Alternate cal hypo and bleach

JessM

Member
Jan 3, 2015
20
Tulsa, ok
Pool Size
19000
Surface
Fiberglass
Chlorine
Liquid Chlorine
Short version: Can I safely use 73% cal hypo one day and then try the bleach method and then switch back if I don’t toting lots of bleach around?

More detailed version: This is my first summer with my 19k fiberglass pool. My pool builder is my pool installer. He said treating water means just three things: use cal hypo, add CYA to make the cal hypo last longer, and once a week use a phosphate cleaner (once called Pool Perfecr, now called Perfect Weekly). He warned repeatedly against trichlor or dichlor pucks and did say I MIGHT someday need borax or muriatic acid, but probably won’t. OK! So I bought a bucket of 73% cal hypo. But I read here (and the pool store repeated it ) that it will raise my CH too much and create scaling. That made me doubt my pool builder, but I tried his method anyway. Now I’ve almost used the entire 25 pound bucket. On May 4th (first day of the season for us) my CH was at 90 - LOW. Today it’s at 140 So the cal hypo is raising my CH, but it’s still so low that Pool Math actually suggests I manually raise it (but I’m not). Since my cal hypo is running out, I want to try the bleach method. Here are my questions if any of you experts have time to offer some advice.
- Can I just use cal hypo one day and then bleach the next?
- Cal hypo has been so easy and worked so well. My water is clear and doesn’t smell at all. No scaling. No problems I see at all. I toss in somewhere between 15 and 24 ounces every night. If I switch to bleach and find the process labor intensive, can I just switch back to the cal hypo?
- I’m guessing that if I continue solely with cal hypo, by late September/early October when I close the pool my CH might creep up to 300. It climbs about 10 ppm each week. That’s still not super high. But does CH lower naturally over the winter after I close? Or will I open next season with high CH?

Thanks for any advice you have time to offer. I feel so lucky to have this resource and did search for previous posts about switching, but could only find advice about switching from pucks to bleach or to cal hypo. I have never used pucks.

My test results from tonight just in case that’s helpful are added as a screenshot from pool math.
 

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magiteck

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May 20, 2020
580
Neenah, Wisconsin
Pool Size
13600
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Perfectly fine to switch back and forth as long as you understand what it’s adding to the water (calcium). You just wouldn’t want to add both at the exact same time.

Calcium will not leave your water except by draining. It does not break down, evaporate, or disappear any other way. So over time, unless you drain and replace water, the levels will continue to climb.
 
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mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
43,640
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
- Can I just use cal hypo one day and then bleach the next?
Yes. Just keep monitoring your CH level.
If I switch to bleach and find the process labor intensive, can I just switch back to the cal hypo?
Yes, see above.
But does CH lower naturally over the winter after I close?
If the high CH water is displaced by rain/snow melt, the CH will fall. Do be aware that low temperatures can increase the scaling tendency.

Your FC is too low. Follow the FC/CYA Chart

Also, no need for the cheap phosphate stuff. Just plugs up your filter. Keep your FC well above minimum for your CYA level and phosphate levels are meaningless.
 
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JessM

Member
Jan 3, 2015
20
Tulsa, ok
Pool Size
19000
Surface
Fiberglass
Chlorine
Liquid Chlorine
@mknauss - Thank you for the advice. The test I shared via screenshot was done at the end of a sunny, high-UV index day. My test results were 2 after a full day of sun, so I added cal hypo to bring my FC to 5, using Pool Math, of course, to help me know how much cal hypo to add. 😀 Are you saying that my FC at the *end* of a full sun day should be at 5 or 6? I typically add chemicals at the end of the day, and my FC climbs then. In the morning it’s typically at 5. But by 9:00 PM it has fallen to about 2. I thought I was all good with that, but maybe I am misunderstanding something.

