Flushing (also called descaling) your tankless water heater removes mineral buildup inside the heat exchanger. If you live in an area with hard water, this maintenance step is critical for performance, efficiency, and lifespan.

Poor maintenance can reduce efficiency over time. See the tankless water heater efficiency guide to learn how buildup affects performance. Regular descaling is part of proper tankless water heater maintenance and helps prevent performance problems. You can also read the tankless water heater maintenance guide to understand recommended yearly service requirements.

Most manufacturers recommend flushing once per year — or every 6–9 months in hard water regions. In this guide, you’ll learn exactly how to flush a tankless water heater safely and correctly.

How Do You Flush a Tankless Water Heater?

Flushing a tankless water heater involves circulating a descaling solution through the heat exchanger to remove mineral buildup. This process typically requires turning off the heater, connecting a flush kit to the service valves, and running the cleaning solution through the system for about 45–60 minutes.

Why Flushing a Tankless Water Heater Is Important

Over time, calcium and mineral deposits accumulate inside the heat exchanger.

This can cause:

Water that goes hot then cold
Reduced flow rate
Rumbling or vibration noise
Higher energy bills
Error codes
Premature unit failure

Regular flushing also helps extend tankless water heater lifespan. See the tankless water heater lifespan guide to learn what affects durability.

Regular descaling keeps your unit efficient and prevents costly repairs. Maintenance cost should also be considered part of total ownership. See the tankless water heater cost guide for typical price ranges.

Tools You’ll Need

To flush a tankless water heater, you’ll need:

A tankless water heater flush kit (pump + hoses)
Descaling solution (or white vinegar)
A 5-gallon bucket
Screwdriver (if needed to access valves)
Towels for minor drips

Many homeowners use a dedicated tankless flush kit because it includes the correct pump and hoses designed for this procedure.

(You can find compatible tankless flush kits and descaling solutions here.)

Note: Always confirm compatibility with your specific model before purchasing tools.

Safety Tips Before Flushing a Tankless Water Heater

Before beginning the flushing process, it’s important to take a few basic safety precautions. Tankless water heaters involve both water and electrical or gas connections, so shutting the system down properly is essential.

Follow these safety steps before starting:

• Turn off electrical power to the unit at the breaker
• Shut off the gas valve if you have a gas-powered system
• Close the hot and cold water isolation valves
• Allow the unit to cool before opening service ports
• Place a bucket or towels nearby to catch small amounts of water

Taking these precautions helps prevent accidental leaks or damage during the maintenance process.

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

Step 1: Turn Off Power and Gas

Turn off the unit at the power button.
Shut off the breaker.
Turn off the gas valve (for gas models).

Safety first.

Do All Tankless Water Heaters Need Flushing?

Most tankless water heaters require periodic flushing to remove mineral deposits that accumulate inside the heat exchanger.

However, the frequency of flushing can vary depending on several factors:

• local water hardness
• household hot water usage
• manufacturer recommendations
• presence of a water softener

Homes with hard water may require flushing every 6–9 months, while homes with softer water may only need maintenance once per year.

Consulting the manufacturer’s manual can help determine the recommended maintenance schedule for your specific unit.

Step 2: Close the Water Valves

Locate your isolation valves.

Close the hot water valve.
Close the cold water valve.

These prevent water from entering the system during flushing.

Step 3: Connect the Flush Kit

Attach the hoses from the flush kit to the service ports.
Place the pump inside a 5-gallon bucket.
Pour descaling solution or white vinegar into the bucket.

Make sure connections are secure before proceeding.

Step 4: Circulate the Cleaning Solution

Turn on the pump.
Allow the solution to circulate for 45–60 minutes.

This breaks down mineral buildup inside the heat exchanger.

If you have severe scaling, you may need a slightly longer circulation time.

Step 5: Flush with Clean Water

After descaling:

Turn off the pump.
Dispose of the used solution properly.
Flush clean water through the system for 5–10 minutes.

This removes any remaining residue.

Step 6: Restore the System

Disconnect hoses.
Close service ports.
Open hot and cold valves.
Restore power and gas.

Run a hot water faucet for a few minutes to ensure proper operation.

How Often Should You Flush a Tankless Water Heater?

General guideline:

Normal water: Once per year
Hard water: Every 6–9 months
Very hard water: Every 6 months

If you notice:

Temperature fluctuation
Error codes
Noise
Reduced pressure

It may be time to descale sooner.

Can You Use Vinegar Instead of Descaling Solution?

Yes. White vinegar is commonly used for tankless descaling.

However:

It may take longer to break down heavy mineral buildup.
Commercial descaling solutions are often more concentrated and faster-acting.

Both options are widely used by homeowners.

Is Flushing a Tankless Water Heater Difficult?

For homeowners comfortable with basic DIY maintenance, flushing is straightforward and typically takes 60–90 minutes.

If you are unsure about gas connections or isolation valves, hiring a licensed professional is recommended. Proper installation makes maintenance easier. See the tankless water heater installation cost guide to understand required setup.

Signs Your Tankless Water Heater Needs Flushing

Over time, mineral buildup inside the heat exchanger can reduce efficiency and affect performance.

Common signs that your tankless water heater may need flushing include:

• inconsistent water temperature
• longer time to heat water
• reduced hot water pressure
• rumbling or vibrating noises from the unit
• error codes appearing on the control panel

If you notice these symptoms, flushing the system may help restore normal operation.

If problems continue after flushing, consult our tankless water heater troubleshooting guide for additional diagnostic steps.

Frequently Asked Questions

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

Most flush procedures take 45–60 minutes of circulation time, plus setup and cleanup.

Can I flush a tankless heater without isolation valves?

It is possible, but much more difficult. Installing isolation valves simplifies maintenance and is strongly recommended.

What happens if you don’t flush a tankless water heater?

Mineral buildup can reduce efficiency, trigger error codes, damage the heat exchanger, and shorten the lifespan of the unit.

Conclusion

Flushing a tankless water heater is an important part of regular maintenance and helps prevent scale buildup, error codes, and performance problems. Most heaters should be flushed once per year, or more often in hard water areas.

Keeping the heat exchanger clean helps maintain efficiency, extend tankless water heater lifespan, and avoid expensive repairs over time.