Bleach vs. SWCG

[RockstarSD]

Trying to figure out if an SWCG would be more cost effective than 12.5% liquid chlorine.

After tax, I get a gallon of 12.5% for 4 bucks.

In a 23,000 gallon pool, it takes 1 gallon of 12.5% to raise FC from 0 to 5.6, which is between 4 and 8, the recommended target for a CYA of ~45.

According to frogabog in this post I should expect to lose 50% of my chlorine during the day for a pool that's in direct sunlight with high temps (90-100 when the pool is open)

So, if I'm trying to maintain a target of 5 fc, i need to dose every night to 10 fc.

If this information is correct, every night I should drop to 5 FC, then add a ~gallon of 12.5% to bring me back up to 10, and repeat the next day.

That means that I'm going to spend 5 months (ish) * 30.5 days a month (ish) * 4 (per gallon) = $610 per season on chlorine.

If the pool calc is correct, if i were to install an SWCG, I'd need to get up to 3000-3500 ppm salt - so 17 40 lb bags at 5 bucks a pop, 85 bucks - and i'd have to replenish here and there, but salt tends to stay, right? Plus the cost of the SWCG system, of 500-1000 bucks.

So - based on this math, it looks like that after a year or two, an SWCG is going to be more cost effective - AND less maintenance.

I could also get a cover, and since I already have the cover roller bar dealy, I may go that route too. The problem with this is that the pool already stays a little too warm for my taste, so I'm not sure that a cover is going to do a lot of good - any recommendations for covers that just block the sun and DON'T increase water temp?

Can anyone provide any thoughts on this?

 

[elenaran]

From what I've read, the main cost downside of the SWG is that you're going to have to replace the salt cell every 3-5 years, and those things are somewhat expensive (~$500 iirc)

 

[bobodaclown]

I like having a SWCG. It's been good. No algae outbreaks. Pool has always been clear. Your salt isn't at zero. There's salt is chlorine so go a little less, it's better to undershoot than over shoot.
Overall I've been happy with my Compupool CPSC48. It hasn't been without issues, but the company has stood behind the product and made things right. (2 Powersupply issues) the last time they replaced both the powersupply and cell. No cost to me, just some time on my end.

The other thing is let your CYA level determine your FC level. With a SWCG you want your CYA 70-80.

What's your current pump run time? A SWCG output is based on 24 hours. You may want to get a unit that is at least double the size recommend. I've got a CPSC48 on my 17K pool. Running at 60% 8 hours a day.

The cells are typically rated at 8000 to 10000 hours.

 

[RockstarSD]

I figure I'd go with the CPSC24 from CompuPool because I'm familliar w/ it and it should be suitable for up to 26000 gallons. Looks like the initial setup is 500. The replacement cell is about 300 bucks, so if I have to replace it every 3 years, my cost looks like this:

Bleach for first 3 years: 600*3 = 1800 bucks
Bleach for second 3 years: 600 * 3 = 1800 bucks
Bleach for 6 years total: 3600

SWCG First 3 years: 500 + (100*3) = 800
SWCG Second 3 years: 300 + (100*3) = 600
SWCG for 6 years total: 1400

And the 100*3 is assuming that i'd have to completely replenish 3500 ppm salt every year, which I don't think I'd have to do...

I mean - for my pool - it really seems like SWCG is the way to go, unless I'm way off on my salt calculations?

 

[linen]

Most analysis on bleach vs. swg indicate that in most cases swgs do not save money over bleach (I think your 50% FC loss per day is a little high for a typical pool with the proper amount of cya). Typically, the number one reason for a swg is convenience...no lugging bottles, being able to leave the pool unattended for a couple days, etc.

 

[RockstarSD]

I've got a CPSC48 on my 17K pool.

The 48 is rated to pools 3.5 times your volume. Why'd you go so large?

 

[linen]

Generally, we recommend getting a swg that is rated 1.5-3 times the pool volume. Typically, the larger the cell, the longer the life for a given pool size.

 

[RockstarSD]

Word. In that case, my SWCG numbers change as follows.

Years 1-3 700 + 300 = 1000
Years 4-6 400 + 300 = 700
Total for 6 years, 1700.

Still half the cost of bleach. Even if I assume 25% loss of FC to sunlight instead of 50%, it's still slightly cheaper over the course of 6 years with less maintenance headaches.

