Water Replacement Interval WITH Ozonator


Well-known member
Apr 28, 2012
Houston, TX
I've seen this formula for water replacement interval (WRI) in some old posts multiple times by chem geek:

WRI (days) = (2/9) x (Spa Size in Gallons) / (person-hours per day)

Each time he says this is for the dichlor-then-bleach method with NO ozonator. I never see where he or anyone else says what the formula is if you have an ozonator, and since I have one, I'd like to know! I'd assume the interval is a little longer? I've also seen Isaac-1 saying he just does it every four months, which is probably a good rule of thumb. I'm on my first fill of a new hot tub right now, and it's not time to replace it yet, but I was just curious.

Also, out of curiosity, has chem geek retired from the forums? I don't see his name pop up much anymore.



TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
You're welcome to contact Richard anytime through his external email account. I believe the link to it is in his profile page. He generally answers emails regarding pool/spa questions and has always been helpful in that way.

As to your main question the answer is hard to calculate. Ozonators, when they are functioning, can cut the amount of FC needed to oxidize bather waste by as much as 50%. The problem is there's no way to really know for sure how well your ozone unit is running or if it is producing any ozone at all.

So you could probably double the water replacement interval based on ozone BUT ONLY if you assume that your ozone system is working at peak efficiency. That would be your max or upper limit.

If it were me, I would just replace the water as Isaac-1 has suggested as the cost of water for the hot tub is probably the least expense associated with running it.

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