your CH level and let us know what that is (just so we have a baseline to understand when corrosion might occur). The Taylor K-2006 has all the tests you need except salt which is K-1766. I would strongly suggest that you then add enough Calcium Chloride to your pool to get to at least 250 ppm Calcium Hardness (CH) which will still be just slightly corrosive, but in a gas-fired heater (when turned on) will only be just slightly scaling which is probably a decent balance. You can then see if this reduces or eliminates your corrosion problem. You are the first user on this forum that I can recall reporting such corrosion, especially so quickly, so I suspect that the lack of calcium may be the issue (in addition to the salt) as most do add at least some calcium even in vinyl pools (when they have gas-fired heaters) even if not to full saturation. For this calculation I assumed you had 3000 ppm salt in your pool.

As for CYA, it is traditionally not needed for indoor pools since there is no sunlight to breakdown the chlorine (and CYA combines with most of the chlorine to form a substance that is more resistent from breakdown from sunlight), but SWG systems usually require high levels of CYA, typically 70-80 ppm, in order to operate more efficiently. Of course, since the chlorine losses in an indoor pool are so much less, the higher efficiency is not as necessary. Just curious, to maintain the 3 ppm FC or so in your pool, how long do you have to run your SWG and at what power level?

Independent of the SWG, have you noticed that without CYA in your indoor pool that your swimsuits wear out faster -- the rubber deteriorates and they start to fade? My wife uses an indoor pool at a community center in the winter time and finds that her swimsuits are ruined after just one winter season while swimming in our own outdoor pool over the summer has no noticeable change in the swimsuits. I believe the difference is the use of CYA in the outdoor pool which significantly cuts down disinfecting chlorine (hypochlorous acid, HOCl) concentration. I personnally am considering recommending that a small amount of CYA (around 10-20 ppm) be used in indoor pools to reduce the chlorine concentration which not only should make swimsuits last longer and the air smell less like chlorine, but should also reduce the disinfection by-products (DBPs) that are produced. The other recommendation, especially for commercial indoor pools with heavy bather load, would be to use a weekly maintenance dose of non-chlorine shock (potassium monopersulfate, KMPS) which should eliminate DBPs, but this use of non-chlorine shock is probably not necessary in a residential environment. Note that these are NOT mainstream recommendations common to this forum -- I'm just letting you know my thoughts on this, that's all.

As you can tell, I'm not giving up on SWG as quickly as you are and want to find ALL the factors that lead to corrosion, not just salt. To convert to a regular non-salt pool, you'll have to not only stop using the SWG, but will have to drain and refill most of your pool water to get rid of most of the salt. As for ORP controllers, the issue isn't so much an automated sensor as "what is the source of chlorine"? Is it chlorinating liquid or bleach injected into the water with a peristaltic pump? Your current SWG is just "set" at a certain chlorine production level so an alternative chlorine system would be similar. An ORP controller adds a new element of measurement to your pool and there are separate issues associated with ORP controllers.

[EDIT] Another factor in your pool that *might* contribute to the faster corrosion is not using CYA since the actual disinfecting and oxidizing form of chlorine (hypochlorous acid, HOCl) will be at levels over 80 times higher than found in the same pool (i.e. with the same measured FC level) with 70-80 ppm CYA in it. As mentioned above, you wouldn't want to use very much CYA in an indoor pool -- only about 10-20 ppm, but that would cut down the oxidizing (and possible corrosion) power of chlorine by a factor of about 10-20. [END-EDIT]
[EDIT] Sean (who works for an SWG manufacturer, is very experienced and knowledgeable, participates in this forum as "PoolSean", and is an upstanding decent human being...well, he seems like he is anyway...) has experience indicating that CYA levels as low as 15 ppm in indoor pool help prevent corrosion. [END-EDIT]

Richard