@magiteck - Thanks for that note about calcium not falling naturally. I wonder how much water I would have to replace next season to get my calcium numbers back down? Any educated guess? Are we talking 1/4 of the water? Half?
 

mknauss

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May 3, 2014
43,640
Laughlin, NV
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6000
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Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Are you saying that my FC at the *end* of a full sun day should be at 5 or 6?
Yes. Do not go below minimum FC if at all possible. Algae can start easily if you do. At 40 ppm CYA your minimum FC is 3 ppm. Do not every go below that level.

CH reduction needs to know the CH of the water you would fill with.
 
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JessM

Member
Jan 3, 2015
20
Tulsa, ok
Pool Size
19000
Surface
Fiberglass
Chlorine
Liquid Chlorine
Yes. Do not go below minimum FC if at all possible. Algae can start easily if you do. At 40 ppm CYA your minimum FC is 3 ppm. Do not every go below that level.
Ok! So I can see two ways of being sure my FC never goes below the minimum as a result of a super sunny high-UV day. One, At night when I add cal hypo or bleach I can raise my FC levels 2 or 3 ppm higher than the recommended level so when it fades during the day, it fades to number above the minimum. Or I have to add chemicals twice a day - midday and then again at night. Right?
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
43,640
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Your pool should be using about 3-4 ppm FC per day. So if you add 4 ppm per night, your FC should never go below minimum. Excluding days you have a big pool party. Always add an additional 5 ppm FC worth of liquid chlorine, at least, before a big party. Especially with kids
 
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Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
10,371
NY
Twice a day is great until you get a feel for what your pool is consuming every day. It changes a lot over the course of the season, but day over day it’s pretty close.

Once you get a feel for it, add enough to lose your daily amount and still be in target range, to ensure you never go below target range.
 
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phonedave

Well-known member
May 30, 2012
921
Montville NJ
Pool Size
17000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Hayward Turbo Cell (T-CELL-5)
@mknauss - Thank you for the advice. The test I shared via screenshot was done at the end of a sunny, high-UV index day. My test results were 2 after a full day of sun, so I added cal hypo to bring my FC to 5, using Pool Math, of course, to help me know how much cal hypo to add. 😀 Are you saying that my FC at the *end* of a full sun day should be at 5 or 6? I typically add chemicals at the end of the day, and my FC climbs then. In the morning it’s typically at 5. But by 9:00 PM it has fallen to about 2. I thought I was all good with that, but maybe I am misunderstanding something.

@magiteck - Thanks for that note about calcium not falling naturally. I wonder how much water I would have to replace next season to get my calcium numbers back down? Any educated guess? Are we talking 1/4 of the water? Half?

If you drained 1/4 (or 25%) of your water and replaced it with pure distilled water (with no calcium in it), then your CH is going to go down by 25%

If you replace 1/2 your water, then it will go down 50% - again if you use water with no calcium

The formula is

New CH = (CH of pool water)(% of water you leave in the pool) + (CH of fill water)(% of water replaced)
 
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JessM

Member
Jan 3, 2015
20
Tulsa, ok
Pool Size
19000
Surface
Fiberglass
Chlorine
Liquid Chlorine
Like always, you guys are so helpful! Thanks for the info about switching from cal hypo to bleach (and maybe back again) and for the extra tips about keeping FC a little higher. I so appreciate all the help!
 
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Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
10,371
NY
Also, seeing how you’ll be alternating chlorine sources, be extra sure to never let them come in contact with each other. Leftover Cal-hypo dust in a bucket that gets liquid chlorine for example. They are one and the same once they go in the pool but make *absolutely* sure that’s the only place they ever mix, and even then, never at the same time.

taken from pool school :


Mixing different chemicals can create very dangerous, toxic and explosive results.

Mixing calcium hypochlorite with anything is one of the most dangerous types of mixtures
 

JessM

Member
Jan 3, 2015
20
Tulsa, ok
Pool Size
19000
Surface
Fiberglass
Chlorine
Liquid Chlorine
@Newdude Thanks! I was worried about the two mixing, even in the pool, which is what originally brought me back here looking for some great advice. I appreciate you guys!
 
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