pH keeps returning to 8+

[pHmadness]

Hi all,
I'm a long time lurker who has learned a lot from these forums. I've been in the pool business over a decade, but I've got a problem that has me stumped.

I have a customer whose pH is consistently around 8.2 or 8.4 every week. It's a salt pool, gunite finish, with a spill over spa. Tuesday the pH was 8.4, TA was 100.

As an experiment I adjusted the pH to 7.4 and then shut the system down overnight. Twenty four hours later, the pool pH was unchanged, but the spa had risen to 7.8. I put a chlorine tablet in the pool, disabled the salt system, and 48 hours later pH was 8.0

My two competing theories are that the spa finish is improperly cured. Or that aeration from the spill over spa is the problem. Or a combination of the two?

This customer is very attentive to his pool and wants a solution. I've had this problem with a handful of other pools in the area, but most people don't care so long as it's safe to swim. He is having significant scaling issues, and I'm trying to use this opportunity to find a long term solution to this fairly common issue.

I'm thinking a borax product may be an option, but I'm open to all suggestions. Any help at all is greatly appreciated.

 

[mknauss]

Welcome to the forum!
The elevated TA of 100 with aeration is forcing the pH higher. If the pool has automation program a Spillover schedule to run the spa spillover for about 30 minutes twice a day. TA of fill water may also be an issue.
Scale implies the water chemistry is not being properly managed. Use CSI in Poolmath. I suspect a TA of 60 will make a big difference. But if this pool is chlorinated with trichlor, that is difficult to maintain.
I suggest you read ABC's of Pool Water Chemistry

 

[duraleigh]

There is a natural tendency for pH rise in most all pools......some more than others.

As mknauss says, lower the TA to 60 ppm and minimize the spillover (aeration of any kind) and you will likely minimize the pH increase.

That said, it appears there are some pools that simply require acid frequently to keep pH down. How old is the plaster?