Deliberately higher CYA to reduce maintenance frequency

[RouteVegetable]

Hey all, I couldn't find any answers to this question, though I feel like I have read about it before somewhere as a legitimate thing to do.


Background:
I use the Dichlor+bleach method to maintain my 450 gal hot tub, and so far things are all good. But a while back I overdid the Dichlor (I ran out of alternatives for logistic reasons, and needed to get something in there); I ended up with ~60ppm CYA.
Since then, I've compensated by keeping my FC on the higher side - 5-7.5ppm, as I read that CYA's chlorine-blocking is linear - 60ppm of CYA will weaken chlorine 50% compared to with 30ppm of CYA. I also have 50ppm borates, 60ppm TA and 350ppm CH.

My question is:
Is double-the-FC-to-compensate-for-double-CYA indeed a legitimate thing to do?
If so, that seems useful/convenient to do anyway for high bather load/chlorine demand - it allows you to add more chlorine, but less frequently (offgassing aside), and get effective sanitization without overchlorinating your tub or bathers.
I also want to know how far can one push this (eg: could I have 200ppm CYA with 30ppm of FC, jump right in the tub and be okay for a week without adding more bleach?). What limits how far you can go?

My guess about this:
Adding too much bleach at once would lead to some intolerable upward swing in pH. Perhaps you could offset this by combining the bleach with additional acid + additional alkalinity, and until the next bleach addition aerate as necessary to 'burn off' that extra alkalinity+acid and keep the pH in a good range. Though that sounds awkward to orchestrate and in most cases far more work that just adding less bleach more frequently.

Many thanks in advance, and thanks to all the helpful people on this forum who have got me this far in comprehending a surprisingly challenging craft!

 

[ajw22]

CYA creates active chlorine and reserve chlorine that is bound to the CYA.

As your CYA level increases you need higher FC levels to have sufficient active chlorine. Study the FC/CYA Levels and your will see that is why the minimum and target FC levels increase as the CYA level increases.

CYA - Further Reading

www.troublefreepool.com

CYA Chlorine Relationship - Further Reading

www.troublefreepool.com

Above FC 10 your pH test becomes invalid.

The higher the FC is the more rapidly it degrades. Your water will not stay at FC for long.

 

[RouteVegetable]

Thanks for the quick response! Maybe I was unclear (I know I said a lot of different stuff) - I was aware of how more CYA = more FC to have sufficient active chlorine. What I was wondering about is whether there is value deliberately maintaining higher levels of both, to deal with higher bather loads without needing to re-add bleach so frequently.
Effectively keeping a whole bunch of chlorine in reserve, which will keep the pool of 'active chlorine' fed. So yeah, a buffer.

I've noticed that nobody seems to talk about doing this, so I am ready to assume there's some issue with it that I'm not aware of - but I haven't yet found what that issue is. Perhaps it is related to what you said:

Above FC 10 your pH test becomes invalid.

Which is interesting! I haven't read this before; could you expand on/link to why exactly that is the case?

 

[mknauss]

could you expand on/link to why exactly that is the case?

PH - Further Reading

www.troublefreepool.com

 

[RouteVegetable]

Excellent; thanks for that link, and makes sense.

That link does mention the workaround of diluting the pool water with distilled water, which also makes sense.
Given it's possible to work around that issue, I am left wondering if the approach of deliberately running with higher FC and CYA to maintain a bigger reserve of chlorine is a worthwhile/effective thing to do in some specific circumstances, where you have high bather load but it's logistically awkward to add more chlorine frequently. For example, an all day hot tub party with a bunch of people where you don't want the FC to drop to 0.

 

[mknauss]

The higher the FC % of CYA, the higher the daily loss. Not sure how that would work with a spa as it is covered most of the time. Seems like those that have the least issues with managing their spas use a SWCG.

 

[RouteVegetable]

I'm no chemist, but I assume that offgassing would increase proportionally to amount of free chlorine, regardless of CYA.
And so even if I kept the *ratio* of FC to CYA somewhat normal (eg, 30ppm FC : 200ppm CYA), I would see a lot of FC loss thanks to offgassing. And that loss could perhaps undo any benefit afforded by having a larger CYA-protected FC reserve - it would be a profoundly "leaky" reserve.

 

[JoeSelf]

FC/CYA Levels follow this. You are trying to do something that we don't subscribe to. We follow a tried and true method.

 

[RouteVegetable]

So it looks thus far like nobody knows a definitive answer to my question. Which is a shame, because I am *pretty* sure I read somewhere about some situation where this was something that was done (perhaps it was a rural pool, with infrequent access for additional dosing?). I haven't managed to find that article/post/whatever, so was hoping to get some confirmation that "it is a thing", and learn about the problems associated with such an approach.

