Getting pH down in spa?

[Prodigal]

We recently moved into a house with a 375 gallon Sundance Cameo spa (I've never done any kind of pool maintenance before), and I'm having trouble getting the pH down. I just drained and refilled it, and took a sample to the local pool supply store to get a reading (I have the Taylor testing kit, and my results are very close to theirs). The results straight from the tap (not great):

TA	40
pH	8.7
FC	0.22
TC	1.58
CH	80
CYA	5
Iron	0
Copper	0.2
Phosphates	0
TDS	400

The pool store recommended getting the TA between 80-120 first, then getting the pH between 7.2-7.8. It took 3.5 oz of Alkalinity Up/sodium bicarbonate (spread over 30 minute intervals) to get the TA up to 90.

So I moved onto pH, but I just can't keep it down. The pool store thought 2.5oz of dry acid would be enough, and I tried the Pool Math calculator, which also said 2.5oz of dry acid in a 375 gallon spa would drop the pH by 1.1 to 7.6, which sounds perfect.

I added 2.5oz of dry acid over a few 30 minute intervals, but the pH was still "above 8" (couldn't get any more granular than that since the color matching in the Taylor kit only goes up to 8.0). So I just kept adding 1 oz every ~30 minutes, and after 9oz total, the pH was down to 7.2.

Unfortunately, that much dry acid totally tanked the TA: it was down to ~10 by the time I got the pH that low. I tried adding 2oz of Alkalinity Up, and the TA went up to 40 but the pH jumped all the way back to 8.0.

Does anyone have any advice for why the pH is so resistant to dry acid, or why the estimates I find aren't working for me? I understand that the lower the TA, the more volatile the pH, so I wonder if I need to wait longer between batches of dry acid, or if I need to move onto another chemical, or something else?

I tried to summarize above, but posted my log below (which may be way too much detail for this!)

Thanks in advance!

Date	TA	pH	Action
Day 1 7pm (#s from pool store)	40	8.7	3.5oz alk up @3 30 minute intervals
Day 2 7am	90	> 8	0.75oz dry acid
Day 2 7:30am	90	> 8	1oz dry acid
Day 2 8am	80	> 8	1oz dry acid
Day 2 9:15am	70	> 8	1oz dry acid
Day 2 9:50am	60	> 8	1oz dry acid
Day 2 10:50am	50	> 8	1oz dry acid
Day 2 11:30am	40	8	1oz dry acid
Day 2 3pm	30	8	1oz dry acid
Day 2 3:45pm	20	7.8	1oz dry acid
Day 2 5:15pm	10	7.2	1oz dry acid
Day 2 6pm	40	8

 

[Texas Splash]

Welcome to TFP! I'm puzzled as well, but before we sound the alarm for others, let's confirm some things:
- You just moved in, you drained and refilled, but did you do an Ahh-Some purge?
- Are the tap results above at the top from the store or your K-2006? If not your kit, please post your tap results.
- How old are your K-2006 reagents?

Let's start there and we'll see what else we may need to consider.

How do I use Chlorine in my Spa (or pool)?

If you have a question about any information within this Sticky, please start a new thread for yourself where you will receive very quick responses to your specific questions. Thank you! Introduction There are two main things to consider when maintaining your hot tub water, Water Balance and...

www.troublefreepool.com

 

[Prodigal]

Thanks for replying!

To answer your questions...

- You just moved in, you drained and refilled, but did you do an Ahh-Some purge?

Sorry, I did not. Just refilled and started testing. Have not heard of this product before...sounds highly recommended.

- Are the tap results above at the top from the store or your K-2006? If not your kit, please post your tap results.

That first section of results is from the pool store. Unfortunately, I just ran with what the pool store gave me and went straight into adding alkalinity up without doing my own test too.

- How old are your K-2006 reagents?

I couldn't find this info when I looked, but each bottle expires late in 2022.

Thanks again for taking a look at this! I'm puzzled by why what the math says should work seems so different from what's actually happening.

 

[jseyfert3]

One thing not outlined the best in the test kit instructions for Ta is you should add drops until the color stops changing, then don’t count that last drop that didn’t change the color anymore.

Since this test goes green to red, usually with a drop or two range of almost colorless, it’s easy to stop too early and thing your TA is lower than it actually is.

 

[RDspaguy]

Aeration will raise ph, so keep your air control valves on the top of the spa closed when not in use. You will still have some aeration from the ozone injection.
I was taught the same, alk 80-120 then go after ph, and gave that advice thousands of times. That works great in my area, since that's how it comes out of the hose. But since frequenting these forums I have heard many struggle with this. Now I say balance your ph and let your alk fall where it will.