Quick Answer: Tankless water heaters need four recurring tasks, not one: an annual descaling flush, quarterly inlet filter/screen cleaning, an annual gas venting inspection (gas units only), and a pre-winter freeze-protection check in unconditioned spaces. Skip them and the failure isn’t gradual — a Battelle Memorial Institute study found gas tankless units on 26-grain hard water “completely failed to function” after just 1.6 years with no descaling. Manufacturer warranties from Navien, Rheem, and EcoSmart all explicitly exclude scale and sediment damage, so a $1,000+ unit can fail outside coverage even mid-warranty if maintenance is skipped.
Most tankless owners have heard of descaling and stop there. That’s one task on a four-item list, and it’s not even the one that causes the most service calls — a clogged inlet filter is. This guide covers the full maintenance schedule manufacturers actually publish, what each task involves, and what happens when you skip it.
The four-item maintenance schedule
| Task | Frequency | Applies to | DIY cost |
|---|---|---|---|
| Descaling flush | Annual (6mo above ~12 grains/gal) | Gas + electric | $78–$156 kit, then ~$12–$16/flush |
| Inlet filter/screen cleaning | Quarterly | Gas + electric | Free (rinse) or $10–$20 (replace) |
| Venting & combustion air check | Annual | Gas only | Visual: free; pro inspection $75–$150 |
| Freeze-protection check | Pre-winter, once | Unconditioned-space installs | Free (verify power/gas stays on) |
Descaling: the task everyone knows about
Descaling deserves its own deep dive — brand-specific rules, which flush kits actually work, and the exact scale-damage numbers from the Battelle study are covered in our full tankless water heater descaling guide. The short version for this checklist: flush annually with a $78–$156 kit (more often above 12 grains/gallon hardness), and never use chemical descalers on a Rinnai unit — vinegar only, per Rinnai’s own manual.
Start with a complete flush kit
- A submersible pump, two hoses, and isolation-valve fittings handle the annual flush without hiring a plumber for a routine task.
- Full brand-by-brand descaling rules and kit comparisons are in our dedicated descaling guide.
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Inlet filter cleaning: the task that causes the most service calls
Every tankless unit has an inlet water filter or screen — a small mesh strainer where the cold supply line enters the unit — that catches sediment, pipe scale, and debris before it reaches the heat exchanger. Rinnai, Navien, and Rheem all specify checking it quarterly, more often on well-water systems or older galvanized plumbing.
A clogged inlet screen is, in practice, the single most common cause of the “my tankless heater lost pressure” or “hot water runs lukewarm” complaint — not a failing heat exchanger. It’s also the cheapest fix on this list: most inlet screens pull out, rinse under a faucet, and reseat in under five minutes with no tools beyond a pair of pliers to loosen the union fitting. Replacement screens run $10–$20 when a rinse alone won’t clear caked sediment.
Venting and combustion air: the gas-only annual check
Gas tankless units draw combustion air and exhaust through dedicated venting — PVC for condensing models, stainless steel for non-condensing — and both Rinnai and Rheem’s installation manuals call for an annual visual inspection of that venting for blockage, corrosion, or animal nesting at the outdoor termination. This isn’t optional maintenance trivia: blocked venting on a gas appliance is a carbon monoxide safety issue, not just a performance one.
What to check, per manufacturer guidance:
- Outdoor vent termination is clear of leaves, snow, insect nests, or anything that reduces the opening.
- Vent pipe joints show no visible gaps, cracks, or white staining (a sign of condensate leakage on condensing models).
- Combustion air intake (on units with a dedicated intake, not just room-air draw) is unobstructed.
A full physical inspection of the vent run inside walls or attics is a job for a licensed technician — Rheem’s manual specifically recommends professional inspection if the vent run is inaccessible for a visual check from outside.
Freeze protection: the one-time pre-winter task
Tankless units installed outdoors or in unconditioned spaces (garages, crawlspaces, unheated basements) rely on built-in freeze protection — small electric heating elements around vulnerable components that activate below a set temperature threshold. That protection only works if the unit has continuous power. The most common freeze-damage cause manufacturers cite isn’t a design flaw; it’s a homeowner who switches off a breaker or shuts off gas to an outdoor unit during a vacant period in winter, disabling the freeze protection along with everything else.
Before the first hard freeze each year: confirm the unit’s power (and gas supply, for gas units) stays on continuously, and that any exposed supply lines are insulated per the installation manual. Rheem and Rinnai both exclude freeze damage from warranty coverage when the unit was deliberately powered down in freezing conditions.
What skipping maintenance actually costs
The warranty language is specific, not vague. Per the manufacturer manuals reviewed for our descaling guide: Navien’s warranty excludes “damage caused by poor water quality,” Rheem’s excludes failures from water conditions outside its specification table, and EcoSmart excludes “product failure caused by liming, sediment buildup” outright. A unit can be well inside its stated warranty period by age and still have a claim denied because required maintenance wasn’t documented.
The Battelle Memorial Institute study for the Water Quality Research Foundation is the primary data point on how fast this compounds: gas tankless units run on 26-grain hard water with no descaling failed completely after 1.6 years of equivalent use, and every 5 grains/gallon of hardness cost roughly 4% efficiency at 50 gallons/day usage — before any hard failure occurred. None of that requires exotic conditions; it’s the default outcome of doing nothing.
DIY vs. professional service, and what each actually covers
| DIY | Professional | |
|---|---|---|
| Cost | $78–$156 kit + $10–$20 filter, first year | $150–$350 per HomeGuide; $150–$250 tankless-specific per Angi |
| Covers | Descale flush, inlet filter | Descale, filter, venting inspection, relief-valve test |
| Best for | Owners comfortable with isolation valves | Gas venting checks, warranty-documentation paper trail |
The practical split most owners land on: DIY the annual descale and filter cleaning with a kit that pays for itself after one use, and bring in a technician every year or two specifically for the venting inspection and a documented service record — useful if a warranty claim ever comes up.
The bottom line
Descaling gets all the attention, but it’s one of four recurring tasks — the inlet filter is the one that actually causes most performance complaints, and skipping the annual gas-venting check is a safety issue, not just a maintenance one. Follow the manufacturer’s quarterly-filter, annual-descale, annual-vent schedule and the failure modes documented in the Battelle study simply don’t happen. For the descaling step specifically — brand rules, kit comparisons, and the full scale-damage data — see our complete descaling guide, and if you’re still sizing or shopping, our best tankless water heater roundup covers current-generation picks across gas and electric.