Tankless Water Heater Maintenance: How to Flush and Descale (and How Often)

• Flush and descale it about once a year, or more often in hard water. Most manufacturers recommend annual service and say hard-water homes may need it more often; check your manual.
• Method: shut off power and the gas supply, close the isolation (service) valves, connect hoses to the service ports, and circulate undiluted white vinegar through the heat exchanger with a small pump for roughly 45 to 60 minutes, then rinse with clean water.
• Clean the cold-water inlet filter each time you flush, and more often if water flow drops.
• Install isolation valves if your unit does not have them. A plumber can add a service-valve kit.
• Gas and venting are not DIY. Leave gas piping, combustion adjustments, venting repairs and carbon-monoxide issues to a licensed technician.
• Source: the U.S. Department of Energy notes that tankless heaters last 20+ years and that regular maintenance, such as flushing, extends life (Energy.gov: Energy Saver Guide).

Minerals such as calcium and magnesium in hard water form scale on the inside of the heat exchanger as water is heated. Scale narrows the passages, which reduces flow, lowers efficiency, makes the unit run hotter, and can trigger error codes. The U.S. Geological Survey classifies water as soft up to 60 mg/L of calcium carbonate, moderately hard 61 to 120, hard 121 to 180, and very hard above 180 (USGS: Hardness of Water). One grain per gallon equals 17.1 mg/L, so about 7 grains per gallon is the edge of hard water.

Find your hardness number from your water utility’s annual water quality report, a test strip, or a lab test. A whole-house water softener or a scale-prevention filter upstream of the heater helps protect it; see our water softener guide.

Manufacturers set their own schedules, so your manual rules. As a starting point:

• Soft water (under about 3.5 gpg): every 1 to 2 years, or the schedule in your manual.
• Moderately hard (about 3.5 to 7 gpg): every year.
• Hard to very hard (7 gpg and above): every 6 to 12 months, and consider a softener.
• Add extra service if you see lower hot-water flow, longer waits for hot water, a rumbling or whistling sound, or scale or error codes.

Rinnai, for example, says that hard water may need more frequent maintenance and has a code for scale buildup. Check your brand’s maintenance page for the details: Rinnai, Navien.

• Two washing-machine style hoses or clear tubing with fittings that match your service ports
• A small submersible or circulation pump (often sold as a descaling kit), and a 5-gallon bucket
• About 2 to 4 gallons of undiluted white vinegar (check your manual for the amount)
• Isolation valves with service ports (add them if missing)
• Towels, a flashlight, and an adjustable wrench or pliers
• A bucket and cloth for the inlet filter

Descaling kits for tankless heaters are commonly in the range of about $50 to $150 for valves and hoses. Check current prices. A professional flush typically costs a few hundred dollars or less, depending on your area.

1. Turn off power (unplug the unit or switch off the dedicated breaker).
2. Turn off the gas supply at the unit’s gas shut-off valve. If you ever smell gas, leave the house and call your gas utility from outside; do not try to repair it yourself.
3. Close the cold and hot isolation valves and open a hot faucet to relieve pressure.
4. Connect the hoses. One hose from the cold service port to the pump, another from the hot service port to the bucket. Open the service port caps carefully; a little water will come out.
5. Fill the bucket with undiluted white vinegar, enough to cover the pump.
6. Run the pump so vinegar circulates through the unit for about 45 to 60 minutes.
7. Drain the vinegar and disconnect the pump. Flush with fresh water for about 5 minutes.
8. Remove and clean the inlet water filter: take out the screen, rinse out sediment, and replace it.
9. Reopen the isolation valves, bleed the air at a hot faucet until the flow is steady, restore power, then turn on the gas, and check for leaks at all fittings.

If the unit shows an error after restarting, see the section below.

• Vent and air intake: check for blockage, bird nests and corrosion. A licensed technician should inspect combustion and venting.
• Condensate drain on condensing models: make sure it drains freely and the neutralizer (if fitted) is replaced as the manual says.
• Temperature settings: keep at 120°F unless you have a reason to change it; the Department of Energy lists 120°F as a typical setting (Energy.gov).
• Annual professional service: recommended for gas units, especially to test combustion and safeties.

Look up your brand’s fault code in our water heater and HVAC error code lookup: Rinnai codes, Navien codes, Rheem codes and A. O. Smith codes. Scale-related codes usually clear after a proper flush. Persistent ignition, gas or exhaust errors need a licensed technician. For other hot-water problems, see water heater not working.

• U.S. Department of Energy: Tankless or Demand-Type Water Heaters
• USGS: Hardness of Water
• Rinnai
• Navien