How to Flush a Tankless Water Heater Safely

Homeowner safely inspecting a wall-mounted tankless water heater before maintenance

A tankless water heater is not maintenance-free. Hard-water minerals can form scale inside the heat exchanger, restrict water flow, reduce performance, and potentially shorten equipment life. The safest approach is to inspect the unit regularly and flush or descale it according to the instructions for your exact manufacturer and model.

Table of Contents

How Mineral Buildup Affects a Tankless Water Heater
How Often Should You Flush a Tankless Water Heater?
Inspect the Heater Before You Start
What You Need for a Typical Flush
Understand the Typical Flushing Loop
Step-by-Step Tankless Water Heater Flushing Process
Manufacturer Instructions Differ More Than You Might Expect
Signs That Scale or a Clogged Filter May Need Attention
Common Flushing Mistakes to Avoid
DIY or Professional Service: Which Is Better for Your Situation?
How to Reduce Future Mineral-Buildup Problems
Tankless Water Heater Maintenance Checklist
Frequently Asked Questions
Key Takeaways

There is no single flushing interval, vinegar quantity, or circulation time that applies to every tankless water heater. As of September 25, 2026, U.S. Department of Energy guidance and current manufacturer instructions show meaningful differences among models, so your model-specific manual should take priority over any general DIY procedure.

Quick answer: A typical flush isolates the heater from the home's plumbing, circulates a manufacturer-approved descaling liquid through the heat exchanger using the service ports, rinses the heat exchanger with fresh water, checks the inlet filter, and then returns the heater to service according to its manual.

Direct Answer & Frequency
Required Tools and Preparation
Step-by-Step Flushing Procedure
Common Mistakes to Avoid
Frequently Asked Questions

How Mineral Buildup Affects a Tankless Water Heater

Tankless water heaters heat water as it passes through a heat exchanger rather than storing a large volume of hot water in a tank. That design eliminates a storage tank, but it does not eliminate mineral deposits. Calcium and other minerals found in hard water can accumulate along the internal water passages.

The U.S. Department of Energy explains that scaling can restrict the water passages inside a tankless heat exchanger and reduce performance. Scale can also affect equipment life. This is why descaling or flushing appears in the maintenance instructions of major tankless water-heater manufacturers.

Mineral accumulation is not identical in every home. Water mineral content matters, but so do the heater model, hot-water demand, operating conditions, and manufacturer requirements. Navien, for example, notes that higher mineral content, higher temperature settings, and heavier hot-water use can increase the need for descaling.

Do not assume that every change in hot-water performance is caused by scale. Bosch identifies reduced flow, error codes, and boiling sounds among possible symptoms associated with severe mineralization, but similar symptoms can have other causes. Treat these signs as reasons to inspect the system and consult its diagnostic information rather than as proof that descaling will solve the problem.

How Often Should You Flush a Tankless Water Heater?

A fixed once-a-year rule is too broad. U.S. Department of Energy technical guidance notes that manufacturer-recommended maintenance intervals can vary depending on the equipment, water quality, and use. The DOE material describes manufacturer recommendations ranging from twice a year to once every three years.

Some current manufacturers do use annual maintenance as a practical starting point. Navien recommends descaling at least once per year and says more frequent service may be appropriate when mineral content, temperature settings, or hot-water use are higher. Rheem also recommends major maintenance tasks at least annually in its maintenance guidance.

However, Rheem's 2026 tankless installation and service guidance provides another example of why the manual matters: it recommends heat-exchanger flushing at approximately 500 operating hours, described as roughly every 1 to 1.5 years. The difference does not mean one recommendation is necessarily wrong. It shows that maintenance instructions can vary by product and document.

Situation	Best Approach
Your manual gives a specific interval	Follow the interval for that exact model.
Your home has higher mineral content	Check whether the manufacturer recommends more frequent descaling.
Hot-water use is unusually heavy	Review the manual because usage can affect the maintenance schedule.
You notice reduced flow or unusual operating symptoms	Inspect the filter and maintenance status, then use the manufacturer's troubleshooting procedure rather than assuming scale is the only cause.
You do not know when the heater was last serviced	Identify the model, obtain its current maintenance instructions, and inspect the system before choosing a flushing procedure.

Avoid using an unsupported universal hard-water number to decide when to flush. The reviewed sources do not establish one water-hardness threshold that applies to every manufacturer and model. If you need a maintenance interval based on your local water, compare your water conditions with the recommendations in the heater's documentation.

Inspect the Heater Before You Start

A useful maintenance session begins with inspection rather than immediately attaching a pump. Confirm the manufacturer and model, locate the current manual, and identify the cold-water inlet, hot-water outlet, isolation valves, and service connections described in that manual.

