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Thread: Size of SWG vs size of pool

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    Size of SWG vs size of pool

    For what its worth, I'm a pool tech in NY, and the majority of my pools run at 40% or lower. If a pool isn't maintaining the FC levels I expect at 40% it tells me to start looking for a problem (i.e. CYA too low, Salt too low, scale on cell, etc).

    You are a bit farther south, so it wouldn't be unreasonable to run at a higher %, but 80% sounds a bit high.

    As Jason suggested, raise up the FC and see what happens. I might go a step farther and suggest raising it up to Shock levels. If you are on the verge of an algae bloom, you will want to head it off before it takes off.
    TreeFiter

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    Re: Intellichlor concerns...

    TreeFiter, in general the "normal" SWG percentage settings can vary wildly from pool to pool. It all depends on the relative size of the SWG and the pool. If you put a SWG for a 50,000 gallon pool on a 15,000 gallon pool you will be running it at a way lower percentage than if you put a SWG for a 15,000 gallon pool on a 50,000 gallon pool.
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    Re: Intellichlor concerns...

    I realize that factors such as the size of the pool will have an effect on how the SWG will need to run, but in my experience it doesn't make that big of a difference. The pools I take care of range in size from about 10,000 to 70,000 gallons, and not a single one of them requires running the SWG above 40% unless there is something wrong. That being said, some of the smaller ones are happy running around 20%, a few even as low as 10% with covers on them. If a pool needs to run at 80% just to maintain a reasonable FC level, I would be a little concerned that the CYA is still a bit low, the salt level is low, or there is algae growing in that pool.

    Also keep in mind that my experience is based on visiting my pools once per week, which requires maintaining a higher FC level to ensure that if chlorine demand increases, the pool has a reasonable amount of wiggle room before algae can start to grow. This would require a slightly higher SWG setting than a pool that is being maintained on a daily or semi daily basis.

    Farther south, I would expect to need a higher setting, but 80% just seems too high. It doesn't leave you much room to go up if there is ever a need. If a pool requires 80% in NC, it would be impossible to maintain with a SWG in Florida, and I'm pretty sure thats just not the case. I realize you can always supplement with liquid chlorine, but that defeats the purpose of getting a SWG in the first place.
    TreeFiter

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    Re: Size of SWG vs size of pool

    All that means is that your pools were built by a builder who more or less matched the size of the SWG to the size of the pool. What you describe is not typical and not good advice for others as the SWG size and the pool size are not typically well matched. My pool runs around 8-12% because I purposely oversized the SWG, and many people posting here have reported situations where they routinely run at 80%, because they got a very small SWG to save on up front costs, and in all cases those results were normal for those pools.
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    Re: Size of SWG vs size of pool

    The other part of the equation is what is the run times for these pools - are the pumps running all day?
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    Re: Size of SWG vs size of pool

    The run times for these pools are typically 8am-8pm.

    Jason, I can't get on board with the idea that I should tailor my advice based on the idea that there are a lot of bad installs out there. If the SWG was not sized properly, it will either over generate, which means that they will easily be able to run below 40%, or it will undergenerate, which means they will have to run at much higher percentages and will burn through cells every other year. As I said, if I see a SWG that can't maintain FC at 40%, I start looking for a problem. Its entirely possible that the problem is an undersized SWG.

    The OP was asking for input from others regarding their experiences with SWGs. I offered the experience I've had with hundreds of different pools over the past decade. I've found my rule of thumb to work across a variety of different SWGs on a huge variety of different pools.
    TreeFiter

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    Re: Size of SWG vs size of pool

    If the SWG keeps up with the chlorine demand, regardless of how high a percentage it needs to do that, how is that "undersized"? And what possible benefit could come from telling someone they have an "undersized" SWG when everything is working just fine?

    Note: I am talking residential pools here. Commercial pools are a different situation.
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    Re: Size of SWG vs size of pool

    Salt Cells don't last forever. They are designed to last for so many hours of operation. If they run at 10% they will last 10 times longer than a cell running at 100%. For the average pool owner that translates into replacing a $500 cell every 10 years vs. every season. The cost benefits of a SWG disappear if the cell needs to be replaced frequently.

