Bleach Harmful To Plumbing?

[NewB@Spa]

For my last fill, almost 4 months ago, I switched to Nitro's Dichlor/Bleach method. I have not had any hint of issues through this whole period. I was speaking to a pool/spa professional and I mentioned that I switched to a bleach method and she stated she would not recommend that and was concerned that the bleach would damage the plumbing/heater element. She did not try to sell me on any product/method so I am not sure what her motive to warn would be. I trust Nitro's method, I just want to verify that someone has used this for an extended period (years) and have seen no harm

[chem geek]

She is probably thinking that you would be using ONLY bleach with no CYA in the water in which case the active chlorine level would be too high and be more corrosive, though usually it's the hot tub cover that sees this effect first (plumbing is much more affected by low pH). The Dichlor-then-bleach method first uses Dichlor to build up the Cyanuric Acid (CYA) level so that the active chlorine level is far lower. At 4 ppm FC with 30 ppm CYA and 104ºF temperature, the active chlorine level is no higher than found with 0.6 ppm FC with no CYA.

Once diluted in the water, the chlorine from bleach is IDENTICAL to the chlorine from other sources including Dichlor. The difference is what else gets added where for every 10 ppm FC added by Dichlor it also increases CYA by 9 ppm.

The chlorine/CYA relationship has been known definitively since at least 1974 as described in this scientific paper, but most people in the pool and spa industry do not understand this relationship and its effect on chlorine's strength in spite of numerous scientific studies demonstrating this as I describe (and link to) in the Chlorine / CYA Relationship section in this post.

[NewB@Spa]

I was positive that you guys would not lead me down the wrong trail. Thanks Chem, btw you said that the cover would see the affects? I would guess that is an affect of outgassing? My tub has been averaging 4 - 6ppm, sometimes more, some less throughout this fill so you would think that this would be okay? I am right at about 4 months on this fill and I have not been running into any balance issues, should I dump and refill now? I have been getting a little pH float (7.4 - 7.7).

[chem geek]

Yes, the effect on the cover is due to outgassing, mostly of hypochorous acid, and that depends on the active chlorine level so is much lower when there is CYA in the water compared to when there is not. 4-6 ppm FC is fine so long as you have 30 ppm or so CYA in the water. If you tend to run your chlorine on the higher side, then you could have your CYA at 50 ppm instead, but I wouldn't go much higher than that. When I say one would see the effects on the cover, that would show up in a timeframe you would think was shorter if you used bleach without any CYA, but in your situation you might not notice the difference between a cover lasting 3 years vs. 4, for example. This post gives one dealer's experience with hot tub cover life as a function of sanitizer type. It pretty much tracks the expected amount of sanitizer/oxidizer outgassing. Dichlor-then-bleach would be somewhere in that #6 category, possibly a little worse, while bleach alone with no CYA would be much, much worse than anything listed.