How to Flush a Tankless Water Heater: The Complete Guide

Flushing a tankless water heater means circulating a food-safe descaling solution through the heat exchanger using a submersible pump, two service hoses, and a 5-gallon bucket. The process takes 45 to 60 minutes, requires no plumbing license, and should be completed annually in hard water areas. Skipping this maintenance causes scale to accumulate inside the heat exchanger, reducing efficiency, raising energy costs, and shortening the unit's service life.

Key Takeaways

• Annual flushing is the single most important maintenance task for a tankless water heater in hard water regions.

• You need a submersible pump, two service hoses, a 5-gallon bucket, and a food-safe descaling solution - all available in the Chromex Tankless Descaler Kit.

• Never flush with household bleach, muriatic acid, or any solution not rated for potable water systems.

• The flush cycle runs for 45 to 60 minutes. Heavily scaled units benefit from a 90-minute flush.

• Most Rinnai, Navien, Noritz, Rheem, Takagi, AO Smith, Bosch, EcoSmart, and Stiebel Eltron tankless heaters have accessible service isolation valves - confirm yours before starting.

• A tankless heater that has never been flushed may need two full flush cycles to clear heavy initial scale buildup.

• After flushing, run a fresh water rinse cycle before restoring the unit to normal operation.

Why Does a Tankless Water Heater Need to Be Flushed?

A tankless water heater heats water on demand as it passes through a compact heat exchanger coil rather than storing it in a tank. That coil is in constant contact with raw water from your supply line. In areas with hard water - water containing elevated dissolved calcium and magnesium - those minerals precipitate out of solution every time the water is heated above approximately 120 degrees Fahrenheit.

The result is limescale: a hard, chalky mineral deposit that accumulates layer by layer on the interior surfaces of the heat exchanger. Unlike a tank water heater where scale settles at the bottom and is removed during a drain and flush, scale in a tankless unit coats the narrow channels of the heat exchanger coil directly. These channels are already compact by design. Scale narrows them further, reducing water flow, forcing the burner or heating element to work harder, and increasing the time it takes for your home to receive hot water.

The EPA WaterSense program notes that water heating is one of the largest energy uses in the home, accounting for a significant share of residential energy consumption. A scaled heat exchanger can reduce thermal efficiency by 10 to 20 percent, which directly raises utility bills and accelerates wear on internal components. Major manufacturers including Rinnai, Navien, Noritz, and Rheem all specify annual descaling in their warranty maintenance schedules. Skipping the flush can void the warranty.

Flushing removes the scale by circulating a food-safe acid-based descaling solution through the heat exchanger under low pressure. The solution dissolves calcium and lime deposits without damaging copper or stainless steel components. The process is straightforward, requires no licensed plumber for standard installations with accessible service ports, and can be completed in under an hour.

Hard water compounds this problem significantly. The EPA Ground Water and Drinking Water program documents that a large majority of US homes receive hard water - with states including Texas, Arizona, California, Nevada, Florida, and Utah experiencing water hardness levels above 180 milligrams per liter. At those hardness levels, meaningful scale accumulation can occur within six months of a flush. If your municipal water report shows hardness above 180 mg/L, treat twice-yearly flushing as a baseline, not a maximum.

What Equipment Do You Need to Flush a Tankless Water Heater?

A standard flush requires five components working together as a closed-loop circulation system.

Submersible utility pump: The core tool. It circulates descaling solution from the bucket, through the heat exchanger, and back. A pump rated at a minimum 500 GPH is sufficient for residential units. The Chromex Tankless Descaler Kit includes a pump rated for full residential heat exchanger coverage. For commercial-grade units or professional plumbers servicing multiple units, higher-flow pumps in the 1,000 GPH range produce faster flush times.

Service hoses (two): One connects the pump discharge to the cold-water service port on the heater. The second connects the hot-water service port back to the bucket, completing the closed loop. Most service ports accept standard 3/4-inch garden hose fittings.

5-gallon bucket: Holds the descaling solution and the submerged pump during the flush cycle. A 5-gallon bucket provides enough volume to sustain the pump without running dry and gives you enough room to monitor the solution color as it returns from the heater.