    In reality, the cell might not need to be replaced every year if you run it at 100%, but it will need to be replaced ten times more often than a cell that runs at 10%.

    As for what benefit could come from telling someone they have and "undersized" SWG when everything is working just fine, there wouldn't be any. However, why would there be a discussion about the sizing of the SWG unless there was a problem? The original post was in regard to a pool running at 80% and still having trouble keeping FC in the pool. I simply stated that if it needs to run that high, I would start looking for a problem. If that problem were to turn out to be an undersized SWG, then the benefit might be that they could install a proper system that would be capable of keeping FC in the pool.
    TreeFiter

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    Re: Size of SWG vs size of pool

    Since larger cells cost almost proportionately more, that reasoning doesn't make much sense. I can get an inexpensive cell that products X chlorine, or a cell that costs 1.8x as much and produces 2X chlorine. Yes there is a slight costs savings from getting the larger cell, but there is also a larger investment that can be lost if the whole thing just breaks for some reason.
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    Re: Size of SWG vs size of pool

    My PB installed a Hayward Salt and Swim which is rated for 30K gallons. My pool is 27,600 and I'm running at 70% right now to maintain an FC of 4 with a CYA of 75. I run my pump from 9AM to 7PM. When I started out at 50% I was only able to maintain an average FCL of 2 to 3. My PH is 7.6, my TA is 70 and my salt level is 3400 ppm. So all my numbers are almost dead nuts on which I take to mean that my SWG is undersized for my pool. I'll probably run the S&S for a year or two then upgrade to something bigger.
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    Re: Size of SWG vs size of pool

    Quote Originally Posted by JasonLion View Post
    Since larger cells cost almost proportionately more, that reasoning doesn't make much sense. I can get an inexpensive cell that products X chlorine, or a cell that costs 1.8x as much and produces 2X chlorine. Yes there is a slight costs savings from getting the larger cell, but there is also a larger investment that can be lost if the whole thing just breaks for some reason.
    I guess it comes down to where you want to put your priorities. If you are banking on the system breaking down prematurely, why invest at all? Would you buy any other product with the expectation that it is simply going to break? Most rational people wouldn't buy something if they thought it was going to fail quickly, so why would this be any different. Most systems come with some sort of warranty anyway. The investment in a salt system is typically justified based on the money you will save on chlorine. A typical pool might use about $200 in trichlor tablets in a summer (here in the northeast). Salt System will cost about $1000 installed. So as long as you can get 5 years out of your SWG, you break even. If you are running that cell at 80% for those 5 years, you might not make it to the break even point, and you aren't going to save any money.

    Bigger cell is more expensive, but it is replacing bigger quantities of chlorine. I don't know the numbers off the top of my head, but I would imagine that it still works out to about 5 years like the example above.

    To be clear, I'm not suggesting everyone should upgrade to an oversized cell. I'm just supporting the idea that the cell should be sized appropriately. If it isn't, chances are you will eat up cells trying to keep up with the FC levels, and replacing cells will eat up any money you saved by installing the smaller system.
    TreeFiter

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    Re: Size of SWG vs size of pool

    Quote Originally Posted by CLW42 View Post
    My PB installed a Hayward Salt and Swim which is rated for 30K gallons. My pool is 27,600 and I'm running at 70% right now to maintain an FC of 4 with a CYA of 75. I run my pump from 9AM to 7PM. When I started out at 50% I was only able to maintain an average FCL of 2 to 3. My PH is 7.6, my TA is 70 and my salt level is 3400 ppm. So all my numbers are almost dead nuts on which I take to mean that my SWG is undersized for my pool. I'll probably run the S&S for a year or two then upgrade to something bigger.
    When you say you started out at 50%, what were the conditions involved? Was the pool already up and running, and you switched to a SWG? I'm wondering if it is possible that there is algae living in the pool preventing the SWG from staying ahead.

