5.1 The Four Balance Factors
Key Takeaways
- The 2025 AFO Exam Candidate Handbook lists four parameters used to calculate pool-water balance: pH, total alkalinity, calcium hardness, and temperature.
- TDS is a common fifth correction in public Langelier-type indexes; it is not taught here as an unpublished extra AFO factor table.
- High pH, TA, calcium hardness, or temperature push water toward calcium carbonate scale; low values push toward aggressive, etching, corrosive water.
- Typical commercial operator targets — always subordinated to the AHJ — are pH 7.2–7.8, TA often 80–120 ppm, CH often 200–400 ppm on plaster (vinyl often run lower), and pool water often 78–82°F versus much hotter spas.
- Handbook-style study pool (360,000 gal, TA 40 ppm, CH 200 ppm, 80°F, pH 7.8): diagnose aggressive/poorly buffered water before any CSI number — TA 40 is the red flag even when pH looks acceptable.
Balanced pool water is not a stack of "in-range" stickers. It is water sitting near equilibrium with calcium carbonate: not hungry enough to dissolve plaster, grout, and metal, and not loaded enough to drop crust on tile lines, sand, and heat exchangers. The 2025 AFO Exam Candidate Handbook lists four parameters used to calculate that balance: pH, total alkalinity, calcium hardness, and temperature. Operators also meet total dissolved solids (TDS) as a common fifth correction in Langelier-type indexes. This independent study chapter treats TDS that way — a public-chemistry correction — and does not invent an unpublished extra AFO factor list.
You already met pH as the disinfectant work-value switch and total alkalinity as the bicarbonate pH buffer. Those acid/base jobs stay in the previous chapter. This section owns saturation. The same two numbers, plus calcium and temperature, are levers on a see-saw. Raise any of the four and the water is more likely to deposit calcium carbonate. Lower any of the four and the water is more likely to dissolve calcium carbonate and attack finishes and metals.
What "balanced" actually means on the deck
Calcium carbonate (CaCO3) is the mineral in plaster, grout, and the scale that coats heaters. Water either:
- Dissolves CaCO3 — aggressive / corrosive / etching water
- Holds CaCO3 near equilibrium — balanced water
- Drops CaCO3 as solids — scaling water
Sanitizer residual, combined chlorine, and filter pressure are separate stories. A pool can have textbook free chlorine and still etch a basin if the saturation levers are low. A pool can be crystal-clear at 9 a.m. and drop a white blizzard on the tile line by afternoon if the levers are high and the heater is running. Balance is the mineral story.
The four handbook factors all push that mineral story in the same qualitative direction:
- Higher pH → more scaling tendency
- Higher total alkalinity → more scaling tendency
- Higher calcium hardness → more scaling tendency
- Higher temperature → more scaling tendency
Flip any factor down and the water becomes more aggressive. That is the entire see-saw. CSI arithmetic in the next section only quantifies what you should already be able to read from which factor is wildly out of band.
The four handbook factors
pH — the fast saturation knob
pH is the only factor that enters a saturation index as the test value itself. The other three become "factors" on a slide rule. High pH makes calcium carbonate less soluble, so heaters, tile grout, and (where used) salt cells collect white or tan crust. Low pH makes the water hungry: plaster turns sandy, grout recedes, copper goes teal, iron goes brown, and bathers complain of sting.
A reading inside a comfort band does not prove balance. pH 7.8 with collapsed alkalinity can still be aggressive once calcium and temperature are in the picture. pH 7.4 with very high calcium in a hot spa can still scale. Common commercial pool targets sit near 7.2–7.8. The 2024 Model Aquatic Health Code (5th ed.) and CDC Healthy Swimming materials discuss a 7.0–7.8 public-health style band. Always subordinate both teaching bands to the authority having jurisdiction (AHJ) and the product labels in the pump room.
Do not re-litigate hypochlorous acid fractions here. Remember only the operator collision: the pH that is kind to chlorine work value is also a saturation lever. You cannot "fix CSI" by ignoring bather comfort, and you cannot "fix chlorine" by parking pH at 8.2.