Descaling solution: Must be food-safe, potable-water-rated, and compatible with copper and stainless steel heat exchangers. Sulfamic acid based solutions are a common choice for tankless descaling. Solutions certified to NSF/ANSI/CAN 60 for drinking water treatment chemicals confirm the product is safe for systems carrying drinking water. The Descaling Solutions collection covers formulations for different hardness levels.

Adjustable wrench and towels: For connecting and disconnecting service port caps and catching any drips.

Everything in the Chromex Tankless Descaler Kit - pump, hoses, and solution - is designed to work as a complete system. Browse the Tankless Water Heater Maintenance collection for the full range of flush and descaling tools.

How to Flush a Tankless Water Heater: Step-by-Step

This process applies to all residential tankless water heaters with accessible service isolation valves. If your unit does not have service ports, a licensed plumber needs to install isolation valves before you can proceed.

Step 1 - Confirm Your Unit Has Service Isolation Valves 

Locate the cold-water inlet and hot-water outlet pipes on your tankless unit. Most modern tankless heaters installed after 2010 include service isolation valves - small shutoff valves with a port cap - on both pipes. If your unit has these valves, you are ready to proceed. If not, contact a licensed plumber to install them before your first flush.

Step 2 - Shut Down the Unit 

For gas units: turn the controller to the service mode or set the thermostat to the lowest setting and turn off the gas supply valve. For electric units: switch off the dedicated circuit breaker. Do not begin the flush with the heater powered on.

Step 3 - Close the Main Supply Valves 

Close the main cold-water supply valve that feeds your tankless unit. Also close the hot-water outlet valve. This isolates the heat exchanger from your main water supply and prevents the descaling solution from entering your household plumbing.

Step 4 - Relieve Line Pressure

 Open a hot water faucet inside the house briefly to relieve any residual pressure in the lines. Close it once the flow stops.

Step 5 - Prepare the Descaling Solution 

Fill the 5-gallon bucket with approximately 3 to 4 gallons of clean water. Add the descaling solution according to the manufacturer's dilution ratio. A common starting ratio is 1 part solution to 3 or 4 parts water. Place the submersible pump into the bucket so it is fully submerged.

Step 6 - Connect the Service Hoses 

Remove the port caps from both service isolation valves. Connect one hose from the pump discharge outlet to the cold-water service port. Connect the second hose from the hot-water service port back down into the bucket. The circuit runs: bucket pump outlet - into cold-water service port - through the heat exchanger - out of the hot-water service port - back into the bucket.

Step 7 - Open the Service Valves

 Open both service isolation valves. The descaling solution will now have a path to flow through the heat exchanger once the pump is running.

Step 8 - Run the Flush Cycle 

Connect the pump to power and start it running. Run the pump for a minimum of 45 minutes for a unit on a regular annual maintenance schedule. For units that have not been flushed in two or more years, extend the flush to 60 to 90 minutes. The solution will typically turn yellow or brown as mineral deposits dissolve into it.

Step 9 - Run a Fresh Water Rinse Cycle 

Power off the pump. Dump the spent solution from the bucket, refill with clean water, and run the pump for an additional 10 minutes to rinse the heat exchanger. Repeat once. This removes any residual descaling solution before the unit is returned to service.

Step 10 - Disconnect and Restore the Unit 

Close both service isolation valves, remove the hoses, and replace the port caps. Reopen the main cold-water supply valve and the main hot-water outlet valve. Restore power or gas. Run a hot tap for one to two minutes to confirm normal flow, temperature, and pressure.

How Often Should You Flush a Tankless Water Heater?

Flush frequency depends primarily on local water hardness and daily hot water volume. The U.S. Department of Energy recommends following the manufacturer's maintenance schedule, which for most major brands specifies annual flushing in hard water conditions.

In areas with very hard water - above 180 mg/L calcium carbonate - consider flushing every six months. In moderately hard water areas (60 to 120 mg/L), annual flushing is sufficient. In soft water areas (below 60 mg/L), flushing every two years may be adequate.

