How often should you drain and flush a tankless water heater?
Updated July 2026
Flush (descale) a tankless water heater once a year, and every 6 months if your water is hard and you haven't installed a softener or scale-inhibitor filter. Unlike a tank, a tankless unit heats water inside a narrow heat exchanger, and mineral scale forms exactly where the heat is: each layer insulates the exchanger, so the burner works harder for cooler water until the unit throws an error code and quits. Most manufacturers recommend annual descaling and several make it a warranty condition.
What changes the interval
| Your situation | Interval | Why |
|---|---|---|
| Soft or softened water | Every 12 months | Minimal scale; the annual flush is mostly inspection and insurance |
| Moderately hard water (3–7 grains) | Every 12 months | The standard manufacturer recommendation |
| Hard water, no treatment (7+ grains) | Every 6 months | Scale forms directly on the heat exchanger at double speed |
| Recirculation loop / high household demand | Every 6–9 months | More runtime means more water evaporated against hot metal |
| Scale-inhibitor filter installed | Every 12 months | The cartridge slows scale, and the cartridge itself needs annual replacement |
| Error codes about temperature or flow | Flush now | Scale codes (e.g. reduced-flow warnings) are the unit asking before it quits |
The cost of skipping it
What skipping it costs: scale quietly taxes every shower first (efficiency drops and output temperature wobbles), then the unit starts short-cycling and throwing error codes, typically at year 3–5 in hard water. A scaled-shut heat exchanger is the expensive ending: $600–1,200 to replace the exchanger, or $2,500–4,500 installed for a new unit when the repair math fails, on hardware that should have lasted 20 years. Skipping the flush can also void the warranty that would have covered some of that. The flush itself: $25 of white vinegar DIY, or $150–250 pro.
How to do it (60–90 minutes · DIY with a small pump, two hoses, and 2–3 gallons of vinegar)
- Turn off power (and gas, if gas-fired) and shut the cold inlet and hot outlet isolation valves. If your install lacks isolation/service valves, have a plumber add them; they turn every future flush into a simple job.
- Connect hoses to the service ports: one from a bucket through a small utility pump into the cold-side port, one from the hot-side port back to the bucket.
- Fill the bucket with 2–3 gallons of food-grade white vinegar (or manufacturer-approved descaler) and circulate it through the unit for 45–60 minutes.
- Dump the vinegar, flush the loop with fresh water for 5–10 minutes, and close the service ports.
- Clean the cold-inlet screen filter (and the water filter, if fitted); a clogged inlet screen mimics scale symptoms.
- Reopen the isolation valves, restore power/gas, and run a hot tap to confirm steady temperature.
Knowing the interval is the easy half. OnOtto reminds you at the right interval, and it won't let you snooze it into next year. Tasks persist until they're done, and completing the annual descale schedules the next one automatically.
Put this on autopilot, free for 30 daysFrequently asked questions
Is the flush really a warranty condition?
Check your manual, but several major manufacturers condition heat-exchanger warranty coverage on regular maintenance, and hard-water scale damage is commonly excluded as neglect. Keep a dated record of each flush; a recurring task with completion history (which OnOtto keeps automatically) is exactly the paper trail a warranty claim wants.
What are the signs a tankless unit needs descaling now?
Water temperature that wanders during a shower ("cold-water sandwich" getting worse), reduced hot-water flow, longer waits, rising gas use, kettle-like rumbling, or maintenance/flow error codes on the display. Any of these in hard-water country: flush first; it's the cheapest fix and the most likely one.
Should I install a water softener or scale filter instead of flushing?
In genuinely hard water (10+ grains), yes: treatment plus an annual flush beats semi-annual flushing forever. A whole-home softener protects every appliance, and a point-of-use scale-inhibitor cartridge ($100–200 plus yearly cartridges) protects just the heater. Neither eliminates the annual flush; both stop the every-6-months treadmill.
Does a tankless heater need anything besides the flush?
A short annual list, usually done the same hour: clean the cold-inlet screen, wash the air-intake filter on condensing models, check the condensate drain runs clear, and eyeball the venting for obstructions or nests. No anode rod and no tank to rust: that's the maintenance you're not doing versus a tank.