Every pool owner has fought pH. Add acid, it comes down, three days later it is back up. Add soda ash, it comes up, a week later it has sunk again. The number that will not hold is a symptom, and the cause is almost always alkalinity or something in the pool that is pushing steadily in one direction. Here is how the two are related and how to make both stay where they belong.
What alkalinity does
Total alkalinity is a measure of the water's ability to resist changes in pH. It is the buffer. With alkalinity in range, small additions of acid or base move pH a little and it settles. With alkalinity too low, pH swings wildly on every addition and every rain. With alkalinity too high, pH drifts upward and resists coming down, and scale forms on tile and equipment. Getting alkalinity right is what makes pH manageable, which is why it is one of the five numbers that matter.
What pushes pH up
- Aeration. Waterfalls, deck jets, bubblers, spillovers, fountains and even a lot of splashing drive carbon dioxide out of the water, which raises pH. A pool with a constantly running water feature will always trend up. Running features on a schedule instead of continuously helps.
- Salt chlorine generators. The cell produces chlorine and a byproduct that raises pH. Acid additions are a routine part of owning a salt pool.
- Liquid chlorine and cal-hypo. Both are alkaline, mildly.
- New plaster. A fresh plaster surface releases hydroxide for months as it cures. The first thirty days of new plaster is a daily pH battle by design.
- High alkalinity. Water with a high buffer naturally settles at a higher pH.
What pushes pH down
- Trichlor tablets. Strongly acidic. A pool on tabs drifts down continuously and eats alkalinity as it goes.
- Rain. Rainwater is slightly acidic and has no alkalinity, so a heavy Piedmont storm dilutes the buffer and lowers pH.
- Acid additions that were larger than needed.
- Low alkalinity, which lets any of the above swing pH further than it should.
Adjusting in the right order
Test both. If alkalinity is out of range, fix that first and let the pool circulate for a day. Then look at pH. Very often pH has moved toward range on its own once the buffer was corrected.
To raise alkalinity, use sodium bicarbonate, which raises alkalinity a lot and pH a little. To raise pH without moving alkalinity much, use soda ash, or simply aerate. To lower both, use muriatic acid or dry acid. To lower alkalinity while keeping pH, add acid in a concentrated spot to drive alkalinity down, then aerate to bring pH back; repeat until alkalinity is in range. This last one is the trick most people never learn.
Dose small, wait, retest
Overcorrection is the most common error. A large acid dose crashes pH, the owner adds base, pH overshoots, and the cycle continues with each step making scale or etching more likely. Dose for half the correction, run the pump for several hours, retest, then dose again. Pool volume matters; know it in gallons.
The chlorine connection
Chlorine is much more effective at the lower end of the pH range than the upper. A pool that drifts high is a pool where chlorine is working at a fraction of its strength, and that shows up as algae and high chlorine demand. Keeping pH in range is a sanitation issue, not just a comfort one. If the drift is coming from the chlorine source, the chlorine comparison shows which products push which way.
Fill water and well water
Municipal water in the Triad arrives with its own alkalinity and pH, and well water varies widely. A pool that gets topped off regularly is being nudged by the fill water every time. Test the fill water once so you know which way it pushes. Filling a pool from a well covers the extra considerations.
A stable pool
Alkalinity in range, features on a timer rather than running all day, a chlorine source whose pH effect is understood, and small corrections tested a day later. That is a pool where pH is a weekly check rather than a daily fight.
We balance every pool we build and start up across the Triad, including Lexington and Pleasant Garden. Get in touch if you are planning a new pool or renovating one that has never quite behaved.