Total alkalinity — buffer and saturation contributor
Total alkalinity (TA), reported as ppm as calcium carbonate, is mostly the bicarbonate system. In the acid/base chapter it was the pH cushion. Here it is also a saturation term. More bicarbonate raises the alkalinity contribution and pushes toward scale. Very low TA drops that contribution and leaves water both aggressive and unstable.
A common commercial working band is 80–120 ppm. Many codes allow a wider window, often discussed around 60–180 ppm depending on sanitizer. Treat 80–120 as a typical operator target, then follow the AHJ.
A TA of 40 ppm is a red flag even when pH looks fine. The water has little carbonate alkalinity to contribute to saturation, and pH will bounce with every acid, carbon dioxide, or hypochlorite dose. That qualitative diagnosis comes before anyone asks you for a Calcium Saturation Index number. Raising TA uses sodium bicarbonate. Soda ash is the wrong first lever when the emergency is TA 40 and pH is already 7.8.
Calcium hardness — the finish-dependent lever
Calcium hardness (CH) is dissolved calcium, also reported as ppm as CaCO3. It is not "total hardness" from a well-water iron strip, and it is not TDS. High CH pushes toward scale. Low CH pushes toward etching, especially on plaster, grout, and grout-adjacent tile.
Typical commercial plaster / gunite / masonry pools are often run near 200–400 ppm, because the finish itself is a calcium matrix that sheds and takes calcium from the water. Vinyl liners are not calcium finishes. Many operators run vinyl — and often fiberglass — at a lower CH so the water is not forced to carry masonry-level calcium that has nowhere to go except the heater and the waterline. Copying a gunite recipe onto a vinyl tank is a common scale complaint. Copying a vinyl-low-CH recipe onto new plaster is a common etching complaint. Follow the basin manufacturer and the AHJ.
Source water sets the starting point:
- Soft surface water may arrive well below 150 ppm CH and need calcium chloride.
- Hard well water may arrive above 400 ppm; dilution, not a calcium drum, becomes the job.
- There is no routine "CH-down" chemical analogous to bicarbonate. Drain-and-fill (and sometimes sequestering as a stain/scale management aid — not a substitute for balance) is the practical path.
Temperature — operational, not a drum of chemical
Temperature belongs in the four-factor list because calcium carbonate is less soluble in hotter water. A typical community pool sits near 78–82°F. Spas, therapy vessels, and some lesson tanks run much hotter, often near 100–104°F. The same pH, TA, and CH that look quiet in an 80°F lap pool can scale a spa. Heat-exchanger surfaces are hotter than bulk water, so positive saturation shows up first as heater efficiency loss and clogged orifices. You do not dose temperature. You set the heater, manage bather comfort, and remember that a temperature rise is a saturation rise.
Cold water at the same chemistry is more aggressive. That rarely stars as the only complaint on a heated commercial deck, but it matters for unheated outdoor tanks in shoulder season and for any vessel whose heater is off while the other three factors sit low.
Typical commercial operating bands — always under the AHJ
Use these as operator targets, then let the posted health code win.
| Factor | Common commercial operator target | Notes |
|---|---|---|
| pH | 7.2–7.8 | Public-health/code-style bands may run 7.0–7.8 (2024 MAHC / CDC Healthy Swimming). Follow the AHJ. |
| Total alkalinity | Often 80–120 ppm as CaCO3 | Wider 60–180 ppm style windows exist depending on sanitizer and code. |
| Calcium hardness | Often 200–400 ppm on plaster | Vinyl often run lower; fiberglass is not a plaster matrix — do not copy gunite CH onto vinyl. |
| Temperature | Pools often 78–82°F | Spas much hotter (commonly near 100–104°F), which pushes saturation toward scale. |
| TDS (common fifth correction) | Watch upward drift | High TDS is a public Langelier-style correction, not an unpublished AFO factor table. |
None of those cells replace the health department. If the AHJ posts different numbers, those numbers govern the deck.
Worked qualitative scenario: 360,000-gallon study pool
The handbook study pool is 360,000 gallons. Today's tests:
- Total alkalinity 40 ppm
- Calcium hardness 200 ppm
- Temperature 80°F
- pH 7.8
Do not jump to a slide-rule number yet. Read the picture.