Your unit's display panel is also a practical indicator. Most Rinnai, Navien, and Noritz models generate a maintenance alert code when scale buildup reaches a threshold the unit's sensors detect. If yours displays a maintenance or descaling code, flush immediately.

For tank-style water heaters that need corrosion protection alongside descaling, see the Water Heater Maintenance collection.

Which Descaling Solution Should You Use?

The solution must carry NSF/ANSI/CAN 60 certification for drinking water treatment chemicals. Sulfamic acid based solutions are a widely approved choice for residential annual maintenance and offer strong descaling performance for units with heavy or long-standing scale buildup.

What to avoid: bleach, muriatic acid, hydrochloric acid, and any product not explicitly rated for potable water system use. Always use a certified, manufacturer-approved solution for appliance maintenance.

The Descaling Solutions collection covers formulations in concentrations appropriate for residential and professional use.

What Are the Signs Your Tankless Water Heater Needs Flushing?

Reduced hot water flow rate: Scale is narrowing the heat exchanger channels.

Longer wait times for hot water: Heat exchanger efficiency has declined.

Error codes on the display panel: Rinnai units commonly display Error 11 or LC codes for scale events. Navien units display Error 001 or 002 for flow-related alerts.

Higher gas or electricity bills: A scaled heat exchanger forces the burner to run longer to achieve the same water temperature.

Rumbling, popping, or crackling sounds: Mineral deposits flaking off the heat exchanger surfaces during operation.

Water temperature inconsistency: The unit struggles to maintain consistent output temperature due to restricted flow.

If you observe any of these, flush the unit at the earliest opportunity.

Tankless vs Tank Water Heater Maintenance: Key Differences

A tankless water heater has no tank to corrode and does not use an anode rod. Flush kits and descaling solutions are the correct tools.

A tank water heater uses anode rods for corrosion protection - either sacrificial rods that deplete every 3 to 5 years, or powered titanium rods using ICCP technology that last the life of the tank. The Powered Anode Rods collection covers the full range. Tank heaters also benefit from annual sediment flushing using the Water Heater Tank Flushing collection.

For complete annual maintenance kits covering both systems, the Complete Kits collection bundles all necessary tools.

Frequently Asked Questions

1. What happens if I never flush my tankless water heater?

Scale builds up progressively in the heat exchanger, reducing flow rate and thermal efficiency each year. Eventually scale deposits can fully block the narrow channels, trigger persistent error codes, or cause the heat exchanger to overheat and fail permanently. Most manufacturers void the warranty for units with documented scale damage from missed maintenance.

2. Can I use white vinegar to flush a tankless water heater?

Household white vinegar at 5 percent acetic acid is generally too mild for effective heat exchanger descaling. A commercial descaling solution certified to NSF/ANSI/CAN 60 for drinking water treatment chemicals is the correct choice for most applications.

3. How do I know if my tankless heater has service isolation valves?

Look for small shutoff valves with a removable port cap on the cold-water inlet pipe and the hot-water outlet pipe directly connected to the heater. If your unit does not have them, a licensed plumber can install them in under an hour.

4. Does flushing a tankless water heater void the warranty?

No - regular descaling using manufacturer-approved solutions is required to maintain the warranty. Using non-approved chemicals or skipping maintenance entirely is what voids coverage.

5. How long does a flush take?

A standard flush cycle takes 45 to 60 minutes. Units with heavier scale buildup may need 60 to 90 minutes. Add 10 to 15 minutes for setup and the rinse cycle.

6. Can I flush a tankless water heater myself or do I need a plumber?

Most homeowners with accessible service isolation valves can complete the flush themselves using a standard kit. A plumber is needed if the unit lacks service ports or if error codes do not clear after a standard flush cycle.

Ready to Flush Your Tankless Water Heater?

Get everything you need in one system. The Chromex Tankless Descaler Kit includes the pump, hoses, and NSF certified descaling solution to complete an annual flush in under an hour. Browse the full range and keep your unit running at peak efficiency.