Running SWG only during the day to minimize FC level swing

[Brett S]

It recently occurred to me that given that the vast majority of chlorine loss happens during the day that it probably makes sense to run the SWG only during the day as well.

Say you have a SWG and variable speed pump that runs 24/7. I think the general recommendation is to run the SWG 24/7 as well. That might be easier to configure since you won’t need a timer for the SWG, but I am thinking that it makes more sense to just run the SWG during daylight hours.

These numbers are oversimplified to make my point clear, but assume you have a pool with a FC loss of 4ppm per day. Obviously you would set your SWG to produce enough chorine gas to add 4ppm of chlorine to your water each day as well. If the SWG is configured to run 24/7 then it would produce 2ppm from 8am to 8pm and 2ppm from 8pm to 8am. However, if we look at how chlorine is used in the pool, nearly that entire 4ppm loss will happen from 8am to 8pm, primarily due to the sun and swimmers. Also assume this pool has a CYA level of 80ppm. According to the chart the target FC level is 6ppm and the minimum level is 4ppm.

Say I wake up at 8am and test the water in this pool and discover that the FC level is perfect at 6ppm. However, by 8pm the pool will have experienced the full 4ppm daily chlorine loss, but the SWG will have only generated 2ppm of chlorine, so now the FC level in the pool is 4ppm and just barely at the minimum level. If it was a particularly sunny day or there were a lot of swimmers it might even dip below the minimum level. Overnight the SWG will continue to run and produce another 2ppm, bringing it back up to 6ppm for my test the next morning at 8am. I’m happy logging 6ppm tests at 8AM every day with no idea that my chlorine is actually dipping so low every afternoon.

On the other hand, say the SWG is configured to run only from 8am to 8pm and to generate the full 4ppm of chlorine during those 12 hours. Again I do a test at 8am and get a result of 6ppm of FC. Again, by 8pm the pool will have experienced the full 4ppm daily chlorine loss, but this time the SWG will also have generated 4ppm of chlorine, keeping the FC level at 6ppm and well within the safe range. There should be no chlorine loss overnight and no additional chlorine generation because the SWG will be off overnight, so the next morning at 8am when I test I will still see a level of 6ppm of FC.

I realize that to some extent this is kind of splitting hairs and a pool will run fine and be successful with the SWG running 24/7... there are already lots of people who run their pools that way without issue. But the is the deep end forum and it seems to me that for the cost of a timer it would be better to run the SWG during daylight hours which should keep the FC levels very consistent all of the time. No more swings up and down or danger of getting close to or even below the minimum level at the end of the day.

Am I missing something here or does this make sense to you guys too?

 

[Jimrahbe]

Brett,

I don't see where it makes any real difference... If your pool used 4 ppm of FC per day, I don't see how it matters when the FC is created. If you run 24/7, then it builds up a little at night, but then the 4 ppm of FC gets burned off by nightfall.. If you shut it off at night, you still have to generate 4 ppm of FC the next day.

I am one of those 24/7 guys.. I run 24/7 because I like generating a little chlorine all the time and I also like constant skimming. If I ran just during the day, I would have to adjust my SWCG to a higher output..

I do not "fear" high FC within reason.. I want my FC to be at my target or higher... I do not try to keep my FC between the minimum and the target. I view the minimum as the edge of a cliff that I never what to fall off...

Thanks,

Jim R.

 

[wireform]

Like Jim said you need what you need what difference does it make when. As long as it's there till your next cycle kicks in.

 

[Newdude]

Also, Over a 24 hour period it matters a lot less. It can rain at night and the SWG will be adding more in the morning. Or vice/versa with the sun and running at night.

The pool company only comes and adds once a week without the SWG so they jack up the FC and hope it makes it until next time, Or they use pucks which jack the CYA and create new problems over time.

Everybody else will probably split hairs in the discussion until they end of time. Add during the day/night or run slow and low 24/7. I’ll show you 3 different people and they all think they are right........ but here’s the best part..... none of them are wrong. Lol.

One thing some people consider is that some electric companies charge more for usage during peak hours and those people usually prefer the night shift.

 

[wireform]

Also, Over a 24 hour period it matters a lot less. It can rain at night and the SWG will be adding more in the morning. Or vice/versa with the sun and running at night.

The pool company only comes and adds once a week without the SWG so they jack up the FC and hope it makes it until next time, Or they use pucks which jack the CYA and create new problems over time.

Everybody else will probably split hairs in the discussion until they end of time. Add during the day/night or run slow and low 24/7. I’ll show you 3 different people and they all think they are right........ but here’s the best part..... none of them are wrong. Lol.

One thing some people consider is that some electric companies charge more for usage during peak hours and those people usually prefer the night shift.

Good discussion. There is one good answer and it's called TESTING

 

[Brett S]

Thanks for the insight guys. I think that to some extent I’m kind of looking at this the wrong way.

Part of my mindset comes from the fact that I have a big saltwater aquarium and for the aquarium stability is key. We try to keep a number of parameters as stable as possible for the fish and corals. After spending years and years of time and effort on the aquarium where stability is king then I think I just kind of have that stuck in my brain.

Additionally in the aquarium too high of a value can be just as bad as too low, so when you’re aiming for a target number you want to keep it stable and you don’t want to go too far out of that range.

But it really sounds like those principles don’t necessarily apply to FC in a pool and it’s kind of hard to break myself from those habIts. Obviously too little FC is bad, but too much isn’t really a problem. (At least not until you get way too much and exceed slam levels)

With a CYA level of 80 then a pool with a FC level of 4 will sanitize just as well as one with a FC level of 6... or a FC level of 8... or even a FC level of 16.

I still like the idea of stability - it makes more sense in my brain, but in the end it seems like I just aim to keep the FC above 6 and below 10 (so I can still do pH tests) and I’ll have a clean and sanitized pool. And that’s all that really matters.