Tankless water heaters are efficient, space-saving, and energy-friendly. But if you live in an area with hard water, mineral buildup (scale) can quickly reduce their performance and shorten their lifespan. That’s why pairing your tankless system with the right water softener or conditioner is a smart investment.
In this guide, we’ll compare the top water softeners for tankless water heaters, break down the science behind why they matter, and highlight both expert insights and real-world test results from leading reviewers.
best water softener for tankless water heater
Why Tankless Water Heaters Need a Water Softener
- Hard water problems: Minerals like calcium and magnesium form scale inside the heating elements.
- Efficiency loss: Even a thin layer of scale can reduce efficiency by 10–30%.
- Lifespan: Without protection, repairs and replacements come sooner (sometimes within 5–7 years). With soft water, lifespans can extend 15–20 years.
- Manufacturer recommendation: Most brands require softeners in hard water regions to keep warranties valid.
Water Softener Options: Salt-Based vs. Salt-Free
| Feature | Salt-Based Water Softeners | Salt-Free Conditioners |
|---|---|---|
| Method | Ion exchange (removes hardness minerals) | Crystallization (alters minerals, prevents sticking) |
| Effectiveness | Best for very hard water (0 GPG after treatment) | Helps reduce scaling, but doesn’t truly soften |
| Taste | May add a slight salty taste (filter fixes this) | No change in taste |
| Maintenance | Requires salt refills (monthly) | Only filter changes (yearly) |
| Appliance Protection | Excellent | Good, but not equal to soft water |
Best Water Softeners for Tankless Water Heaters (Compared)
1. Whirlpool WHES40E 40,000 Grain Water Softener
- Best for most households (2–5 people)
- What reviewers found (BHG test):
- Water hardness dropped to 0 GPG after installation
- Clothes, skin, and hair noticeably softer
- Easy-to-use digital controls & automatic regeneration
- Specs: 40,000 grain | NSF Certified | Size: 15 x 48 x 20.5 in
- Pros:
- High efficiency
- Affordable & widely available
- Certified by NSF
- Cons:
- Slight salty taste in drinking water (needs filter)
👉 Best Pick if you want guaranteed protection and soft water feel.
2. Aquasure Serene Whole House Salt-Free Conditioner
- Best for low-maintenance & eco-friendly users
- What testers noticed (BHG test):
- Less scale buildup on faucets & coffee makers
- Easier cleaning in tubs and showers
- Softer hair, but soap still didn’t lather like soft water
- Specs: 10 x 59.5 x 10 in | Filter replacement every 9–12 months
- Pros:
- No salt, no electricity
- Reduces visible scaling
- Filters chlorine, rust, pesticides
- Cons:
- Doesn’t actually soften water
- Not effective in very hard water (>30 GPG)
👉 Best Pick if you want minimal maintenance and don’t mind keeping some hardness.
3. Aquasure Harmony Series 48,000 Grain Softener (Honorable Mention)
- Larger capacity, great for big families.
- Downsides: more complicated installation, shipping issues reported.
Key Factors When Choosing a Water Softener for Tankless Heaters
- Water Hardness in Your Area
- Measure using test strips or check your city’s water report.
- If hardness is high (15+ GPG), choose salt-based.
- Household Size & Usage
- Small households (1–3 people): 32,000–40,000 grain models.
- Larger families: 48,000+ grain capacity.
- Certifications
- Look for NSF, WQA, or IAPMO approval (guarantees real performance).
- Maintenance Needs
- Willing to refill salt? → Go with ion exchange.
- Want low effort? → Salt-free conditioner.
Maintenance Tips for Long-Lasting Protection
- Check salt levels monthly (for salt-based systems).
- Clean brine tank yearly.
- Change filters every 9–12 months (salt-free systems).
- Annual professional check-up helps prevent hidden issues.
Final Verdict: Best Choice for Tankless Water Heaters
- For complete scale protection & true soft water, the Whirlpool WHES40E is the best balance of price, capacity, and proven performance.
- For those who want a salt-free, low-maintenance option, the Aquasure Serene conditioner is a solid pick, though it won’t fully soften your water.
If you want your tankless heater to last 15+ years while staying efficient, a water softener isn’t just an option—it’s a must-have investment.
