Evan (waterbear),

Well every pool is different, but a cover should have really kept your pH much more stable. My own pool that is usually covered hardly changes pH at all now that I only use liquid chlorine, though it obviously dropped in pH and needed regular base addition when I used to use Tri-Chlor tablets.

Most of the users that report a sudden stop in their pH rise have reported using a pool cover at the time of this "freeze" in pH, so I don't know why you don't see the same thing. The borates should slow down the pH rise, but when you do eventually need to add acid, you'll have to add as much as you did before because the borates will make it just as hard to lower the pH (by adding acid) as they make it hard for it to rise. Of course, report back when this occurs and prove me wrong!

If you find that your rise in pH over time is not accompanied by a slow drop in TA, then there is some source of pure base in your system. I forgot -- is your pool plaster/gunite and when was it last redone? The curing of the cement in plaster/gunite will raise the pH and will also raise the CH though this may not be as measurable as pH. A pool cover won't do anything for this source of pH rise. Also, if you add any chemical to your pool during winter, such as a polyquat algaecide, then perhaps this is basic, though I do not know (I thought polyquat was relatively neutral).

Richard