Field guide

Plumber Tankless Flush Pages: The Hour That Decides Twenty Years

Direct answer: Flush a tankless water heater once a year, every six months on hard water with no softener. The job is a 45 to 60 minute vinegar circulation through the heat exchanger with a small pump, and skipping it is how a twenty-year unit dies in seven.

Scale is the whole story

A tankless heater has no tank to absorb abuse. Every gallon of water passes through a narrow heat exchanger, and the minerals in that water plate out on the hot metal as scale. Scale insulates. The burner works harder to push the same heat through a coated exchanger, the water runs cooler, the gas bill climbs, and the unit starts throwing codes. Manufacturers are blunt about this in their manuals: scale damage is generally excluded from heat exchanger warranty coverage, because the flush is owner maintenance, not a defect.

The page earns its keep by naming the hardness number, not by saying hard water is bad. Navien sets the line at 4 grains per gallon: at or over that, with no softener or scale filter, descale once a year. Rinnai puts its water quality limit at 200 milligrams per liter of total hardness, about 12 grains per gallon. The US Geological Survey classes water under 60 milligrams per liter as soft, 121 to 180 as hard, and anything above 180 as very hard. A homeowner who looks up their utility report and finds 9 grains per gallon now knows they are on the six-month schedule, and the page that gave them the number is the page they trust.

The flush, in the order the page should list it

The procedure is the same across the major brands, and the page should list it in order so the homeowner can see it is a real job with real steps, not a vague recommendation. Close the isolation valves. Hook a small submersible pump and hoses to the service ports so the solution runs from a bucket, through the heat exchanger, and back into the bucket. Fill the bucket with three to four gallons of undiluted white vinegar. Circulate for 45 to 60 minutes. Drain the solution, then run fresh water through for five to ten minutes to rinse it out. Pull and clean the cold water inlet filter while you are there.

Two details separate a useful page from a generic one. First, never use CLR or bleach: both damage the rubber seals and copper inside the heat exchanger, and every manufacturer says so. Second, the flush only works if the unit has service valves with ports under it. No valves means no clean way to circulate the solution, and that fact belongs on the install page too, because it is a reason to pay for the valves at install time rather than discover the problem at the first flush.

How often, by hardness

The interval follows the water, and the page should print the table instead of a single number. Soft water, under 3.5 grains per gallon: every one to two years, annually if the warranty asks for it. Moderately hard, 3.5 to 7: once a year. Hard, 7 to 10.5: every six to twelve months. Very hard, over 10.5: every six months, and treat the water. Navien advises at least annually, adjusted for mineral content and use. Rinnai suggests every 12 to 24 months, dropping to roughly every six months on very hard water or heavy use. A household that sets its thermostat at or above 120 degrees Fahrenheit will scale faster on the same water, which is worth a line, because it is the kind of specific the competition leaves out.

The codes that mean scale

The homeowner who searches is usually staring at a code. Rinnai displays LC, short for lime scale, and its manuals title the procedure Flushing the Heat Exchanger, Error Code LC or 00, with 00 used on some older models. The page that names the code, says what it means, and gives the flush as the fix is answering the exact search that brought the reader in. A page that talks about maintenance in general while the reader's unit flashes LC is a page that gets closed.

What the page must not promise

Vinegar handles routine scale. A unit that has gone years between flushes, or one already showing a scale code, may need a commercial descaler, usually citric or sulfamic acid based, which works faster on heavy buildup. And a flush does not fix a failed heat exchanger. The honest line is that the flush is the cheapest insurance on the most expensive part of the unit, and the page that says what it cannot do is believed about what it can.

Frequently asked questions

How often should a tankless water heater be flushed?

Once a year in most homes. Move to every six to nine months on hard water with no softener, and soft-water homes can stretch to two years if the manual allows it. The number follows your water hardness, not the calendar alone.

How long does a tankless flush take?

About an hour. Circulate vinegar for 45 to 60 minutes, then rinse with fresh water for five to ten minutes, and clean the cold-water inlet screen while you are there.

Can I use CLR or bleach to descale a tankless water heater?

No. Both damage the rubber seals and copper inside the heat exchanger. Use undiluted white vinegar for a routine flush, or a descaler made for tankless units when the buildup is heavy.

What does the LC code on a Rinnai mean?

Lime scale. The unit is telling you the heat exchanger needs a flush. Older Rinnai models show 00 for the same condition.