Look for obvious leakage, damaged accessible plumbing, or a valve arrangement that does not match the procedure you intend to follow. A common flushing setup depends on isolation valves and service connections. Bosch and Noritz procedures, for example, describe systems equipped with the connections needed to isolate and circulate descaling liquid through the heater.

If those service connections are absent, do not improvise a connection based on a generic online diagram. The installation may require a different maintenance method, so check the model-specific instructions or use qualified professional service.

The cold-water inlet filter or strainer also deserves attention. Rheem instructs owners to remove and rinse the inlet filter in its maintenance procedure, while Navien also includes cleaning the cold-water inlet screen. Bosch explains that a clogged mesh filter can contribute to reduced flow, water-temperature problems, or even prevent operation.

If you are also checking the rest of your home's water controls, knowing how to find and test your main water shutoff can make future plumbing maintenance and emergency response easier.

Do not open combustion components, gas piping, or other internal gas-system parts as part of a routine homeowner flush. Bosch specifically advises that work involving live gas equipment should be handled by qualified personnel. If the required procedure goes beyond the water-side maintenance described in your manual, use a qualified service professional.

Tankless water heater isolation valves and service ports ready for inspection

What You Need for a Typical Flush

The exact supply list comes from your model's manual. Across the reviewed manufacturer procedures, a common setup includes a bucket, a submersible or transfer pump, hoses, accessible service or isolation valves, and a descaling liquid that the manufacturer allows.

Bucket: A 5-gallon bucket appears in manufacturer procedures including Navien's guidance.
Pump: Use the type of submersible or transfer pump specified or suitable for the manufacturer's flushing arrangement.
Hoses: A typical closed-loop procedure uses hoses between the pump, service connections, and return bucket.
Descaling liquid: Use only a liquid allowed by your exact manufacturer and model instructions.
Model-specific manual: Keep the procedure available during the entire job rather than relying on memory or a generic sequence.

White vinegar is common in official procedures, but even vinegar instructions are not universal. Takagi documentation specifies 3 gallons of 5% acidity food-grade white vinegar in the reviewed procedure, and an A. O. Smith service handbook also specifies 3 gallons of 5% white vinegar. One Rheem manual instead specifies up to 2 gallons of virgin food-grade white vinegar.

Other manufacturers differ again. A reviewed Noritz procedure uses 3 to 4 gallons of vinegar, while Rinnai's consumer flushing guidance specifies 4 gallons, or 15.1 L, of undiluted white vinegar. These differences are a strong reason not to copy a vinegar quantity from another heater.

The type of cleaner also matters. Navien allows an approved descaling solution or household white vinegar in its guidance. Rinnai's reviewed consumer procedure instructs users not to use chemical solutions and instead calls for undiluted white vinegar. A product marketed as a tankless-water-heater cleaner therefore should not automatically be assumed compatible with every heater.

Rule to follow: Use the descaling liquid, amount, concentration, and handling procedure specified for your exact heater. Do not substitute a different cleaning product unless the manufacturer permits it.

Understand the Typical Flushing Loop

The general concept is simple even though the details vary. In a common service-valve arrangement, the pump sends descaling liquid from the bucket into the cold-side service connection. The liquid passes through the heat exchanger, exits through the hot-side service connection, and returns to the bucket.

A. O. Smith's service guidance provides an example of this arrangement: the pump outlet connects to the service connection on the cold-water inlet side, while a hose on the hot-water outlet side returns liquid to the vinegar bucket. This creates a closed circulation loop through the heat exchanger.

That description is a general orientation, not permission to rearrange your heater's valves. Valve positions and shutdown procedures differ among installations. Before operating any valve, match it to the diagram or written instructions for your model.

Step-by-Step Tankless Water Heater Flushing Process

The following sequence explains the common structure found across manufacturer procedures. It intentionally does not assign one universal vinegar quantity, circulation time, or valve sequence. Wherever your manual differs, the manual controls.

Step 1: Read the Model-Specific Procedure

Locate the installation, service, or maintenance instructions for the exact model number. Bosch explicitly directs owners to consult the installation manual, and this should be the starting point for any tankless heater. Confirm whether homeowner flushing is supported, what cleaning liquid is permitted, and whether the unit has any special scale-control components.

This check is especially important for equipment with specialized scale-control technology. A. O. Smith's guidance for X3-equipped models includes a specific exception: the X3 cartridge must not remain installed during descaling, and a bypass cartridge is required if descaling is performed. A generic flushing article cannot safely replace such model-specific requirements.

