Tankless water heater maintenance comes down to one main job: descale (flush) the heat exchanger at least once a year, twice in hard water. Rinse the cold-water inlet filter on the same schedule, soften hard water if you can, and book a yearly professional inspection for the gas, venting, and burner side. Skipping it invites scale, error codes, and a voided warranty.
Manufacturers like Rinnai and Navien each publish their own maintenance pages, and the details get scattered across PDFs and support articles. This is the consolidated version: what you can do yourself, what belongs to a plumber, and what each skipped task quietly costs you.
The tankless maintenance tasks at a glance
Most of the year-round upkeep is two small jobs plus one annual pro visit. Here's the split.
| Task | Who does it | How often |
|---|---|---|
| Descale / flush the heat exchanger | You (or a plumber) | Once a year; twice in hard water |
| Rinse the cold-water inlet filter | You | Every few months; with each descale |
| Professional gas, venting, and burner inspection | Plumber / heating contractor | Once a year (gas units) |
| Check water hardness, add or service a softener | You / plumber | Once a year, or when descaling gets frequent |
| Watch for warning signs (flow, temp, error codes) | You | Ongoing |
Descaling: the one task that matters most
Every time you run hot water, your home's water passes through a heat exchanger. Minerals in that water cook onto the metal as scale. Scale insulates the exchanger, so the burner works harder, efficiency drops, and eventually the unit throws error codes or fails early. Descaling clears it out.
Both Rinnai and Navien recommend descaling at least once a year. In hard water, twice a year is common. Softer water can stretch the interval. Rinnai's procedure circulates roughly 4 gallons of undiluted white vinegar through the unit for up to 45 minutes, then a fresh-water rinse. Rinnai is clear on one point: never use a chemical descaling solution, because household water flows through the same unit. Plain white vinegar only.
The flush relies on the isolation (service) valves on the cold and hot connections. If your unit doesn't have them, adding a valve kit is a plumber's job, and it's worth doing once so every future flush is easy. For the full timing logic by water hardness, see our tankless water heater descaling schedule.
If you also run a traditional tank somewhere in the house, the cadence is different there. Our guide on how often to flush a water heater covers that side.
Rinsing the cold-water inlet filter
Tankless units have a small screen on the cold-water inlet that catches grit and sediment before it reaches the heat exchanger. When it clogs, hot-water flow drops. Cleaning it takes a few minutes: shut off the water, pull the screen, rinse it under cold water, and scrub gently to clear debris.
The screen and its housing are plastic, so finger-tighten it on reassembly and give it only a small extra turn. Cranking it down can crack the fitting. The easiest time to do this is right after a descale, since the water is already off and the unit is open.
What to leave to a pro
You can handle descaling and the inlet filter. The combustion side is not a DIY job. Leave these to a qualified plumbing-and-heating contractor:
- Gas supply and gas-pressure checks
- Venting and combustion-air inspection
- Burner cleaning and adjustment
- Condensate line and trap service (on condensing units)
- Anything inside the heat exchanger beyond a vinegar flush
Gas units should get a yearly professional inspection covering venting, gas pressure, and the burner. There's a warranty angle too: many manufacturers, Navien among them, require documented annual service for the warranty to stay valid. Keep the receipt or service report. The Department of Energy makes the same point: periodic maintenance significantly extends a tankless unit's life and minimizes efficiency loss, and installation or service should be done by a qualified contractor.
Soft water cuts how often you descale
Scale is a hard-water problem. The harder your water, the faster minerals build up and the more often you'll descale. In a hard-water area, a whole-home water softener is the single best way to slow scale and stretch the interval between flushes.
As a rule of thumb, water above about 7 grains per gallon is considered hard. That's a general guideline, not a manufacturer spec, so treat it as a starting point rather than a hard line. A cheap test strip tells you where you stand. If you find yourself descaling twice a year and the unit still struggles, a softener usually pays for itself in fewer flushes and a longer-lived heat exchanger.
Signs your tankless heater needs service
A few symptoms tell you maintenance is overdue or something needs a pro's eyes:
- Reduced hot-water flow or pressure, which often points to scale or a clogged inlet filter
- Temperature swinging between hot and lukewarm during a single shower
- An error code on the display
Error codes are model-specific, so check the manual that came with your unit (or the manufacturer's site) for what your exact code means. Don't guess from numbers you read online. If a flush and a filter rinse don't clear a flow or temperature problem, that's the cue to call a contractor.
Common questions
Can I descale a tankless water heater myself?
Yes, if your unit has isolation valves. You connect a small pump and a bucket of white vinegar, circulate it through the heat exchanger for up to 45 minutes, then rinse with fresh water. If your unit lacks the valves, have a plumber add them first.
What happens if I never descale it?
Scale builds on the heat exchanger, efficiency drops, hot-water flow falls, and the unit starts throwing error codes. Left long enough it shortens the heater's life and can fail the heat exchanger. Skipping documented annual service can also void the warranty.
How long does a tankless water heater last?
Well-maintained tankless units commonly outlast tank models, which is part of why descaling matters. If yours is failing despite upkeep, our guide on the cost to replace a water heater walks through what a swap runs.
Do tankless heaters have an anode rod?
Most don't, because there's no storage tank to protect. That's a tank-heater part. If you also run a tank, see how often to replace a water heater anode rod.
White vinegar or a descaling chemical?
White vinegar. Rinnai specifically warns against chemical descaling solutions because your household water flows through the same unit. Undiluted white vinegar does the job safely.
Good maintenance rhythm
- Every few months: rinse the cold-water inlet filter under cold water.
- Yearly: descale the heat exchanger with white vinegar (twice a year in hard water).
- Yearly: book a professional inspection of gas, venting, and the burner, and keep the service record for your warranty.
- Yearly: check water hardness and service your softener if you have one.
- Ongoing: watch for weak flow, swinging temperatures, or error codes and act early.