How to Maintain a Tankless Water Heater

Published: August 11, 2026

Tankless water heaters are sold as low-maintenance, and compared with a tank they do have fewer parts to wear out. But "low-maintenance" is not the same as "no maintenance." The single biggest reason tankless units lose capacity, throw error codes, or fail early is mineral scale building up inside the heat exchanger.

The good news is that a modest yearly routine keeps most units running efficiently for well over a decade. Here is what to do and how often to do it.

Why Tankless Units Need Regular Service

Tankless heaters pass water through a narrow heat exchanger and heat it on demand. Any dissolved minerals in the water — calcium and magnesium especially — can settle onto those hot surfaces as scale. Over time the buildup insulates the water from the flame or element, so the unit works harder, raises the temperature less, and eventually triggers a fault code.

Hard water accelerates the process. If you already deal with scale on faucets and showerheads, your water heater is dealing with it too.

How Often Should You Service a Tankless Water Heater?

• Annual descaling: A reasonable baseline for most homes.
• Every 6 to 9 months: If you have hard water or heavy daily use.
• Filter cleaning: Every few months, more often if you see debris.
• Professional inspection: Yearly, or any time the unit shows a persistent error code.

Manufacturers often tie the warranty to documented maintenance, so keep a simple record of when you descale and who serviced the unit.

Tools and Supplies for Descaling

You will need two short washing-machine hoses, a small submersible pump, a bucket, and a descaling solution — either a commercial product or plain white vinegar. Many tankless heaters have dedicated service valves and purge ports that make the job much easier. Check your manual before you start, because connection points vary by brand.

Step-by-Step: Descaling a Tankless Water Heater

1. Shut Down the Unit

Turn off the power at the breaker for electric models, or close the gas valve for gas models. Close the cold-water inlet and hot-water outlet valves so you can isolate the unit.

2. Connect the Pump Loop

Attach one hose from the pump to the cold service port and another from the hot service port back to the bucket. Fill the bucket with descaling solution or a vinegar-and-water mix.

3. Circulate the Solution

Open the service valves and run the pump so the solution circulates through the heat exchanger. Follow the product's dwell time — commonly 30 to 60 minutes. You may see sediment and cloudy water return to the bucket, which means the process is working.

4. Flush with Clean Water

Drain the solution, refill the bucket with clean water, and circulate for several minutes to rinse the exchanger. Do not skip this step; residual acid left in the unit can cause problems.

5. Restore and Test

Close the service valves, remove the hoses, reopen the water valves, and restore power or gas. Run hot water at a fixture and confirm the unit fires and reaches temperature. Check the hose connections for drips.

Don't Forget the Inlet Filter

Most tankless units have a small screen filter on the cold inlet that traps grit before it reaches the heat exchanger. Turn off the water, remove the filter, rinse it under running water, and reinstall it. If it is damaged or heavily clogged, replace it — it is inexpensive and protects far costlier components.

Other Maintenance Checks

• Vent and air intake: Keep the intake and exhaust clear of lint, leaves, and nesting material. Blocked venting is a safety concern on gas units.
• Freeze protection: In cold climates, confirm the unit's freeze-protection mode is active and that exposed pipes are insulated.
• Error codes: Write down any code when it appears and check the manual. Repeated codes often point to scale or a flow issue.
• Water pressure: Very low flow can prevent the unit from firing. See our guide to low hot water pressure if fixtures run weak.

Signs Your Tankless Unit Needs Attention

• Hot water temperature fluctuates or never gets hot enough
• Reduced flow at fixtures compared with the past
• Rumbling or banging sounds during heating
• Frequent shutdowns or recurring error codes
• Visible scale around fittings or the service ports

When to Call a Professional

Descaling is a reasonable DIY task for a confident homeowner with the right fittings. However, gas connections, venting repairs, circuit-board faults, and stubborn error codes should be left to a licensed technician. If your unit is over ten years old and needs a major repair, compare the cost against replacement — the same logic in our repair cost guide applies to tankless systems.

Staying ahead of scale is the difference between a tankless heater that lasts and one that disappoints. If you would rather hand off the job, a qualified technician can descale, inspect, and document the service for you.