Step 2: Shut Down and Isolate the Heater as Directed

Follow the manufacturer's shutdown procedure for the heater's energy source and water connections. The required sequence can differ between gas and electric equipment and among manufacturers, so do not assume that a valve or switch serves the same function on every installation.

The goal is to put the unit into the maintenance condition specified by its manufacturer and isolate the heat exchanger from normal household water flow before opening service connections. Do not perform work on internal gas components as part of this DIY procedure.

Step 3: Connect the Pump, Hoses, and Bucket

With the unit safely isolated according to its manual, connect the flushing equipment to the designated service ports. In the common closed-loop arrangement, the pump supplies the cold-side service connection and the hot-side hose returns the liquid to the bucket.

Check the hose connections before starting the pump and follow the connection arrangement specified by the manufacturer. If the installation does not provide the service connections shown in the manufacturer's procedure, stop rather than improvising.

Step 4: Add Only the Approved Descaling Liquid

Fill the flushing bucket with the liquid and quantity specified by the manufacturer. Do not assume that 2, 3, or 4 gallons is correct simply because that amount appears in another manufacturer's documentation.

The same caution applies to concentration. Some verified procedures specify food-grade white vinegar at 5% acidity, while Rinnai's reviewed consumer guidance specifies undiluted white vinegar and says not to use chemical cleaning solutions. Follow the wording for your heater.

Step 5: Circulate for the Specified Time

Run the pump for the duration given in your manual. Navien specifies at least 45 minutes in its reviewed descaling instructions. Rheem documentation also includes a minimum 45-minute circulation period, and A. O. Smith describes approximately 45 minutes in its service handbook.

Noritz provides an example of a longer procedure, specifying at least one hour of circulation in the reviewed instructions. It also describes replacing the vinegar after 30 minutes and repeating the process for maximum effectiveness. Because official procedures differ, a generic statement such as “always circulate for exactly 45 minutes” would be inaccurate.

Pump, bucket, and hoses connected for a tankless water heater flushing procedure

Step 6: Drain the Used Solution and Rinse with Fresh Water

Descaling does not end when circulation stops. Manufacturer procedures include a fresh-water rinse after the descaling stage.

Navien calls for at least 10 minutes of fresh-water flushing in its reviewed instructions. One Rheem procedure specifies 5 minutes, while A. O. Smith specifies approximately 10 minutes. Rinnai's SENSEI technical procedure directs users to pass water through the heat exchanger for 5 minutes to remove vinegar.

Use the rinse duration and valve sequence specified for your heater so the cleaning liquid is removed according to the manufacturer's procedure.

Step 7: Inspect and Clean the Inlet Filter

Check the cold-water inlet filter or screen as instructed by the manufacturer. Rinnai's SENSEI procedure specifically includes cleaning remaining debris from the cold-water inlet filter after flushing. Rheem and Navien also include inlet-filter maintenance in their guidance.

Rinse an approved removable screen according to the manual and reinstall it correctly before returning the unit to normal operation. Bosch notes that a clogged inlet mesh filter can contribute to reduced flow and other operating problems.

Step 8: Restore the Heater Exactly as Directed

Remove the flushing equipment, close service connections, return isolation valves to their operating positions, and restore the heater using the manufacturer's restart sequence. Check accessible connections for leakage before considering the job complete.

Do not simply restore power or fuel immediately after stopping the circulation pump. Complete the manufacturer's rinse, filter, hose-removal, valve-restoration, and restart steps first.

Manufacturer Instructions Differ More Than You Might Expect

The table below illustrates why a single internet flushing recipe cannot accurately cover every tankless heater. These are examples from the reviewed official materials, not specifications for products outside those particular procedures.

Manufacturer Example	Verified Procedure Detail	What It Shows
Rheem	One manual allows up to 2 gallons of food-grade white vinegar; a reviewed procedure uses at least 45 minutes of circulation and 5 minutes of rinsing.	Even familiar maintenance advice must be matched to the applicable manual.
A. O. Smith	A reviewed service handbook specifies 3 gallons of 5% white vinegar, about 45 minutes of circulation, and about 10 minutes of fresh-water flushing.	Its procedure should not automatically be transferred to another brand or model.
Takagi	A reviewed procedure specifies 3 gallons of 5% acidity food-grade white vinegar.	Vinegar concentration and quantity can be explicitly defined.
Noritz	A reviewed procedure uses 3 to 4 gallons of vinegar and at least one hour of circulation.	Circulation time is not universally 45 minutes.
Rinnai	Consumer guidance specifies 4 gallons (15.1 L) of undiluted white vinegar and directs users not to use chemical solutions in that procedure.	Cleaner choice and quantity can differ substantially.
Navien	Guidance permits an approved descaling solution or household white vinegar and specifies at least 45 minutes of circulation.	Some manufacturers permit options that another manufacturer's procedure may not.

