Ken,

The problem is not CYA, but too much CYA relative to the Free Chlorine (FC) level (or conversely, too little FC for the CYA level).

Without any CYA in the water, the FC level in most pools will drop by 50% every hour around noontime in direct sunlight. In your situation with only two hours of sun, the FC might drop by 75% (assuming the sun is on the pool near noon in the summer), but for most pools what you propose is impractical unless one has an automatic dosing and measurement system.

Also, without any CYA in the water, normal FC levels are too strong and would oxidize swimsuits, skin and hair too quickly and produce more of the most volatile and irritating disinfection by-products such as nitrogen trichloride (see this post) and increase metal corrosion rates as well. The problem is that it is very difficult to maintain only 0.1 ppm FC in the pool (with no CYA) which would be a more-than-adequate level to prevent algae growth and kill pathogens quickly (the min. FC level in most of Ben's chart is roughly equivalent to 0.06 ppm FC with no CYA).

The CYA acts as a chlorine buffer holding the chlorine in reserve as needed so its use is much more than just protecting chlorine from breakdown from the UV in sunlight. The fact that it moderates chlorine's strength (i.e. the active chlorine, hypochlorous acid, level) is a good thing. In fact, I recommend that it be used, in moderation, in indoor pools -- at a level of 4 ppm FC with 20 ppm CYA to give a little more oxidation power in indoor pools since there is no assistance from UV in that case.

Every year, my wife experiences the difference of having CYA in our outdoor pool vs. swimming over the winter in a community center indoor pool with no CYA (and around 1-2 ppm FC). Her swimsuits degrade over just one winter season (the elasticity gets shot) and her skin is flakier and hair frizzier while in our outdoor pool none of these problems occur. Our pool has typically had around 3 ppm FC with 30 ppm CYA (it has an opaque electric safety cover) so is roughly equivalent to 0.1 ppm FC with no CYA. It's no wonder that with the indoor pool having 10-20 times the active chlorine level that the degradation of swimsuits, skin and hair was so noticeable.

Richard