A tankless water heater has no tank to drain, so owners assume there's nothing to maintain. Wrong — it has something fussier: a heat exchanger with narrow water passages that slowly coat with mineral scale, especially on hard water. Scale insulates the metal, the burner works harder, temperatures swing, and eventually the unit throws error codes or shuts down mid-shower. The yearly routine below is what most manufacturers call for in the manual, and it's mostly a vinegar flush plus three quick checks.
This is the heart of the job. Homes with soft water can often stretch the interval; hard water areas should treat it as non-negotiable every year, and every six months isn't overkill. If your unit has no isolation valves with service ports, that's a one-time plumbing addition worth paying for — it turns every future flush into a hose hookup instead of a pipe-cutting project.
Shut the unit down. Turn off power at the breaker or unplug it, close the gas or water valves per your manual, and let a hot tap run briefly to take the scald edge off the water in the lines.
Close the isolation valves. Turn the hot and cold isolation valves so the heater is separated from the house plumbing, then open the service ports and relieve the pressure. Expect a cup or two of water — towel down.
Hook up the pump loop. Connect one hose from the pump (sitting in the bucket of vinegar) to the cold service port, and a return hose from the hot service port back into the bucket. Open the service port valves.
Circulate for 45 to 60 minutes. Run the pump and let the vinegar loop through the heat exchanger. It comes back cloudy and bluish-green as it strips scale — that's the job working. Check the connections once for drips, then let it run.
Flush with clean water. Dump the vinegar, refill the bucket with clean water (or connect the cold supply per your kit's directions), and circulate several minutes to rinse the acid out of the exchanger.
Put it back in service. Close the service ports, open the isolation valves, restore power, and run a hot tap until the water runs hot and steady. Recheck the ports for drips.
Why the screen matters more than it looks. The filter screen is a two-minute job that prevents the most misleading symptom in tankless ownership: the unit shutting down or surging because it can't get enough water. Owners replace sensors and call for service over a screen clogged with pipe grit. Clean it every time you descale and you'll probably never meet that error code.
Why the condensate trap matters. High-efficiency condensing units produce acidic water as they run, and the trap keeps exhaust gases from pushing back through the drain line. A trap that's dried out, clogged, or draining uphill will shut the heater down or corrode the drain it feeds. It takes one minute to look at. Look at it.
| Task | How often | Time | What skipping it costs you |
|---|---|---|---|
| Descale flush | Yearly; every 6 months on hard water | 45–60 minutes | Scale buildup, temp swings, early exchanger failure |
| Inlet screen rinse | With every flush | 2 minutes | Low-flow shutdowns and false error codes |
| Condensate trap check | Yearly | 5 minutes | Shutdowns, corroded drain piping |
| Vent inspection | Yearly, and after big storms | 10 minutes | Blocked exhaust — a genuine safety issue |
Skipping years because the water still runs hot. Scale builds silently. By the time showers turn inconsistent, the exchanger has been cooking itself for a long while, and a flush may no longer restore it.
Forgetting to reopen the valves. The classic post-flush panic: no hot water, one closed isolation valve. Before you call anyone, walk the valve positions.
Using the wrong cleaner at the wrong strength. Follow the manual — standard white vinegar or a manufacturer-approved descaler. Random acids and mystery chemicals can attack seals and void warranties.
Ignoring a repeating error code. A code that clears and returns is the unit telling you something specific. The manual's code table is the cheapest diagnostic tool you own.
Tankless maintenance is one annual hour: flush, rinse the screen, glance at the trap and vents, read the codes. Owners weighing the switch can see how this routine compares with tank upkeep in our tankless vs tank water heater breakdown, and the flush discipline itself is a cousin of the annual drain and flush a tank water heater routine. If your unit is already flashing warnings, run through the signs a water heater is failing before you spend money on parts.
Once a year is the standard recommendation from most manufacturers. On hard water — or if you notice temperature swings or longer waits for hot water — every six months is the safer rhythm.
Yes. Undiluted white vinegar circulated for 45 to 60 minutes is the method most manuals describe. Commercial citric-acid descalers work faster and are also fine. Either way, flush with clean water afterward.
Mineral scale coats the heat exchanger, forcing the burner to run hotter for the same output. Symptoms creep in: temperature swings, reduced flow, error codes, shutdowns. Left long enough, the exchanger fails — and it's the most expensive part in the unit.
Typically $150 to $300 in most areas, depending on your market and whether isolation valves are already installed. A pump kit for doing it yourself costs less than one pro visit and lasts for years.
Most manufacturers require documented periodic maintenance — especially descaling in hard-water areas — as a warranty condition. Keep receipts or a dated note of each flush. The manual for your model states the exact requirement.