Tankless Water Heater Maintenance: How and When to Descale
Why tankless water heaters need descaling, how often to do it, the steps involved, and the other checks that keep a tankless unit running for 20 years.
In this guide
Tankless water heaters need descaling to remove mineral scale from the heat exchanger, usually about once a year and more often with hard water. Descaling means closing the isolation valves, connecting a small pump and hoses to the service ports, and circulating a descaling solution through the unit, then flushing it with clean water. Also clean the inlet water filter screen and check the air intake and vent. Scale warnings or error codes on the display are a sign it’s overdue.
A well-maintained tankless water heater can last 20 years or more, roughly twice as long as a typical tank. The key is regular descaling. Skip it, and mineral scale slowly chokes the heat exchanger, cutting efficiency and eventually causing errors or failure.
Why tankless heaters need descaling
As water is heated, dissolved minerals like calcium and magnesium can come out of solution and form scale. In a tank heater, that settles as sediment at the bottom. In a tankless heater, it builds up as a hard coating inside the narrow passages of the heat exchanger. That coating insulates the water from the heat, so the unit works harder, uses more gas or electricity, and may start showing scale or overheating error codes.
How often should you descale?
- About once a year for most homes.
- Every six months or more often with hard water or heavy use.
- Whenever the unit shows a scale warning or error code, or hot water temperature becomes unstable.
- Follow your manufacturer’s schedule; it may also affect warranty coverage.
How descaling works
- Turn off the power to the unit (and the gas, on gas models).
- Close the isolation valves on the hot and cold lines. Most installations have service valves with small hose ports for this purpose.
- Connect a small circulation pump and hoses to the service ports, with a bucket of descaling solution or food-grade white vinegar.
- Circulate the solution through the heat exchanger for the time your manufacturer recommends, often around 45 minutes to an hour.
- Flush with clean water to rinse out the solution and loosened scale.
- Clean the inlet water filter screen, reopen the valves, restore power and gas, and check for leaks and error codes.
No service valves? Some older or improperly installed tankless units don’t have isolation valves, which makes descaling much harder. Adding them is a quick plumbing job that makes every future service easier.
Other tankless maintenance checks
- Air intake filter: clean dust and lint from the intake screen, if your model has one.
- Venting: make sure vent and intake terminations outside aren’t blocked by leaves, snow or nests.
- Condensate drain (on condensing models): check that it flows freely and any neutralizer is still effective.
- Error codes: write down any code the unit displays before you call; it speeds up diagnosis.
Will a water softener help?
In areas with hard water, a water softener or scale-reduction system can slow buildup and extend the time between descalings. It doesn’t replace maintenance, but it helps.
Want us to service your tankless?
We descale tankless heaters, add service valves where they’re missing, clean filters and check venting and gas pressure in one visit. If your unit is showing an error code, we can diagnose that too.
For more on tankless units, see tankless vs. tank water heaters. If your unit isn’t heating at all, start with no hot water: common causes.
Key takeaways
- Tankless heaters need descaling to remove mineral scale from the heat exchanger.
- Descale about once a year, more often with hard water.
- Service (isolation) valves make descaling quick; add them if they’re missing.
- Also clean the inlet filter and check the vent and condensate drain.
- Regular descaling helps a tankless unit reach 20+ years.
Frequently asked questions
How often should a tankless water heater be descaled?
About once a year for most homes, and every six months or more with hard water. Follow your manufacturer’s schedule.
Can I use vinegar to descale a tankless water heater?
Many manufacturers allow food-grade white vinegar, while others recommend a specific descaling solution. Check your manual.
What happens if you don’t descale a tankless water heater?
Scale builds up, efficiency drops, hot water temperature becomes unstable, error codes appear, and the heat exchanger can eventually fail.
Do I need service valves to descale?
They make it much easier. Units installed without them can be retrofitted by a plumber.
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