Where Did My Chlorine Go?

[parsog]

I have a new (filled 2 months ago) pool. Prior to yesterday, I had things going along pretty well chemistry-wise but both my CYA and calcium levels were a bit low. I had recently bumped up the SWG to 70% output because the FC had been around 2.5. After running at 70% for a couple of days (10 hours/day), the FC rose to 4.0.

Yesterday I added cyanuric acid (to bring levels from around 60 to 70) and calcium chloride (to bring levels from 325 to 460). When I tested this morning, those levels are good but my chlorine barely registers. I ran the pump all night since it’s recommended to run for 24 hours after adding those chemicals but at a flow rate that likely wasn’t high enough for the SWG to activate. However, the SWG has been running for 10 hours a day at the 70% output and I know it’s working because I can see both the wattage of the circuit increase for 70% of every 5-minute period and can also see hydrogen bubbles coming out of one of my returns while it’s on.

I just bumped up the SWG to 100% but I’m wondering whether adding either of those two chemicals would cause the chlorine to drop so much. Any other ideas?

 

[SJPoe]

Calcium Hardness - Further Reading

www.troublefreepool.com

See the section on 'Calcium Chloride may reduce FC to 0'

 

[reggiehammond]

Overnight Chlorine Loss Test to see if you have algae (which seems likely given low FC)
SLAM Process to eliminate it

I don't believe you need to add CH; test your fill water first. There have been some reports over the years of CH additions causing FC to register lower, but it's never been validated that I know of...

 

[parsog]

I don’t think I have an algae problem because FC levels were a bit low but not zero before this morning. I’m hoping it’s related to @SJPoe answer and running the SWG at 100% for a couple of days will bring my FC back up.

 

[SJPoe]

I would recommend running much 'hotter' with your FC/CYA - at a target CYA of 70, while 4ppm FC is technically ok, your range is 5-10 - aiming for more like 7, 8 or 9 gives you much more 'wiggle room' should there be any issues.

There is no real advantage to running lean - as long as your FC/CYA is in balance, you'll never notice it.

 

[Rancho Cost-a-Lotta]

Don't count on your SWG to increase FC quickly. Get some liquid chlorine into the pool as soon as possible to get you back up to target level.

 

[Newdude]

I don’t think I have an algae problem

Don't think. Know.

Overnight Chlorine Loss Test

If you pass an OCLT, then it allows us all the time in the world to fiddle with anything else because you won't be gifted a swamp in the meantime. (keep the FC f-r-e-e and clear from min at that point while we look into anything else, of course)

 

[zz28zz]

Yesterday I added cyanuric acid (to bring levels from around 60 to 70) and calcium chloride (to bring levels from 325 to 460). When I tested this morning, those levels are good but my chlorine barely registers.

Had the same thing happen to me recently. Added calcium chloride and the next day FC dropped to zero. CC also zero. Added the correct amount of 12% bleach per the Pool Math app and it came right back up. Weird..

 

[parsog]

I also added the 10% liquid chlorine yesterday per pool math and things look good this morning. I’ll turn my SWG back down a bit and hope levels maintain.

Thanks to everyone for the quick responses.

 

[Mdragger88]

I don’t think I have an algae problem because FC levels were a bit low but not zero before this morning. I’m hoping it’s related to @SJPoe answer and running the SWG at 100% for a couple of days will bring my FC back up.

They don’t have to go to zero for algae to get a foothold- just below minimum
FC/CYA Levels
Don’t depend on your swcg to make up a deficit in fc- it simply takes too long and nasties can grow in the meantime.
Use liquid chlorine to reach target fc levels then set the swcg to maintain that.