How Low pH & Alkalinity Ruin Heat Exchangers (and How to Fix It)
When customers tell us “my heater just failed out of nowhere,” nine times out of ten the autopsy points to water chemistry, not a bad part. Specifically, running the pool with low pH and low total alkalinity (TA) turns the water acidic. Acidic water is hungry—it dissolves metal, and your heater’s heat exchanger (the maze of metal tubes the water flows through) becomes lunch. That corrosion leads to leaks, stained water, and a heater that needs replacing long before its time.
Below we’ll explain exactly why this happens, how to test and correct your water, and which heater models use nickel-alloy (cupronickel) heat exchangers that better tolerate occasional chemistry slip-ups.
Pool Water Chemistry - What does it all mean?
First thing that we need to cover is what all the chemistry jargon means before we explain why low pH & low alkalinity eats up you heater.
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pH |
A measure of how acidic or basic the water is, on a scale from 0 (acidic) to 14 (basic). |
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Total Alkalinity (TA) |
The measure of dissolved carbonates and bicarbonates in the water that buffer pH (help prevent rapid changes). |
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Calcium Hardness (CH) |
The amount of dissolved calcium in the water. |
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Free Chlorine (FC) |
The active chlorine available to sanitize and kill bacteria. |
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Cyanuric Acid (CYA) |
Also called stabilizer or conditioner, it shields chlorine from UV sunlight degradation. |
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Langelier Saturation Index (LSI) |
A calculated value that determines if water is corrosive (negative) or scale-forming (positive). |
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Acidic Water |
Water with low pH (<7.2) or low alkalinity (<80 ppm). |
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Salt (NaCl) |
Used in saltwater pools, where the salt cell converts salt to chlorine through electrolysis. |
Why low pH & low alkalinity destroy heat exchangers.
Now that your head is about to explode with all this high school chemistry knowledge, let's get into why this matters.
Think of Total Alkalinity (TA) as the seatbelt for pH. If TA is low, pH can slam from “okay” to “heater-eating” fast.
- pH measures how acidic or basic the water is. The lower the pH, the more acidic and corrosive the water becomes.
- Total Alkalinity is your water’s buffering capacity—it resists swings in pH. When TA is low, pH can crash, sometimes overnight, especially in salt pools or after heavy rain and splash-out.
- Result: Acidic, poorly buffered water aggressively dissolves copper-based metals, thinning the heat-exchanger tubes, creating pinhole leaks, blue-green stains, and ultimately a ruined heater.
What you should do to stop your pool water from destroying your pool heater.
1. Test your pool water regularly.
Use a good drop-based kit (or a fresh-strip brand you trust) for pH and TA at least twice a week during the season. We carry a large selection of water testing products to choose from.
Shop for Water Testing Products
Log your numbers. Spikes and trends tell you more than one-off tests.
After heavy rain, big pool parties, or large top-offs, re-test the same day.
Salt pools: test weekly; SWGs can slowly nudge pH up, but if TA is too low the pH can still dive after rain/additions.
2. Target this water chemistry (day-to-day)
These are solid, industry-standard targets for protecting heaters and finishes:
- pH: 7.4–7.6 (acceptable 7.2–7.8)
- Total Alkalinity: 80–120 ppm (90–110 ppm is a sweet spot for most pools)
- Calcium Hardness: 200–400 ppm (higher for plaster pools in cold water)
- Free Chlorine: 1–3 ppm (with CYA 30–50 ppm outdoors)
- Salt (if SWG): Follow your cell’s range, typically 2800–3500 ppm
- LSI (Langelier Saturation Index): Aim roughly –0.3 to +0.3
3. Correct your pool's chemistry immediately
If your water is acidic and TA is low, raise Total Alkalinity (TA) first, then fine-tune pH, then stabilize. To avoid “see-saw dosing.” Make one adjustment, circulate, then re-test before the next move.
4. Use these Heater-saving habits, through-out the pool season.
- Never run the heater with off-balance water. Test first when opening or after big weather events.
- Ensure good flow (clean baskets/filters) so the heater doesn’t overheat locally.
- Salt pools: Keep salt, pH, and TA in range; low pH + salt accelerates corrosion.
- Winterize correctly—trapped acidic water can quietly chew the exchanger all off-season.
5. Look for signs your exchanger has been attacked
- Blue/green staining at returns or in the pump basket
- Heater tripping on HLS/Hi-Limit or showing SFS/AFS errors (after leaks start)
- Weeping seams or that “penny” smell from the equipment pad
- Sudden water loss when the heater runs
If you see those, test pH/TA immediately, then shut the heater down until chemistry is corrected.
6. Consider having these essentials on hand to help fix the issue
- Drop-based pH/TA test kit (or high-quality strips), plus CYA & Calcium tests
- Alkalinity Increaser (sodium bicarbonate)
- pH Increaser (soda ash) and pH Reducer (muriatic or dry acid)
- Scale/metal control (optional helper for tough source water)
7. The best offence is the best defense, consider cupronickel heat exchangers.
Some manufacturers build heaters (or “HD” variants) with cupronickel heat exchangers. The added nickel improves resistance to corrosion and erosion—especially helpful in salt pools and for folks who occasionally miss a week of testing. It’s not a free pass to ignore chemistry, but it’s an extra layer of protection.
Here are our top Pool Heaters that offer the extra protection using cupronickel heat exchanger.
Hayward — Universal H-Series
All current Universal H-Series gas models are marketed with cupronickel heat exchangers for added durability.
SHOP HAYWARD CUPRONICKEL POOL HEATERS
Pentair — MasterTemp (HD models)
MasterTemp comes in standard copper and Heavy-Duty (HD) cupronickel versions; the HD models use the cupronickel exchanger.
SHOP PENTAIR CUPRONICKEL POOL HEATERS
Sta-Rite — Max-E-Therm
Sister platform to MasterTemp: available with cupronickel exchangers on heavy-duty and commercial/ASME variants.
SHOP STA-RITE CUPRONICKEL POOL HEATERS
Jandy — JXi
JXi models are offered with copper or cupronickel heat exchanger options; the cupronickel SKUs are clearly labeled.
SHOP JANDY CUPRONICKEL POOL HEATERS
Raypak — AVIA HD / Digital Gas cupronickel variants
Raypak’s AVIA HD with NiTek uses an enhanced nickel-alloy exchanger; Raypak also sells Digital Gas models with cupronickel exchangers.