    A few thoughts based on how I keep my pools:
    -CYA is right about where I try to keep mine, but you might find that if you bump it up, your FC will maintain a higher level as well. I would save this as a last resort.
    -If the salt is dead on, and the CYA is high enough, I would inspect the cell. If there is scale building up on the cell, it will reduce the efficiency at which it produces. You might need to acid dip the cell.
    -If all of these other things check out, I would suspect algae. It might be worth it to bring up the FC for a day or two, and see what happens. Maybe do a Chlorine drop test overnight. I have seen many pools where they seem to struggle to keep up, but after a good dose of chlorine, I end up having to turn them down 10-20%

    Its entirely possible that the cell you have is marginal for the size pool you have, but from what you say, it falls within the range. You might not get it to run at 40% like I try to keep mine, but I would think it could run below 70%. When mine run at 40%, I'm typically maintaining FC of 7-10, sometimes higher (it needs to be higher because it is only checked once per week).

    Out of curiosity, what kind of SWG are you running? We deal with the old EcoMatics and CompuPools quite a bit.
    TreeFiter

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    Re: Size of SWG vs size of pool

    well 40% for 12 hours would generate the same FC as 80% for 6 hours
    so you are all correct
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    Re: Size of SWG vs size of pool

    My thought exactly
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    Re: Size of SWG vs size of pool

    It does not work out to about 5 years. The number of years is completely disconnected from the numbers that actually matter. The numbers that matter are the cost of the SWG and the amount of chlorine it produces. That is what a SWG is, a device that produces chlorine. It will produce a fixed amount of chlorine over it's lifetime regardless of what size pool you install it in. It makes no difference to the SWG if it products that chlorine in 6 months, or over ten years. Baring accidents the total amount of chlorine is constant regardless of how quickly you have the SWG produce it.

    Smaller SWGs cost less, and produce less total chlorine over their lifetime. Larger SWGs cost more and produce more total chlorine over their lifetime. The total cost per unit chlorine works out to be similar in both cases. There is a small cost advantage to buying the larger SWG (for most brands, a few work out the other way), but it isn't a significant advantage in this context.

    Using the smaller SWG on a small pool for more years has exactly the same cost implications as using that same small SWG on a larger pool for a shorter number of years In both cases you are producing exactly the same total amount of chlorine and paying exactly the same amount of money for that chlorine. A SWG is a tradeoff in money spent up front for a fixed amount of chlorine produced over the lifetime of the SWG, vs paying for some other form of chlorine over time. It makes no difference if you are getting that from a larger SWG or a smaller SWG or how many years they last for. What matters is how much you paid and how much chlorine you get out, which are both unrelated to the size of the pool you are running the SWG on.

    Undersizing is about having to manually add chlorine because the SWG is already at 100% and simply isn't producing enough chlorine. As long as the unit can keep up with the pool, even if that means running at 100%, then all is good. Obviously failing to keep up and having to add chlorine manually matters. It might not be a problem for everyone, but that is something you want to know about. Everything else is trivia, possibly interesting, often debated, but not really important.

    Yes, some brands are less expensive to use if you buy their larger SWG, but that remains true regardless of what size pool you use that larger SWG on. And for that matter, some brands get more expensive to use when you buy their larger model, and again that remains true regardless of what size pool you use those units on.
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    Size of SWG vs size of pool

    Quote Originally Posted by aussieta View Post
    well 40% for 12 hours would generate the same FC as 80% for 6 hours
    so you are all correct
    Exactly
    Percentage is not what the cell puts out.
    But the time it's on.
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    Re: Size of SWG vs size of pool

    Quote Originally Posted by MBG75 View Post
    Percentage is not what the cell puts out. But the time it's on.
    That actually varies by brand. A few brands modulate their chlorine production and remain "on" all of the time. Most brands work the way you describe, but not all. The way aussieta said it remains true regardless of brand.
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    Re: Size of SWG vs size of pool

    Quote Originally Posted by aussieta View Post
    well 40% for 12 hours would generate the same FC as 80% for 6 hours
    so you are all correct
    This is true, but at 80% you really don't have too many options when something happens. For example; you are maintaining your pool by running at 80% and then you have a pool party where the bathing load is increased significantly. You would be limited to only an additional 20% to try to accommodate the extra chlorine demand. If you are at 40% you have a lot more room to adjust. Of course you could always add chlorine to the pool, but that defeats the purpose of having a SWG.

