Constant pH rise in older plaster pool

[BasicProblems]

I have a 3000 gallon plaster pool with a SWCG and a bluestone waterline tile (sealed) that has now been in service for 15 months. I was not too surprised that the pH would rise quickly during the time the the plaster is curing, but nothing has changed after 18 months.

I've recently tried going with a higher CYA (80 instead of 50) and managed to get the SWCG down to 6%. at this setting, the pH will rise from 7.0 to 8.0 in 2.5 days. Even with the SWCG off and using tricolor (because it will not raise the pH) for sanitizer, the pH will rise from 7.0 to 8.0 in 4-5 days. i've considered adding borax, which might help a little bit, but I really want to figure out what the heck is causing the ph to rise.

This link has a spreadsheet of what has been going on with the pool chemistry over the past few weeks Pool chemistry - Google Sheets. Like I stated, this problem has been going on since the pool was put into service, but I'm finally trying to get some answers as the plaster should have cured by now. Any help is greatly appreciated.

 

[Texas Splash]

Hello and welcome to TFP! A few things can effect a raise in pH, and one of them is TA. It would be interesting to know your current TA . If it's at 100 or more, you might consider lowering the TA down to about 70-80 and see if that helps maintain control better. Pool School - Lower Total Alkalinity. Also, if you have any fountains or spillways that cause aeration, that will let the pH rise as well.

Nice to have you with us.

 

[pooldv]

Welcome to TFP!

What is your TA? Sorry, I won't click in to your external link.
Do you run any bubblers or waterfalls?

 

[BasicProblems]

Welcome to TFP!

What is your TA? Sorry, I won't click in to your external link.
Do you run any bubblers or waterfalls?

The TA typically runs about 60. The pool does have a spillover from the hot tub. The design is an 8x20 rectangle with a spillover separating the hot tub from the pool. The water level in the hot tub is about 1/2 inch higher than the pool when full.

 

[pooldv]

60 is pretty good. You could drop it to 50, but any lower usually results in unstable PH. Cut the runtime for the spillover down to 30 minutes or so, that will help.

You can also add borates, that will help to stabilize your PH. More here on that, Borates - Why and How

 

[Meadow]

OP, what’s the PH on your fill water and how often you fill?

I have a 15 yr old 20k gals plaster pool with 2 months old city water. I have been adding about 18 oz of MA every other day but that is when my TA was at 90. After some serious aeration my TA is now down to 50. Yet, I have to add 18 oz of MA twice a week or whenever I topped off the pool.

Socal weather is dropping my water level by about 2 inches within 3 days due to evaporation. Made me think the fill water is a major factor in battling PH rise, no?