Just curious. I know about the formula that calc's usage vs when to drain and re-fill but what actually happens to water quality if you don't fresh-fill your hot tub accordingly? If pools can go all season why not hot tubs - if balance and key numbers are maintained?
Barry

I've personally been able to keep the water in the hot tub a lot longer than the recommended refill. But I think I've read that the soap from bathing suits gets in the water and will start to cause a lot of suds over time, not sure of any others reasons except maybe certain ways of maintaining the chems might do better with periodic refills. I use BBB for the hot tub as well as the pool.

Thanks for the reply.........
So it may be a phosphate build-up issue in a relatively low volume container coupled with higher temps?

If you had a hot tub as large as a pool and with the same number of people in it then it probably would last all season.
The problem is the ratio of people to small amount of water and all the dead skin cells, sweat, urea, etc etc.
If you fitted a reverse osmosis filter maybe it would last a bit longer.

Hey there, Jen from DEL Ozone chirping in. On a personal note, not relating to ozone at all, instead of washing your bathing suits in soap, wash them in white vinegar. Clothes carry enough soap to wash 5 more loads. Vinegar is gentle on clothes and the environment.

Also, make sure your ozone generator is still working. Most spas are sold with ozone, but customers don't know to maintain their ozone generator which is responsible for sanitizing your water plus clarifying it, de-greasing it, and dramatically decreasing your chemicals for an over-all better spa experience.

Make your water live longer. Replacing your ozonator ensures your water lives healthier and longer.

Take care, and happy hot tubbing!

Jennifer Engelmeier
Manger, Aftermarket Spa Sales
800-676-1335 x232
www.delozonespa.com
jen@delozone.com

Just so everyone is on the same page here, the industry formula for when to replace spa water is:

Water Replacement Interval (WRI) in days = (1/3) x (Spa Volume in Gallons) / (# of Bathers)

The soak time that is assumed for each "bather" is probably around 20 minutes, maybe 30 minutes tops. The above formula seems to work reasonably well in spas using Dichlor-only with no ozonator. For spas being maintained using Dichlor-then-bleach, one typically gets twice the length of time between water changes and the water quality is better at the end of that time as well (so one could probably go longer if one wanted to).

As was noted in earlier posts, there is a buildup of unoxidized organics over time. A spa with a functioning ozonator might do better since ozone will oxidize some chemicals that chlorine won't. The ammonia and urea from your sweat and urine should get fully oxidized by chlorine, but other chemicals, especially fats and oils that don't have nitrogen sites, don't get oxidized readily by chlorine.

Don't forget the salt - chlorine adds salt to the water and it builds up....

-Chak

That's true, but it doesn't build up to unreasonable levels. If we assume there is no ozonator and using the Dichlor-then-bleach method, then doubling the WRI formula gives:

Water Replacement Interval (WRI) in days for Dichlor-then-Bleach = (2/3) x (Spa Volume in Gallons) / (# of Bathers)

and assuming 20 minute soaks then for a 350 gallon spa this gives a WRI of (77.8 / person-hours-per-day). The amount of chlorine needed to oxidize bather waste is around 7 ppm FC per person-hour in 350 gallons. So that implies a cumulative FC of 7*77.8 = 544 ppm which would result in an increase in salt level of around 900 ppm which is quite manageable.

So, bottom line, salt should not be an issue unless one is going well beyond the WRI formula.