Do we have any numbers anywhere (perhaps ChemGeek can chime in?) that describes some kind of formula for determining chlorine loss based on sun exposure?

 

[Bama Rambler]

We normally see about 2 ppm per day loss in FC as about the best you can do if you don't have an opaque cover on it most of the time.

I bet most folks here lose between 2 and 3 ppm FC per day in a well kept pool.

 

[Isaac-1]

Is anyone considering electricity consumed by the SWG?

 

[jblizzle]

We have been through the process a few times. Seems that it always come out to have the costs be roughly the same over the long term. Just less bleach lugging with the SWG.

The SWG will likely require more acid to keep the pH in check as well and additional electricity.

One example (although admittedly a lot of guessing):
cost-comparison-between-the-liquidator-and-a-salt-system-t42718.html

 

[JasonLion]

Chlorine usage varies with how much shading the pool has. A heavily shaded pool will use less chlorine that a pool in direct sunlight from dawn to dusk.

 

[RockstarSD]

Jason, i'd say that the pool is in direct sunlight 80% or so of the day.

This math was wrong, see the correct energy usage calculation below.
[s:3syi6js0]Re: Electricity: I may be way off here on calculating electricity, but the CPSC48 indicates that it has a running cost of 460 watts. I assume that means 460 watts per 24 hours since its other statistic, chlorine output, is listed at X kg/day.

So assuming I run the thing 24/7, which I won't, I'm looking at 167900 watts a year = 168 kilowatts. Looks like I pay about .10 per kilowatt, so - 17 bucks a year to run the thing? If that's true, it's really not even worth calculating.[/s:3syi6js0]

 

[RockstarSD]

Calculation was incorrect. CPSC48 indicates 460 watt "running cost."

E(kwh) = P(w) x t(hr) / 1000

kwh = 460 * (24*365.25) / 1000
kwh = 460 * 8766 / 1000
kwh = 4032360 / 1000
kwh = 4032.36

4032.36 * .1 = 403

So I have to add an additional up to 400 bucks a year to run the SWCG, if you assume it runs 24/7/365.

 

[JasonLion]

460 watts is worst case peak usage. When the cell isn't energized the power usage is drastically lower.

 

[jblizzle]

But we up size the SWG generally, so in reality the SWG is not ON 24/7/365. Is that 460W only when the SWG is actively generating? If you have it set to 50% and only "on" for 8 hours / day ... then that would only be a generating time of 4 hours per day.

 

[JasonLion]

460 watts is worst case peak usage. When the cell isn't energized the power usage is drastically lower. Even when the cell is energized, actual electrical usage will be somewhat less in most situations.

 

[RockstarSD]

Right - 460 watts is the peak, so I figured I'd do the worst case scenario calculation. If you run it generating 24/7/365, it's going to cost you 400 bucks a year.

if you run it generating 8 hours a day, it's going to be 133 a year. If you run it 8 a day and generating 4, it's going to be less than that even.

I guess we've proven again that there's not much of a difference, besides maintenance ease, of an SWCG and bleach. Oi. Who knew that owning a pool was so freeking expensive? I sure didn't. I think it's worth it though.

 

[chem geek]

As others have noted, with the proper CYA setting for your pool, your daily chlorine loss should be closer to 2 ppm FC per day or 3 ppm tops. So in 23,000 gallons that's 5.9 to 8.8 cups of 12.5% chlorinating liquid or roughly half or less of what you were assuming. For 5 months and $4 per gallon that's $220 to $330 per year.

Also note that SWCG sizings are generally woefully inadequate. The CPSC24 outputs 1.3 pounds chlorine per day (around 24 grams per hour) which in your 23,000 gallon pool is 0.28 ppm FC per hour so it would take 7 hours to output 2 ppm and 10.7 hours to output 3 ppm if on 100% of the time. So your chlorine level may very well drop during peak noontime sun since your SWCG may not output quickly enough and that's ignoring any significant bather load. This is why we say you should get an over-sized unit so that you can run at a lower on-time percentage so that your cell will last longer. Larger cells do not cost proportionately more so you end up saving in the long-run.

Where an SWCG generally comes out ahead is in larger pools with extended or year-long seasons. Otherwise, as others have noted, it's more or less a wash in costs (though obviously not in convenience).

 

[harleysilo]

I've Added borates this year to see if it will help manage creeping PH and reduce acid usage, but it costs money too....