Sorry if this is the wrong place - I posted in "The Deep End" initially, as I know it is largely academic.
It looks like a moderator/admin moved it here, and this actually happened twice, so I suspect it's deliberate (and I take that to mean this is the right place for such academia).
I'm definitely not trying to discourage 'tried and true' methods, and I'll continue to keep sensible numbers in my own hot tub until I get some reason to do otherwise.

 

[Newdude]

Being a spa it complicates this. When open all day in the case of a party, it would behave like a pool and the extra FC would burn off faster in the 20-30 range to erase a good deal of your buffer FC. When the spa was closed (and the leftovers were sanitized already) you'd keep almost all of the FC for next time.

Most would just go to SLAM levels for a party which is entirely safe for bathers and the spa. You couldn't possibly know beforehand how much FC that party would use, or how quickly SLAM level would drop so you'd have to check and redose a couple of times throughout the day either way. Doubling the CYA for the sole purpose of doubling the FC may or may not last the party day, and you'd be checking/ adding anyway, so I don't see any benefit to trying here.

An oversized SWG would constantly add throughout the party, but you'd have to check that as well so it didn't run away from you with unsafe high/low levels.

So no matter what, you need to take a beak every 2(?) hours and reasses however you do it. A small price to pay for your guests safety. If that seems like too much of a pain, wait until you send 6 guests home with 'hot tub rash' that only goes away with prescription ointments.

Your choice if you'd rather go to SLAM with bleach or SWG, but keep tabs on either.

 

[RouteVegetable]

Ah, thanks; that all makes sense to me - it does sound like, as you say, high CYA is just not worth doing in real life; and at least in the scenario I suggested, there would be a lot of FC loss anyway and yeah, it wouldn't actually make anything easier in reality, particularly given the taylor pH test stops working at >10ppm!

Most would just go to SLAM levels for a party which is entirely safe for bathers and the spa

I was surprised by this; my newbieness is showing, perhaps - my understanding was that >10ppm is really not good for bathers. But I did a search and found that it seems accepted that you can go all the way up to shock level without problems for bathers.

 

[Newdude]

You can go to SLAM level (40% of CYA) per the FC/CYA Levels without harm. This would be the easiest way to ensure plenty FC for longer than normal. There is zero leeway under minimum FC for the CYA. Keep in mind that a spa is like making people soup for bacteria / viruses / pathogens. Lots of residual soaps / shampoos / body oils or fluids to eat up the FC and open the door to rash town.

 

[Newdude]

Fun fact. 4 people in a 500 gallon spa is the equivalent of 200 in a 25k gallon pool. And the spa is 104 degrees with occupants sweating away and a perfect breeding ground for nasties.

The simplest thing ever if you see it correctly. But everybody just thinks Meh, I put 1/4 cup of bleach in.... should be good all day.

 

[Mdragger88]

I had a very lengthy post for u yesterday in the thread that was moved so when i went to post - poof! It was gone
It basically entailed everything that @Newdude has covered above
& that 20-40ppm cya is the proven sweet spot for a reason :
Cya too low =
Fc can be too harsh on people/equipment,
This doesn’t allow for fc to be high enough at the beginning of a soak to last through the soak & prevent person to person transmission
Cya too high=
maintaining ph gets hard to coordinate with keeping proper fc levels according to the FC/CYA Levels as fc above 10ppm makes ph test invalid,
Increasingly Large doses are needed to raise the tub to shock level, keep cc’s down, & to maintain adequate fc levels.

 

[Mdragger88]

To make sure things aren’t unclear- cya only protects fc from the sun - not organics. A higher cya will not make fc last longer in a covered spa/indoor pool if sun is not in the equation.
It’s purpose in a covered spa/indoor pool is only what I described above.

 

[Newdude]

maintaining ph gets hard to coordinate with keeping proper fc levels according to the FC/CYA Chart as fc above 10ppm makes ph test invalid,

And don't get too concerned about this. In practice, your PH would only be unreadable for the party. The leftovers from the party would put you back under 10, possibly even needing an after SLAM or 2. But most of the time you'd be running a 4-8 FC and could test the PH.

Everything will be out-of whack with lots of bathers in an all day event. Pick your battles and focus on keeping the FC up for that.

I had another thought too, parties mean lots of folks all slathered up in sunscreen. It can be enough of a chore to soak it up / filter it out / kill it with FC in a large pool. You may find it much easier to just plan on dumping the tub after large parties, wiping the snot out of it both literal and figurative, and starting with clean water.

 

[Mdragger88]

The extra bodies, hair conditioner, sunscreen, & suits that have been washed with fabric softener can definitely wreak havoc on a hot tub in short order or a pool for that matter. Keep that fc up at all times then deal with the clarity later. I have luckily never had to drain after a gathering - maintaining higher fc levels & washing out the filter has always kept the situation under control for me. I do keep my scum bug floating - it makes me feel better & that I’m doing my part to catch rogue oils.
But I haven’t thrown any ragers yet either