Hot tub chemistry numbers

digger0713

Active member
Sep 8, 2015
29
Brownstown/Michigan
Hello everyone. I've been using this site for quite a few years with my pool and hot tub and love all the advice given. My question is about recommended levels for Bromine, PH, Alkalinity, and Hardness for hot tubs. It seems like everywhere you read on the internet and even on the spa chemical bottles, they seem to recommend 4-6 ppm bromine, 7.2 - 7.6 PH, 80-120 ppm alkalinity, and 150-300 ppm hardness. I've also seen alkalinity recommended from 100-150 ppm and hardness from 100-200. I know I've seen some recommendations on here to be different than these numbers as well. Why is there so much varying agreeance on what the proper numbers should be? I understand I can just look on this site and use those numbers and be fine, but you would think there would be a standard agreeance that should work across the board for everyone based on your hot tub type.
 

5tan

Active member
Jan 9, 2021
37
Canada
Pool Size
1643
Chlorine
Liquid Chlorine
I think you probably know the answers already since you have several years of reading about spas and hot tubs. I'm only 6 months into this game, and only use chlorine so can't speak about bromine, but I'll sum up what I've figured out so far:

There is no one exact, proper number for any of these measures. It's called an acceptable range, because anything within that range is acceptable. If you try to target one specific number for any of those readings, you are going to waste tons of time chasing something that is always changing, for zero noticeable benefits.

pH 7.2-7.8 is the common range because below 7.2 is harsh on your metal components, and above 7.8 promotes mineral scaling on your metal components. Anything in between is perfectly acceptable. While pH 7.6 is reportedly the most comfortable for human skin and eyes, there is no need to adjust pH unless it goes below 7.2 or above 7.8. Perfectly fine to be anywhere in between. It's easy for me to lock into 7.6 because I use 50ppm borates and keep my TA in the 30-60 range.

For most spa owners, it's easier to target 7.2, then with the eventual pH rise, add acid after exceeding 7.8, and then figuring out how often to add X amount of acid based on Y hours of the jets being on. Before I used borates and before keeping my TA below 60, I would go from the 7.4-7.6 range to the 8.0-8.2 range after a few hours of running the jets. Now I can keep my pH at 7.6 for weeks before it starts drifting upwards (but when it does, the pH rise is exponential, not linear, meaning it could take days to go from 7.6 to 7.8, but going from 7.8 to 8.2 can happen in hours). Adding a small scoop/teaspoon/tablespoon (3-6g) of dry acid after each soak is another way to dampen the pH rise. With zero borates and TA above 80, you'll be adding lots of acid because the pH will rise quickly after only a couple of hours of running the jets.

Chlorine is much more effective at lower pH than higher. I recall seeing a big difference in hypochlorous acid effectiveness where pH=7.2 is much better at sanitizing than pH=7.8.

TA 80-120 is common for pools and scenarios where plaster, grout, tile, concrete, etc are involved, either inside the spa, in the surrounding deck, or if a pool is attached. This affects your Calcium Saturation Index (CSI). For standalone acrylic spas, you can run a lower TA (closer to 50) in order to slow down the pH rise from aeration. Adding 50ppm borates greatly helps in slowing down the pH rise from aeration. Adding acid lowers TA, adding baking soda increases it. The more TA, the more your pH will rise after running the jets. I keep my TA in the 30-50 range.

CH is also a factor for CSI, same reasons as above. In a standalone acrylic spa, you should set your CH to 130-150 to reduce foam. I previous ran mine at 100-120, and noticed the foaming went down a lot after reaching the 130-150 range. You don't want to go higher than 150 in a standalone acrylic spa situation as to prevent scaling on your heater and metal components. However, if you have an attached pool or any grout/tile/stone/concrete nearby, you'll need to use a higher range so you don't damage those things. Never mix CH increaser in a bucket or jar, it is an exothermic reaction and gets very hot very quickly. Never add TA increaser (baking soda) within 24 hours of adding CH increaser - the 2 will bind together, precipitate out (cloud) into the water, and you'll have less TA and less CH than you calculated.

To calculate what you need, just use PoolMath and The Pool Calculator. If you have an attached pool, concrete, grout, tile, stone, etc, you need to pay attention to the CSI numbers and ensure it says "Balanced".
 

A.O.

Well-known member
Apr 12, 2016
310
Kershaw, South Carolina
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Looks like 5tan above pretty much nailed it. While I don't use borates I keep my TA right at that 50 mark and very easy to keep PH in that 7.2-7.8 range. My well water is pretty soft here but it has never caused me any grief and my hardness it 100 or below all the time.. pool and hot tub.
 

digger0713

Active member
Sep 8, 2015
29
Brownstown/Michigan
Thanks for the replies. That's what I'm talking about though. I've been keeping my hot tub alkalinity around 50 and it's been doing good keeping my PH under control from drifting, but on most other sites and even the chemical bottles or testing kits, they all tell you to target between 80-120. I've had good luck with that range for my pool, but my hot tub doesn't like those high numbers. Most people would probably scold you for keeping the alkalinity that low in a hot tub, but if it doesn't do any harm to you or the tub then it shouldn't really matter. I've at least been glad I found this site a few years ago because it has helped me so much with pool and hot tub issues. Never would have thought to keep my alkalinity down around 50 but it seems to be the magic number range. I'm due for a water changeout shortly anyways and I'm going to try a lower number for my calcium hardness. I've always been in the 150-200 range and I do deal with foaming issues after awhile. Sounds like keeping a lower calcium hardness number could help with that.
 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
4,969
NW Ohio
Most people would probably scold you for keeping the alkalinity that low in a hot tub
Why would you care? I understand what the TA level actually means chemically and it's relationship with pH. "Most people" know nothing beyond "number doesn't match chart = bad". Those aren't people I'm taking advice from.
 
Last edited:
  • Like
Reactions: jimmythegreek