    There is also the issue of consistency. If you run 40% for 12 hours, the FC levels will remain within a narrower range, whereas if you run at 80% for 6 hours, you will essentially be building FC levels to a high point, and then letting them fall back down to a minimum. So at 40% your FC might be as low as 3, but as high as 5, whereas at 80% it would be more like as low as 1 and as high as 7. In both cases it averages out to 4, but by running at a lower % for a longer period of time you maintain more consistent FC levels throughout the day.

    I really don't understand why there is even a debate here. There is a reason the manufacturers specify a pool size for a given system.
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    Re: Size of SWG vs size of pool

    Quote Originally Posted by JasonLion View Post
    It does not work out to about 5 years. The number of years is completely disconnected from the numbers that actually matter. The numbers that matter are the cost of the SWG and the amount of chlorine it produces. That is what a SWG is, a device that produces chlorine. It will produce a fixed amount of chlorine over it's lifetime regardless of what size pool you install it in. It makes no difference to the SWG if it products that chlorine in 6 months, or over ten years. Baring accidents the total amount of chlorine is constant regardless of how quickly you have the SWG produce it.

    Smaller SWGs cost less, and produce less total chlorine over their lifetime. Larger SWGs cost more and produce more total chlorine over their lifetime. The total cost per unit chlorine works out to be similar in both cases. There is a small cost advantage to buying the larger SWG (for most brands, a few work out the other way), but it isn't a significant advantage in this context.

    Using the smaller SWG on a small pool for more years has exactly the same cost implications as using that same small SWG on a larger pool for a shorter number of years In both cases you are producing exactly the same total amount of chlorine and paying exactly the same amount of money for that chlorine. A SWG is a tradeoff in money spent up front for a fixed amount of chlorine produced over the lifetime of the SWG, vs paying for some other form of chlorine over time. It makes no difference if you are getting that from a larger SWG or a smaller SWG or how many years they last for. What matters is how much you paid and how much chlorine you get out, which are both unrelated to the size of the pool you are running the SWG on.

    Undersizing is about having to manually add chlorine because the SWG is already at 100% and simply isn't producing enough chlorine. As long as the unit can keep up with the pool, even if that means running at 100%, then all is good. Obviously failing to keep up and having to add chlorine manually matters. It might not be a problem for everyone, but that is something you want to know about. Everything else is trivia, possibly interesting, often debated, but not really important.

    Yes, some brands are less expensive to use if you buy their larger SWG, but that remains true regardless of what size pool you use that larger SWG on. And for that matter, some brands get more expensive to use when you buy their larger model, and again that remains true regardless of what size pool you use those units on.
    Lets take a pool that uses $200 worth of chlorine tablets per season. We could fit a cell that would be considered too small for it, or one that is properly sized. With the undersized cell, FC levels can be maintained at high outputs like 80%. Whenever the chlorine demand increases, due to increased bather load, heavy rain, or anything else, the SWG isn't likely to be able to handle this on its own, and you will have to add more chlorine. Adding chlorine is going to add to the cost.

    In the other case, the properly sized unit is running around 40%, and the chlorine demand increases for the same reasons. You can still more than double your output if necessary in order to accommodate the higher chlorine demand. You are far less likely to need to add more chlorine.

    Yes you will pay more for the larger unit, but you will get more out of it. Even if the cost per unit of chlorine generated is identical between the two cells, an undersized cell is likely to cost more in the long run because it puts you in jeopardy of things like algae blooms that require extra care to correct.
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    Re: Size of SWG vs size of pool

    Except that I was specifically talking about a SWG that was large enough so that it never needed manual chlorine additions, and yet peaked near 100% on the hottest days of the year. Such a unit would run well over 40% through much of the summer, but never require manual chlorine additions. With such a unit, the risk of an algae bloom is exactly the same as it is with a much larger unit, coming entirely from the user setting the percentage wrong.

    No SWG should ever be used to fight algae without manual chlorine additions. My SWG is massively oversized, and even that isn't enough to maintain shock level, let alone get to shock level in the first place while fighting algae.
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