Hard water is the main reason tankless water heaters lose performance: minerals coat the heat exchanger, flow drops and error codes appear. A scale inhibitor filter on the cold line slows that buildup, and a clean inlet screen keeps sediment out. These five filters and replacement parts are the best we found for protecting a tankless heater between yearly flushes.
We compared filter media, connection size, cartridge format and fit using manufacturer specifications and owner feedback. For flushing gear see the best tankless water heater accessories kit, or see the best tankless water heater easy maintenance guide; our Editorial Policy explains how we choose. These filters reduce scale buildup; they are not water softeners and do not purify drinking water. Follow the maker’s replacement schedule.
Top 3 Tankless Water Heater Filters
3M Aqua-Pure AP430SS Whole House Scale Inhibition Inline System
- Whole house
- Polyphosphate
- Inline
KleenWater KW2520SCALEX Tankless Hot Water Filter System
- See-through
- Bracket + wrench
- All brands
IPW Industries Heater Treater 20 5-Micron Sediment and Scale Filter
- 5 micron
- Scale
- Corrosion
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Tankless Water Heater Filters Compared
| # | Product | Key Features | |
|---|---|---|---|
| 1 |  3M Aqua-Pure AP430SS Whole House Scale Inhibition Inline System |
| Check Latest Price |
| 2 |  KleenWater KW2520SCALEX Tankless Hot Water Filter System |
| Check Latest Price |
| 3 |  IPW Industries Heater Treater 20 5-Micron Sediment and Scale Filter |
| Check Latest Price |
| 4 |  MEASUREMAN MD051 3/4″ NPT Whole House Scale Inhibitor Filter |
| Check Latest Price |
| 5 |  IGIUA H98-510-S Inlet Screen Filter for Rinnai Tankless (2-Pack) |
| Check Latest Price |
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1. 3M Aqua-Pure AP430SS Whole House Scale Inhibition Inline System
Best Overall
- Polyphosphates bind hardness minerals
- Helps reduce scale in heaters and pipes
- Inline whole house design
- Mounts inline on the cold supply
- Not a water softener
- Cartridges need replacing
- Type: scale inhibition system
- Model: AP430SS
- Cartridge: AP431
3M’s AP430SS is a common choice for protecting water heaters from scale.
Install it on the cold supply.
Who the 3M Aqua-Pure AP430SS fits best
Most hard-water homes.
Who should skip the 3M Aqua-Pure AP430SS
Soft-water homes.
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2. KleenWater KW2520SCALEX Tankless Hot Water Filter System
Best Clear Housing
- Transparent housing to see cartridge
- 2.5 x 20 in polyphosphate cartridge
- Bracket and wrench included
- Compatible with all brands
- Needs wall space
- Cartridge replacement
- Type: polyphosphate filter
- Cartridge: 2.5 x 20 in
- Model: KW2520SCALEX
KleenWater’s clear housing shows when media runs low.
It can protect the whole house.
Who the KleenWater KW2520SCALEX fits best
DIY-minded owners.
Who should skip the KleenWater KW2520SCALEX
Tight spaces.
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3. IPW Industries Heater Treater 20 5-Micron Sediment and Scale Filter
Best Sediment + Scale
- 5-micron sediment filtration
- Polyphosphate core for scale and corrosion
- Fits 20-inch housings
- Protects hot water systems
- Replacement cartridge only
- Housing sold separately
- Type: sediment + scale
- Rating: 5 micron
- Model: GSSR25-20005
IPW’s Heater Treater 20 combines sediment and scale protection.
It suits existing 20-inch housings.
Who the IPW Heater Treater 20 fits best
Well water homes.
Who should skip the IPW Heater Treater 20
No housing.
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4. MEASUREMAN MD051 3/4″ NPT Whole House Scale Inhibitor Filter
Best Whole House
- 3/4″ NPT whole house connection
- Salt-free polyphosphate media
- Protects pipes and fixtures
- No sodium added
- Newer brand
- Not a softener
- Type: scale inhibitor
- Connection: 3/4″ NPT
- Model: MD051
MEASUREMAN’s MD051 protects the whole plumbing system.
Install before the heater.
Who the MEASUREMAN MD051 fits best
Whole-home protection.
Who should skip the MEASUREMAN MD051
Point-of-use only.
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5. IGIUA H98-510-S Inlet Screen Filter for Rinnai Tankless (2-Pack)
Best Inlet Screen
- H98-510-S inlet screen assembly
- Made for Rinnai tankless heaters
- 2-pack for spares
- Simple swap during service
- Rinnai only
- Not a scale filter
- Type: inlet screen
- Fits: Rinnai
- Part: H98-510-S
IGIUA’s screens replace clogged Rinnai inlet filters.
Clean or swap yearly.
Who the IGIUA H98-510-S fits best
Rinnai owners.
Who should skip the IGIUA H98-510-S
Other brands.
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Filter Tips
- Install scale filters on the cold inlet.
- Replace cartridges on schedule.
- Still flush the heater yearly.
- Check local codes for filters on drinking water lines.
Frequently Asked Questions
Do tankless water heaters need a filter?
In hard water, a scale inhibitor helps reduce buildup.
How often to replace the cartridge?
Follow the maker’s schedule.
Does a filter replace flushing?
No, flush yearly.
Is a scale filter a softener?
No.
Final Verdict: Best Tankless Water Heater Replacement Filter
The 3M Aqua-Pure AP430SS is the best tankless water heater filter for most homes. The KleenWater KW2520SCALEX has the clearest housing, and the IPW Heater Treater 20 adds sediment filtration.
Best overall: 3M Aqua-Pure AP430SS Whole House Scale Inhibition Inline System Check Latest Price
Clear housing: KleenWater KW2520SCALEX Tankless Hot Water Filter System Check Latest Price
Sediment + scale: IPW Industries Heater Treater 20 5-Micron Sediment and Scale Filter Check Latest Price
A good filter makes every flush easier.
Looking for something else? Browse all of our Water Heaters guides and the wider Heating & Cooling section.
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