why change hot tub water?

[GuinnessPhish]

If the water remains clear, doesn't smell, and you're on top of monitoring and maintaining it by adding bleach to oxidize waste after use, is it really necessary to change the water every 3 - 4 months?


I changed the water at the beginning of August. It gets used by us pretty lightly, but fairly regularly....once or twice a week for about 15 - 30 minutes for one person usually, sometimes two.


The water looks and smells fine...so what is the reasoning behind changing it?

 

[linen]

What method of sanitation are you using? Assuming you are using chlorine in you tub, with some cya (20-30 ppm), chemgeek says this here:

As for how often to replace the water, it doesn't have to be after 3 months. When using Dichlor-then-bleach, you can usually go twice as long vs. using Dichlor-only and even then the water is in better shape when you do end up changing it. With Dichlor-only, you usually have to change the water after cumulatively around 39 person-hours (in a 350 gallon spa -- 28 person-hours in a 250 gallon spa) while with Dichlor-then-bleach you can usually go to cumulatively 80 person-hours or longer. So if one is not using the spa every day, one can usually go longer than 3 months between water changes.

He also says this here in response to a question of shocking a hot tub:

You can, of course, measure the Combined Chlorine (CC) level just before your soak to see that in fact the bather waste seems to be getting handled. You can also monitor the chlorine demand to see that it isn't increasing over time which would indicate a buildup of unoxidized organics. This does happen slowly for the slowest-to-oxidize organics, especially if you are not clean when getting into the spa, so you may see the daily chlorine loss with no bather load go from 25% to 50% over many months until your next water change. These aren't organics that would oxidize quickly even if you shocked so there's not much point in doing that. It's just easier to change the water. At least with Dichlor-then-bleach you can go about twice as long between water changes and the water quality is still better when you change it. This is because there is no buildup of CYA which reduces the active chlorine level.

My understanding (poor as it is) is the build up of these hard to oxidize organics are what end up using more chlorine as the water ages and drive the water change. At some point you are not able to maintain FC between chlorine additions (even if you add chlorine daily) and your FC level drops to zero between. With that said, I am using chlorine with ~30 ppm cya and have been able to go a full winter without changing my water (and in the process of doing it again this year). The load is about 5 person hours/week and I have gone ~ 7 months. This works out to about 150 person hours. Near the end of that time, I am adding chlorine at least every two days, and sometimes every day. It is not very convenient to switch out water here during the winter.

 

[toddbailey]

I rarely dump a hot tub and only then to do a surface cleaning. During the summer months I tend to do a partial water change, about 25% once a month in order to keep skin oils down to a minimum.