I started on BBB 4 months ago with our new hot tub. It worked very well for us. Low maintenance, low cost, great water results. After we got the hang of it we only had to test once or twice per week. Here is where we typically kept the water:

CH 190
TA 60
Ph 7.6
CYA ~20
Borates 40
FC 2 - 6
CC .5 - 1

We would add 5 oz of regular clorox after each person/hour of soaking. We occasionally threw in 5 tsp of MPS if we had a big load of kids in the tub just to give the chlorine some help.

The water always looked, smelled and felt perfect.

In the last two months we always seemed to have a lingering .5 - 1 CC level. So yesterday, I did the recommended 100ppm decontamination procedure and drained and refilled the tub. That seemed to do the trick as I shocked it to 10ppm last night and awoke to 5.8FC and only .2CC. I am finishing the balancing today and looking forward to another 4 - 6 months of troublefree-ness. Here are the measurements:

FC 5.8
CC <.2
TA 65
CH 130
pH 7.6
CYA ~10 (estimated from dichlor addition)
Borates 0

I am adding Borax and Dry Acid to get the Borates up to 40ppm to lock it all in.

Thanks to TFP and all of you for all the good advice!

4 months without draining seems pretty good to me... that's only 3 times a year with what sounds like quite a bit of use. In my portable spa days, I would drain that often or more using a Di-Chlor only regimen... low usage, and the water was not very healthy during that last month. Congratulations on mastering the tub!

We probably use it 8 person hours per week. We don't shower before using it. If it's only me at night, I go "al fresco" but otherwise we wear bathing suits but we don't wash them after every soak. As we got near the 3 month mark, we had a little bit of foaming when the jets were on full and the air was on, but otherwise the foam cleared quickly. I have it set to filter 4 hours per day -- 2 hours in the am and 2 hours in the evening.

So 8 person hours a week in a 355 gallon tub is a fairly high bather load. The traditional formula for water replacement interval (WRI) is as follows:

WRI (days) = (1/3) x (Spa Volume in Gallons) / (Number of Persons per Day)

The soak time is not specified but is probably assumed to be 20 minutes (certainly not more than 30 minutes). So in your case you have around 69 person-minutes per day or the equivalent of 3.4 persons per day soaking 20 minutes each. The above formula gives a WRI of 35 days. Using Dichlor-then-bleach, one usually gets double that time, so around 70 days. You're going for 4 months is truly exceptional -- probably a combination of your being more clean, possibly sweating less, and pushing the limits further than most in terms of time before water change.

Wow. Great formula. Thanks.

The first fill was November 1 and I drained it yesterday so that's 112 days. I maybe overestimating the person/hours per week. I also erred on the side of adding a little more Clorox than a little less after each soak. If I thought I was in it for 45 minutes I'd round up to an hour. And, I was definitely dealing with a lingering CC level of around .5 to 1 toward the end.

I was hoping I could make this a 6 month cycle but it sounds like 3 to 4 months is more likely.

Thanks again.