When you run a pool and spa retail store you see a lot of things when it comes to how people treat their personal body of water, and a lot of it will scare you! Yet I know it is not the fault of my customers for treating their hot tub water like a witches cauldron, its our fault for not educating them properly.
With all the latest in hot tub technology some new spa owners may think their spa will take care of itself and the water you add to it will stay forever clean. The problem is it just does not work that way.
No matter how much chlorine you put in or how good the spas filtration system is there is only a certain amount of time before your spa water starts to get pretty narly.
For the average spa that gets moderate use we recommend draining the water and re-filling with fresh water every 3-4 months. Most people who come into our store with water clarity or odor issues are usually right at that 3 month mark since the last time they refreshed the spa water.
Usually when I tell people this I get a nasty look as they know that means they will have to do a little bit of work, but if you have a submersible pump, it is not that bad.
With all the money you spend on high end spas with fancy ozone and UV sanitation systems, as well as the money you spend on your weekly chemicals you may be asking why the need to drain it?!
That all comes down to a little piece of chemistry called Total Dissolved Solids or TDS as we call it…’in the biz’.
From the wonderful world of wikipedia:
Total Dissolved Solids (often abbreviated TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid in: molecular, ionized or micro-granular (colloidal sol) suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a sieve the size of two micrometer.
Reference Link: Dissolved Solids Wiki
What ends up happening a small body of water like a hot tub over time, is all the chemicals ans bather waste that has been oxidized in the spa builds up and prevents our sanitizers from working properly. We usually see this happen right around the 2000 parts per million level.
Because TDS is usually made up of particles in the spa that have already been oxidized they are very small and can not be picked up by the filter, so they remain free floating. Because they remain free floating your chlorine and bromine particles go after them. So therefore the higher the TDS the more sanitizer you need to maintain a desired residual in your spa. Once we get 3-4 months down the road the tipping point has been reached and our chemicals as well as our ozone systems can no longer keep the hot tub adequately clean.
“Jay, I have a pool and I have never drained it due to high TDS? Wouldn’t it be the same?” -Joe Spa Customer
Well Joe spa customer, yes and no.Yes TDS does build up in a pool but due to its size not nearly as fast. Also with pools we see a lot more natural water turnover due to back washing, spalsh out, evaporation, and we drain a bit out when we close it for the winter.
There is also this little fact of chemistry from David Dickman in Pool and Spa News
We’ll call it 8. That is, every pound of material added to a 15,000 gallon pool will contribute about 8 parts per million.
How about our 450 gallon spa? Take 1,000,000, divide it by 3,753 (the weight of the water in the spa), and you get 266.45. We’ll settle on 266. So for every pound of stuff that you add to a 450 gallon spa, you will be increasing the TDS by 266 parts per million.
So 8 vs. 266 is a bit of a difference showing that TDS will rise faster in a small spa. Yes we do need to keep in mind we are only adding about 6-8oz of chemicals combined per week (if that) in a spa but over 3 months that adds up.
Now you know if you are having funky, smelly water problems in your hot tub and your water has reached the 3 month mark…you know what to do. If you are not sure, bring a sample down to your local pool store and ask them to test your TDS and see where you stand.
If there are any pool and spa professionals that would like to add to the science of Total Dissolved Solids I would love to hear your input in the comments below.