The practical lesson is not to average these values. If one manufacturer says 2 gallons and another says 4 gallons, the correct answer is not automatically 3 gallons. The correct amount is the one specified for the heater being serviced.

Signs That Scale or a Clogged Filter May Need Attention

A maintenance schedule is useful, but changes in operation can also prompt an inspection. Bosch technical information associates heavy mineralization and heat-exchanger scale with reduced flow rates, error codes, and boiling sounds. Navien also notes that mineral buildup can reduce heat-exchanger efficiency and water flow through plumbing and filters.

Reduced hot-water flow: Inspect the inlet filter and review the manufacturer's scale-maintenance guidance.
Unexpected performance changes: Check the manual's troubleshooting information rather than assuming the heater only needs vinegar.
Error codes: Record the code and use the manufacturer's explanation for that exact model.
Unusual sounds: Investigate according to the manual, especially if the sound is new or persistent.
Unknown maintenance history: Inspect the heater and establish a schedule based on its manual and your water conditions.

A hidden plumbing leak can also create water-pressure or moisture symptoms that have nothing to do with heat-exchanger scale. If the evidence points beyond the heater itself, checking for a hidden water leak before opening a wall can help you narrow down the source without immediately starting destructive work.

Common Flushing Mistakes to Avoid

Using Another Brand's Numbers as Universal Specifications

This is one of the easiest mistakes to make. Official procedures in the reviewed sources differ in vinegar volume, circulation duration, rinse duration, and recommended maintenance interval. Treat manufacturer examples as examples, not industry-wide specifications.

Assuming Every Heater Needs an Annual Flush

Annual maintenance is recommended by major manufacturers including Navien and Rheem, but it is not a universal rule for every tankless heater. DOE guidance documents a broader range of manufacturer recommendations. Your model and operating conditions determine which schedule applies.

Using an Unapproved Chemical Descaler

Do not choose a cleaner solely because it is sold for water-heater descaling. Navien documentation allows approved descaling products or household white vinegar, while Rinnai's reviewed consumer procedure instructs users to avoid chemical cleaning solutions and use white vinegar. Compatibility must be verified before the cleaner enters the heater.

Skipping the Fresh-Water Rinse

Circulating the cleaner is only part of the process. Manufacturer instructions include a separate fresh-water rinse to remove the cleaning liquid. Follow the specified rinse process before restarting the heater.

Ignoring the Inlet Filter

Descaling the heat exchanger while leaving a clogged inlet screen untouched can leave a flow restriction unresolved. Several reviewed manufacturers include inlet-filter maintenance, and Rinnai's SENSEI procedure specifically calls for cleaning remaining debris from the inlet filter after flushing.

Working on Gas Components as a DIY Maintenance Task

A water-side flush should not become an excuse to disassemble combustion or gas equipment. If diagnosis requires work on live gas equipment or internal gas-system components, use a qualified professional.

Flushing a Special Scale-Control System Like a Conventional Unit

Some models have special maintenance requirements. The A. O. Smith X3 example demonstrates why identifying the exact equipment matters: its cartridge requires special handling if descaling is performed. Never assume that a standard service-valve diagram overrides a model-specific exception.

Professional checking a tankless water heater after flushing and filter maintenance

DIY or Professional Service: Which Is Better for Your Situation?

Some manufacturers, including Navien, indicate that owners with the appropriate equipment and skills may be able to perform descaling. That does not mean every installation is a good DIY candidate. Navien also warns that damage caused during DIY work can affect warranty coverage.

Your Situation	Practical Choice
The manual clearly provides an owner maintenance procedure, service valves are accessible, and you understand the shutdown and restart sequence.	A careful DIY flush may be reasonable if you follow the manual exactly.
The heater lacks accessible isolation or service valves.	Do not improvise the standard two-hose flushing setup. Consider professional service.
The procedure requires opening or servicing gas equipment.	Use a qualified professional.
The unit has a specialized scale-control component or an unfamiliar cartridge.	Check the model-specific service instructions before doing anything.
The heater shows persistent error codes or poor performance after maintenance.	Use manufacturer diagnostics or qualified service rather than repeatedly flushing it.
You cannot positively identify the valves or service ports.	Stop and obtain the correct documentation or professional assistance.

The best DIY decision is sometimes deciding not to proceed. A flush is relatively controlled when the manufacturer provides an applicable procedure and the installation has the expected service connections. Uncertainty about gas equipment, unusual plumbing, specialized components, or the correct shutdown sequence is a reason to get qualified help.

