Why is CYA kept higher with a SWG?

[troy1]

I have done a lot of reading on the forums over the past couple weeks.

I have read that pools with a SWG generally are kept at a higher CYA level than pools without a SWG.

But I can't seem to understand WHY that is done.

Has this been explained already somewhere that my search has not been able to find?

Can someone point me to an explanation?

 

[JohnT]

Higher CYA allows the SWCG to maintain the appropriate chlorine level without having to run so hard that it shortens cell life. Most manufacturers recommend CYA levels around 80ppm.

It appears that the normal issues with high CYA are mitigated by the very high chlorine levels that the water is exposed to within the cell. This effectively shocks all of the water in the pool with each turnover.

 

[chem geek]

Also, by having a lower SWCG on-time, that reduces the rate of pH rise caused by factors that occur when the SWCG is running (undissolved chlorine gas outgassing; hydrogen gas bubbles causing more carbon dioxide outgassing). A higher CYA level lowers the amount of chlorine lost to the UV in sunlight so lets one turn down the SWCG on-time percentage.

 

[troy1]

OK, I think I am understanding it, sorta......

So the higher CYA level allows the SWCG to run less, yet still keep a high FC level because it is not losing as much FC to the UV light.

But the pool is in a screen enclosure, so that should cut the UV exposure by about half wouldn't it? Also the pool is to the west side of the house, so it will only get the afternoon sun, but none in the morning. That should lower the UV exposure even further.

So if I have a somewhat lower than normal UV exposure, do I still need to keep the CYA level that high?

I am just concerned that by having it that high (80ppm), that it will require inordinate amounts of Chlorine to shock it once in a while.

 

[chem geek]

You shouldn't need to shock it, but yes if you want to try a lower CYA (say 50 ppm) due to the sun shade, you can try it. You can see what sort of on-time you need and what sort of pH rise you get. It's your pool and you can adjust it any way that works for you. We're just giving you the facts from which you can make your own rational decision consistent with your own needs and situation.

 

[troy1]

chem_geek,

I appreciate that you are giving me the options. I am just trying to understand the chemistry and why some things are done the way they are.

So, you are saying I should almost never have to shock the pool? A friend of mine also has a SWG pool and he has not shocked it in the nearly 2 years he has had the pool. So I guess it is true.

So, within the SWG element the clorine levels get really high?. Like.... they get way higher than the FC shock level? Even with the CYA at 80 then? WOW, that is impressive and it explains why you almost never need to shock it then.

Please correct me if I've got it wrong.

Thanks,

 

[JasonLion]

You have it about right. The only thing I would add is that we are not 100% sure exactly what happens inside the SWG cell that causes SWGs to work with lower FC levels. What you describe is the leading theory, just not proven yet. Whatever is going on, there is plenty of experience showing that it works.

 

[harleysilo]

Hi Troy! I have a SWG and this year I've got my CYA at 50, and I think I'm going to keep it there. I have an opaque cover on my pool when it is not being swam in. Last year and this year I've had trouble getting the SWG dialed in. I'm not sure what % of the time sun is hitting my pool, maybe 14% for a week. That's 14% figurin 8 hours swim over the weekend. In the evenings my pool is shaded. So if I had the potential for 56 hours of sun, I only get 8 due to cover....I don't know how many hour per week of direct sunlight is figured into CYA levels and chlorine loss to the sun, would be interesting to know.

 

[chem geek]

So, within the SWG element the clorine levels get really high?. Like.... they get way higher than the FC shock level? Even with the CYA at 80 then? WOW, that is impressive and it explains why you almost never need to shock it then.

Even in a pool without an SWG and that is manually dosed with chlorine one hardly ever needs to shock the pool. I haven't shocked mine in at least 2 and maybe 3 years. The key is to properly maintain the proper FC/CYA ratio to prevent algae growth.

The chlorine level near the chlorine generation plate in the SWG is very high (depends on where you measure it) and also at low pH so it's definitely super-chlorinating the water in that area, though it's only a portion of the water that goes through the SWG. This might be killing enough free-floating algae to allow for a lower FC/CYA ratio in SWG pools, but that shouldn't affect algae growing on pool surfaces that doesn't get circulated. So there might be something else going on such as algae being slightly inhibited in its growth from the salt level though that would have higher salt pools without SWG have the same benefit (unfortunately, there aren't enough of those pools for us to make any conclusions). And then there's having 50 ppm Borates in many SWG pools to help control the rise in pH and we know that borates do somewhat inhibit algae growth.

 

[troy1]

I think the reason I am struggling to understand all this is that my family had a pool 30 yrs ago when I was the teenage pool boy. Stabilizer was a new idea back then. Automatic chlorinators where for rich people. And the rest of us low lifes struggled to keep chlorine levels in range.

We must have lost the battle fairly often, so shocking the pool was a once a week ritual.

All these rules have changed in 30 yrs.

Now I am the 50 yr old guy trying to come up to speed on CYA and borates and Salt Water generators - it's a whole new world.

Frankly - I love a challenge and am looking forward to it.

If we sort out some final difficulties, we close on the house on Wednesday.

Wish me luck.