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Water Change Frequency.


robquick

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Hi,

I use the Dichlor/bleach method and maintain the water on a frequent basis. The current water I have in the tub is crystal clear with no smell and is now six months old. The tub is 1500 litres and is used for the equivilant of 2-3 bather hours per week.

Should I change the water and if so, why ?

Thanks.

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1500 liters is around 400 gallons. If you have no ozonator, then 2-3 bather-hours per week would use around 12 to 18 ppm FC in the tub and I'll assume the soaks are nearly every day so that there isn't any additional dosing needed in between soaks. The standard water replacement interval (WRI) calculation used by the spa industry is the following:

WRI = (1/3) x (Spa Size in Gallons) / (# of persons per day)

where the soak time is presumed to be around 20 minutes. If I use person-hours and double the time for the Dichlor-then-bleach method I get:

WRI = (2/9) x (Spa Size in Gallons) / (person-hours per day) = (2/9) x (400) / (2 or 3 divided by 7) = 207 to 311 days = 7 to 10 months

So you're getting to 6 months without seeming to need to change the water sounds about right. You have built up at most 18 * 6 * (30/7) * 1.65 = 764 ppm salt which is not too much.

So there is no reason for you to change the water at this point, but you'll probably want to change it by the time you get to 1 year. Let us know how it goes so we can keep track.

Note that if you have an ozonator and use the spa every day or two, you may be able to go even longer because the amount of chlorine you use would be about half and so the salt buildup would be slower. The only question is on how much buildup there would be of unoxidized organics that both the ozonator and chlorine did not oxidize. Even so, I would think that changing the water once a year was not that much of a burden and there's nothing like starting out with fresh water.

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  • 2 weeks later...

I have a question on this topic. Is monitoring TDS also a good way to determine the time to replace water?

I have been operating off of the information that I found here (http://www.rhtubs.com/whentodrain.htm) for some time now.

It has been my method to drain and refill when my TDS meter reads between 1000-2000 ppm TDS.

We have a 1525 gal swim spa (with ozonator) which my wife uses, at most, every other day for about 20-30 minutes. When I run the numbers with the WRI formula I get:

WRI= (1/3)x(1525)/(0.5)= 1016.67days = 2.79 years!

I use the dichlor/bleach method. Water loss is replaced with water collected from dehumidifiers which is very low in TDS (essentially distilled water).

I actually did let it go for over 2 years one time and the final TDS was 1124ppm. The water was behaving very well but I had to believe that 2 years was excessive.

So, is TDS monitoring as described a reasonable alternative to the formula? Can I really go over 2 years without worry?

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Yes, you can likely go 2 years without worry -- the main reason being that the swim spa is significantly higher volume than a typical spa. And yes, the TDS can be used as a rough proxy since it will be a rough measure of the cumulative amount of chlorine you use since that is the source of the sodium chloride buildup. As to whether the water becomes more of a problem, that depends on the bather load so if you use the spa more frequently, then the TDS that correlates to chlorine usage also correlates to bather load since most of the chlorine is being used to oxidize the bather load.

If you were using the spa only once a week or less, then a good amount of the chlorine would be just in maintaining a background dose and would not be related to bather load. In that situation, the TDS will fool you and the water may be in better shape in spite of a high TDS. Nevertheless, you want to change the water when the salt level (TDS is mostly the salt level) gets high because of the higher conductivity that increases the rate of metal corrosion. Something on the order of 2000 or so would be high enough to change, though it really depends on the quality of the spa materials that are used.

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