How to Reduce Future Mineral-Buildup Problems

Start by keeping a maintenance record for the heater. Record when it was inspected, whether the inlet filter was cleaned, and when the heat exchanger was flushed. Then compare future service timing with the manufacturer's recommendations and your household's operating conditions.

If your water has substantial mineral content, check the manufacturer's guidance on scale-control or water-treatment options rather than selecting a system based on a generic hardness rule. Rheem's 2026 guidance notes that a scale-control water-treatment system can reduce the need for flushing, but compatibility and maintenance requirements should be verified for the equipment in your home.

Also pay attention to the broader plumbing system. Knowing where water can be isolated helps when servicing appliances or responding to leaks. For a separate maintenance task involving a conventional storage water heater, inspecting and maintaining a water heater anode rod applies to tank-style equipment rather than the tankless heat-exchanger procedure described here.

Tankless Water Heater Maintenance Checklist

Identify the exact manufacturer and model before choosing a flushing procedure.
Read the current installation, service, or maintenance instructions.
Confirm the manufacturer's recommended inspection or flushing interval.
Check whether water quality or heavy hot-water use changes that interval.
Identify the isolation valves and service connections before attaching equipment.
Verify which descaling liquid the manufacturer permits.
Use the specified liquid quantity and concentration rather than a generic amount.
Follow the model-specific shutdown and isolation sequence.
Connect the pump and hoses only to the designated service connections.
Circulate the solution for the time specified in the applicable instructions.
Complete the required fresh-water rinse instead of stopping after descaling.
Inspect and clean the cold-water inlet filter when instructed.
Return all valves and components to their normal operating configuration.
Restart the heater according to the manufacturer procedure.
Check accessible connections for leakage and verify normal operation.

Frequently Asked Questions

Does every tankless water heater need to be flushed once a year?

No. Navien and Rheem provide annual maintenance recommendations in current guidance, but the U.S. Department of Energy notes that manufacturer flushing recommendations can vary with model, water quality, and use. Rheem's 2026 service material also includes an approximately 500-operating-hour, or roughly 1-to-1.5-year, recommendation for heat-exchanger flushing. Follow the instructions for your exact model.

Should I flush more frequently if I have hard water?

You may need to. Navien states that higher mineral content can increase the required descaling frequency, along with factors such as higher temperature settings and greater hot-water use. Do not rely on a universal hardness cutoff unless your manufacturer's documentation provides one for your equipment.

How much white vinegar should I use?

There is no universal amount. Verified manufacturer examples range from up to 2 gallons in one Rheem manual to 3 gallons in reviewed Takagi and A. O. Smith procedures, 3 to 4 gallons in a Noritz procedure, and 4 gallons (15.1 L) in Rinnai consumer guidance. Use the quantity specified by your manufacturer for your model.

How long should vinegar circulate through the heater?

Again, check the manual. Reviewed Navien, Rheem, and A. O. Smith procedures use approximately or at least 45 minutes, while the reviewed Noritz procedure calls for at least one hour. These differences are why one circulation time should not be presented as an industry-wide rule.

Can I flush a tankless water heater myself?

Some manufacturers provide procedures that a properly equipped and capable homeowner can follow. However, model-specific instructions come first, and DIY damage may affect warranty coverage. If your installation lacks appropriate service valves, requires work on gas components, contains specialized scale-control equipment, or you are uncertain about the shutdown and restart procedure, use a qualified professional.

Key Takeaways

Tankless water heaters can develop mineral scale inside the heat exchanger, especially under higher-mineral water conditions.
There is no universal flushing interval. Manufacturer, model, water conditions, temperature settings, and hot-water use can affect maintenance needs.
Do not copy vinegar quantities or circulation times from another brand. Verified manufacturer procedures differ substantially.
A common flushing arrangement circulates an approved solution from a bucket and pump through the cold-side service port, heat exchanger, and hot-side service port before returning it to the bucket.
Fresh-water rinsing is part of the verified manufacturer procedures reviewed here and should be completed as directed.
Inspect the cold-water inlet filter as directed because a clogged filter can contribute to restricted flow and other operating problems.
Do not treat internal gas-system work as routine DIY flushing. Use qualified service when the procedure extends beyond the water-side maintenance permitted by the manufacturer.

As of September 25, 2026, the most reliable rule is straightforward: identify your exact tankless water heater, follow its current manufacturer instructions, and use general flushing advice only to understand the process. Model-specific requirements should always override a generic maintenance recipe.

References

Information in this post was checked as of 2026-09-25. Policies and prices may change, so please verify important details with official sources.