Why Drain?

[Supermasterpig]

This may seem like a very silly question, but as I was relaxing in my hot tub tonight a thought occured. Why do we have to drain the water and refill the tub every 3-4 months or so? I mean of course its to keep the water fresh and clean. And yes I do drain it and clean it, I just want to know exactly what the major reasons why are. If your water is balanced shouldn't it be safe? Does water that sits in the tub for months become harder to balance? Does it ruin the plumbing? These are just things I don't know about and thought you guys would be able to tell me why. No matter your answers I will continue to drain clean and refill every couple months.

[Spanky]

Over time the TDS (Total Dissolved Solids) of the water will continue to rise. Bromine tablets, for example, contain binders which remain in the water after the tab dissolves. Other sources include oxidized wastes, salts and other impurities from routine chemical additions. Filters do a reasonable job of keeping the water clear but they are not 100%. There are claims that using a SWCG will extent the life span of the water, due to the fact that they require less chemical addition, but you still have a slow build up of oxidized wastes. Regular water changes are the best way to keep your tub running trouble free.

[chem geek]

For those who use Dichlor, there is a buildup of Cyanuric Acid (CYA) which makes chlorine less effective over time. This is usually the limiting factor for how long one can use the same water. For those using Dichlor-then-bleach, there is a faster chloride salt buildup, but salt is relatively innocuous so the main reason for a water change is the buildup of slow or difficult to oxidize organics. This latter effect occurs with a saltwater chlorine generator as well though the salt level is constant (usually 1500 or 2000 ppm) in this case.

There isn't a set time that the water is changed; it's a function of bather load. The standard Water Replacement Interval (WRI) formula is:

WRI (days) = (1/3) x (Spa Size in U.S. Gallons) / (# of Bathers per Day)

where the soak time is assumed to be around 20 minutes. So in person-hours it is:

WRI(days) = (1/9) x (Spa Size in U.S. Gallons) / (Person-Hours per Day)

With the Dichlor-then-bleach method, you can usually go around twice as long and the water is in better shape at the point you end up changing it (so if you really wanted to, you could go longer).

[spartan]

Hi, this is a great question as I have often wondered this myself.

I use the "bromine for beginners" spa method used on this forum which has worked really well over the last 18 months (thankyou). Over the last 7 months I have not emptied the spa and the water seems to stay as balanced as in the first 3-4 months and the only thing I have noticed is that I use maybe 5% more liquid chlorine (mine is 12.5%) which I thought may have also been as my chlorine is getting older. Our spa is used infrequently and I test at least one a week with a proper test kit.

My spa is 1100 litres (250 gallons) and is used approximately 1 hour per week in total by 1 person.

If I have read the previous post correctly then I should change my water about every 6 months.

Is that correct? Thanks for any assistance.

[chem geek]

If you use the spa infrequently then there won't be much buildup of organics and because you aren't using Dichlor then there won't be buildup of CYA. So there may be no reason for you to change your water even after 6 months. You could just do it once a year. The WRI for your spa is (1/9) x (250) / (1/7) = 194 days but with no CYA buildup figure double that or over one year. Note that if you are using bromine tabs that there can be a buildup of DMH from the tabs and that can lower the active level of bromine. Some commercial/public spa recommendations say to change the water when the DMH hits 200 ppm. If your background bromine usage per day is around 1 ppm, then the DMH would hit 200 ppm after around 500 ppm bromine were added so that's also over one year. So from all indications, including your own assessment of your spa's water quality, you can probably go a year between water changes, mostly due to your low bather load.