Volume is large, so any chemical correction is a serious quantity — Pool Size Factor math already lived in the treatment-dosing section. Balance diagnosis comes first.
- pH 7.8 sits at the top of the common 7.2–7.8 comfort band. It is not automatically "high and scaling."
- CH 200 ppm sits at the low end of a typical plaster 200–400 ppm band. For vinyl it might even look comfortable. It is not the loudest alarm.
- 80°F is a normal pool, not a spa. Temperature is not the villain today.
- TA 40 ppm is far below the 80–120 ppm working band. That is the red flag.
Qualitative diagnosis: aggressive / poorly buffered water — hungry for calcium carbonate — even though pH looks acceptable. Expect pH bounce, elevated etching risk on plaster and grout, and a saturation index that will come in negative or barely inside a typical window once you run the slide rule next. The first chemical conversation is sodium bicarbonate to restore TA, then a retest of pH, then a decision about calcium chloride if CH still sits low for that finish.
Do not add soda ash first just because someone wants "pH insurance" at 7.8. Soda ash is the pH-up chemical. With TA at 40 and pH already at the top of the comfort band, soda ash treats the wrong lever and can drive pH out of the top while the real hole — alkalinity — is still empty.
If a coworker argues "pH is fine, leave it," they are reading one factor. Balance is four factors, plus an optional TDS correction. The study-pool TA of 40 ppm is why alkalinity keeps showing up in handbook-style chemistry questions: a pretty pH can hide aggressive water.
Field symptoms: too low versus too high
Operators diagnose with eyes and with tests. Use both. Cloudy water is not automatically a CSI problem — combined chlorine, poor filtration, and air leaks cloud water too — but scale-cloud usually arrives with a positive index, a rough waterline, and a heater that used to make temperature easily.
| Factor | Too low — field symptoms | Too high — field symptoms |
|---|---|---|
| pH | Etched plaster, grout loss, corroding ladders and light rings, metal staining (copper teal, iron brown), eye/skin sting | Crust on tile lines, rough ladders, cloudy water, reduced heater efficiency, salt-cell scale where used |
| Total alkalinity | Wild pH swings after a single acid or chlorine dose; etching that "shouldn't" happen at a mid pH | pH stuck high; heavy acid demand with little lasting pH drop; scale as stuck-high pH and high TA feed saturation |
| Calcium hardness | Sandy plaster, pitted grout, aggressive water even when pH is in band | White scale on tiles, in pump baskets, on heater tubes; cloudy water filtration struggles to clear |
| Temperature | Cold, unheated water at low chemistry can etch — uncommon as a sole heated-deck complaint | Spa scale, heater scale, clogged feeder orifices, flow restriction; hot skins scale first |
Metal staining deserves a special note. Aggressive water puts metals into solution (heater tubes, heat exchangers, copper piping, ladders). Later, when pH or saturation rises, those metals plate out as stains. The stain is the crime scene; the low-saturation period was the burglary. Logging CH, TA, pH, and temperature together is how you stop arguing about "mystery stains" that are actually a balance history.
Order of operations on the deck
- Test with a rinsed sample from the proper depth and location.
- Identify which factor is wildly out of its operating band. In the study pool, that is TA 40 ppm.
- Move that factor with the matching action: bicarbonate for TA, calcium chloride for CH, soda ash for pH-up when TA is already usable, acid or CO2 for pH-down. Temperature is a heater/operations choice.
- Circulate and retest. A 360,000-gallon vessel does not finish mixing because you dumped a bucket at the deep end and waited four minutes.
- Only then interpret CSI — next section — to see whether the four factors now sit near equilibrium.
Chasing a saturation index while TA is 40 ppm is like balancing a see-saw with one rider missing. Put the missing rider on the board first. The next section turns this qualitative see-saw into the slide-rule number the open-book exam lets you bring.
The 2025 AFO Exam Candidate Handbook lists four parameters used to calculate pool-water balance. Which set is that list?
A 360,000-gallon handbook-style study pool tests TA 40 ppm, CH 200 ppm, 80°F, and pH 7.8. What is the correct qualitative diagnosis before any CSI arithmetic?
How should calcium hardness targets differ among commercial finishes, always under the AHJ?
Which field-symptom pairing matches the saturation see-saw?