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Descaling & Flushing

Tankless Water Heater Descaling & Flushing

Regularly descaling a tankless water heater dissolves hard water calcium deposits, restoring full heat exchanger efficiency and preventing thermal burnout.

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Tankless Water Heater Descaling & Flushing diagnostic overview
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Quick answer

What does descaling a tankless water heater do?

Descaling (flushing) involves circulating a non-toxic descaling solution through the internal heat exchanger using isolation valves and a submersible pump. The solution dissolves calcium carbonate and mineral scale crust from copper or stainless steel tube walls.

Updated ยท Reviewed by the TanklessFixer technical team

Technical Overview

The Importance of Descaling & Flushing

When hard water is heated inside narrow tankless heat exchanger tubes, dissolved minerals like calcium and magnesium precipitate out of solution, coating metal walls with a rock-hard scale crust.

Because scale acts as thermal insulation, it reduces heat transfer efficiency, forces burners to consume more fuel, restricts water flow, and eventually causes metal heat exchangers to crack from localized overheating.

Symptoms & Warnings

Common Signs Your System Needs Attention

  • Hot water taking longer to reach set temperature at faucets
  • Noticeable decrease in water flow volume (GPM) at hot taps
  • Rumbling, knocking, or high-pitched whistling noises during firing
  • Error codes indicating thermal high-limit switch trips (Code 14 or E5)
  • System shutting down unexpectedly during long shower cycles
  • More than 12 months since the system was last flushed with descaling solution
Core Factors

Mineral Scale Effects

Thermal Insulation

Scale crust insulates heat exchanger metal, requiring higher burner firing to heat water.

Flow Restriction

Thick scale deposits narrow internal tube diameter, reducing GPM water flow.

Localized Overheating

Heat trapped behind scale crust causes metal stress cracking and heat exchanger failure.

Sensor Coating

Scale coating thermistors causes inaccurate temperature readings and system errors.

Diagnostic Categories

Key Focus Areas

01

Submersible Pump Circulation

Using a 1/6 HP pump to circulate 3 to 4 gallons of food-grade virgin descaling solution through heat exchanger tubes.

02

Isolation Service Valves

Connecting washer hoses directly to cold and hot isolation valve service ports for sealed flushing.

03

Cold Water Filter Service

Removing and rinsing cold water inlet sediment screen after completing the flushing process.

04

Fresh Water Purge

Flushing heat exchanger with fresh tap water for 5 minutes after descaling to clear residual solution.

Service Process

Step-by-Step Descaling Process

  1. Isolate Unit & Connect Hose

    Turn off gas/electric power, close cold/hot main isolation valves, and attach pump hoses to service ports.

  2. Circulate Solution

    Submerge pump in descaling bucket, open service valves, and circulate solution for 45 to 60 minutes.

  3. Flush Fresh Water

    Close cold service valve, open main cold supply valve, and flush fresh water through hot service port to drain.

  4. Clean Filter & Re-energize

    Remove and rinse cold water filter screen, close service ports, open hot main valve, and restore power.

Brands

Brand Compatibility & Service

Tankless systems from major manufacturers can differ in components, diagnostics, maintenance procedures, and error-code definitions. Service resources cover major brands found in service areas across the country.

  • Condensing & non-condensing gas systems
  • Single-phase & 3-phase electric tankless
  • Whole-home & point-of-use equipment
  • RinnaiCondensing & non-condensing gas
  • NavienCondensing units & combi boilers
  • NoritzIndoor & outdoor gas models
  • RheemGas & electric tankless
  • TakagiResidential gas tankless
  • A. O. SmithCondensing gas tankless
  • BoschGas & electric tankless
  • Bradford WhiteInfiniti gas series
  • Stiebel EltronElectric tankless
  • EcoSmartElectric tankless
  • RichmondGas tankless
  • StateGas tankless
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FAQ

Frequently Asked Questions

Can I use vinegar to descale a tankless water heater?

Food-grade white vinegar can be used, but professional non-toxic descaling solutions (like Haymaker or Flow-Aide) dissolve scale significantly faster.

How long does it take to flush a tankless water heater?

Circulating the descaling solution takes 45 to 60 minutes. The entire process, including setup and purging, takes about 75 to 90 minutes.

How often should I descale my tankless water heater?

Descaling is recommended once every 12 months for average water hardness, or every 6 months in areas with severe hard water (>12 grains per gallon).

Will descaling void my tankless warranty?

No, routine descaling is actually required by manufacturers to maintain warranty protection on heat exchangers.

What happens if scale is never flushed from a tankless heater?

Unflushed scale eventually causes heat exchanger tube cracking, reduced flow, high gas bills, and permanent unit failure.

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