Cyanuric acid vs algae??

[George in Georgia]

I visited my local pool store today for some skimmer socks, and while chatting, the tech said that one should eliminate algae before adding CYA, and strongly urged adding the CYA to the skimmer.

I've always filled a stocking with the CYA, and suspended it/them in front of the filter return jets. This way if I have to backwash before the CYA is dissolved I don't lose much.

What's with the caution against raising CYA in the midst of algae battle? Keeping high chlorine levels to kill the algae without CYA would be difficult. I don't recall entering battle in the past without a proper a CYA level.

[chem geek]

There is absolutely nothing wrong with dissolving CYA in a stocking in front of a return flow. You are right that adding it to the skimmer and having the CYA caught in the filter means that any backwashing will remove the CYA still in the filter. Also, the CYA will take longer to dissolve in the filter since the flow rate is much slower per area. For me, I put an old T-shirt in the skimmer and add my CYA to that. This way it gets a lot of flow but pretty much fully dissolves before it gets to the filter. I always register the nearly full CYA reading after the CYA dissolves and it usually only takes 8-12 hours to dissolve (depending on how much I'm adding). Of course, if you block up the skimmer in this way, you need to make sure there's other flow to the pump as with a floor drain or a skimmer bypass (I have both in my pool).

The caution regarding CYA and algae is that CYA reduces chlorine's effectiveness. If you had no CYA and were maintaining an FC level, you could kill the algae quickly but you are right that with no CYA you'd lose about half the FC per hour around noon just from sunlight (and an additional amount from killing algae) so you'd have to be there to replenish the chlorine to maintain it (you'd be maintaining a low FC, usually less than 5 ppm if there were truly no CYA). With a small amount of CYA (20-30 ppm), you'd be able to significantly slow down it's rate of loss from sunlight, but it would also slow down the rate of algae kill. You partially compensate for this by having a higher FC level of 12 or 15 ppm for that CYA range based on the Best Guess Swimming Pool Chlorine Chart. So while the pool store may be technically correct if you were to be constantly feeding chlorine into the pool, from a practical point of view it's easier to have a small amount of CYA in the water when killing algae in an outdoor pool. It's up to you, though, and if you have time and want to try it the pool store way you can let us know how that works out.

[George in Georgia]

Many thanks for the reply! Since I'd had success with my prior method I'll disregard the pool shop guy and go with what's worked before. Good thing I didn't tell him I use bleach for shock! I plan to use the Pool Calculator to blast to about 15 ppm till the algae is/are gone, then rely on the SWG for maintenance.

As far as the pool store's method I have other things to do...photography, cooking, bee keeping, etc.