How to Flush a Tankless Water Heater

1. What Flushing Does, and What You Need

A tankless heater pushes every gallon of hot water through a narrow heat exchanger, and hard water leaves a little calcium and magnesium carbonate behind on every pass. Scale forms fastest where the metal is hottest, which is exactly the surface that is supposed to be transferring heat into the water. A thin layer acts as insulation, the burner or element has to run harder for the same outlet temperature, and the exchanger runs hotter than it was designed to. Flushing dissolves that layer with a mild acid before it gets thick enough to matter.

It is not the same job as draining a tank heater. A tank collects loose sediment at the bottom and you drain it out with a garden hose. A tankless unit has almost nothing to drain; the deposits are bonded to the inside of the exchanger tubes and have to be chemically dissolved by circulating a descaling solution in a loop. Running plain water through it for ten minutes does nothing useful.

The kit is short. You need a five-gallon bucket, a small submersible utility pump, two washing-machine style hoses with three-quarter inch female hose fittings, and three to four gallons of undiluted food-grade white vinegar or a descaler sold for tankless heaters. Noritz specifies three to four gallons of vinegar and two three-quarter inch hoses in its own descaling instructions, and Rinnai's technical bulletin for its Sensei condensing units calls for four gallons of undiluted vinegar circulated at three to four gallons per minute. A small pump of around one-sixth horsepower delivers that easily, because you are only circulating, not lifting water to a second floor.

You can assemble that from parts, but a prepackaged tankless flush kit with the pump, hoses and fittings in one box is the easier buy and costs less than a single service call. It is worth owning outright, because you will use it every year for the twenty-odd years the heater should last. Add a towel, a flashlight, a small adjustable wrench for the filter cap, and a pair of gloves, and you have everything.

The one thing you cannot improvise is a way to connect those hoses to the heater. That is what the isolation valves are for, and whether you have them decides whether this is a one-hour DIY job or a plumber visit. Check before you buy anything else.

2. Isolation Valves: Check for Them First

Look at the pipes directly under the heater. A properly installed unit has two isolation valves, one on the cold inlet and one on the hot outlet, each with a main handle that shuts the line and a smaller service port with a threaded cap and its own little handle. The service ports are where your hoses screw on. Most sets are color-coded blue for cold and red for hot, and the hot-side valve usually carries a port for the pressure relief valve as well.

If you see those valves, you are set. If you see plain copper or PEX running straight into the heater with ordinary shutoffs, you cannot flush it properly without cutting into the plumbing. That is the point to call a plumber and have an isolation valve kit fitted. The kits are made by the major heater brands and by valve specialists such as Webstone, come in sweat, threaded and press versions to match your pipe, and are a modest parts cost; most of the bill is labour.

Treat that as a one-time upgrade rather than a flushing cost. Once the valves are in, every future flush is DIY, and the valves also let a plumber swap the heater out in the future without draining the house. If you are still at the quote stage, insist they are included, which is the same advice we give in our tankless water heater buyer's guide. Leaving them out is a classic way for an installer to shave a quote.

Before relying on old valves, exercise them. Isolation valves that have sat untouched for years can be stiff, and the small service-port handles in particular can weep once disturbed. Turn each one closed and open again gently a couple of times. If a valve will not close fully, you will see water running into the bucket from the house side during the flush, and that one needs replacing before you carry on.

3. How to Flush a Tankless Water Heater, Step by Step

Read your own manual once before you start, because brands differ on one detail: whether the unit stays powered. Rinnai tells you to unplug the unit, and on its Sensei condensing models to plug it back in for 20 seconds and unplug it again so the internal valves park in the right position for flushing; Noritz has you close the gas valve but leave the unit powered and warns that an error code will appear during the flush, which you should ignore. Follow your manual on that point. Everything else below is common to virtually every unit.

1. Shut off the energy

On a gas unit, close the manual gas valve on the supply line to the heater and, if your manual says so, unplug it or switch off its circuit. On an electric unit, switch off every breaker that feeds it at the panel. Whole-house electric units are often fed by two, three or four double-pole breakers, and every one of them has to be off. Then let the heater cool for a few minutes.

2. Isolate the heater and relieve pressure

Close the main handles on both the cold and hot isolation valves. Open a hot tap somewhere in the house briefly to relieve pressure, then close it. Put the bucket under the heater and slowly remove the caps from both service ports; a cup or so of water will come out, which is normal.

3. Connect the pump loop

Screw one hose from the pump outlet onto the cold-side service port. Screw the second hose onto the hot-side service port and lay its free end in the bucket. Pour in three to four gallons of vinegar or the descaler mixed to its label, and drop the pump into the bucket. Now open both small service-port handles. The solution will travel from the pump, in through the cold inlet, through the heat exchanger, and out of the hot outlet back into the bucket.

4. Circulate for at least an hour

Switch on the pump and let it run for a minimum of 60 minutes. Noritz suggests swapping in fresh vinegar after the first 30 minutes and repeating for maximum effect, which is worth doing if the returning solution looks cloudy or you have not flushed in years. Check the hose ends occasionally to make sure the return is flowing and nothing is leaking.

5. Rinse with clean water

Turn off the pump, empty the bucket, and point the hot-side hose into the empty bucket or a drain. Close the cold-side service port, remove that hose, and slowly open the cold isolation valve so fresh water pushes through the exchanger and out of the hot-side hose. Run it for at least five minutes, or about 20 gallons, until it no longer smells of vinegar.

6. Clean the inlet filter screen

Close the cold valve again. Leave the filter in place during the circulation itself, as Rinnai advises, so loosened debris is caught rather than pushed into the exchanger. The cold inlet on almost every tankless unit has a small mesh filter screen, usually behind a cap at the bottom of the unit where the cold line connects. Unscrew it, rinse it under a tap, scrub off any grit with an old toothbrush, and screw it back in hand-tight. A clogged screen produces low flow and fluctuating temperature that looks exactly like a failing heater, so do not skip it.

7. Restore service

Close both service ports and refit their caps, then open the cold and hot main valves fully. Open a hot tap and let it run until the flow is smooth with no spitting air. Only then restore power and gas. Run a hot tap for a minute to confirm the unit fires and holds temperature, and check every fitting you touched for drips.

4. White Vinegar vs Commercial Descaler

Plain white vinegar works, and it is the solution both Rinnai and Noritz specify in their own flushing procedures. Household vinegar is about five percent acetic acid, which is strong enough to dissolve carbonate scale slowly and weak enough that it will not attack copper, stainless steel or brass seals during a one-hour circulation. It is cheap, available at any grocery store, safe to handle, and can go down a household drain afterward. For a heater flushed every year, it is all most people need.

Commercial tankless descalers are usually built on citric, sulfamic or phosphoric acid with corrosion inhibitors added. They dissolve scale faster than vinegar, which matters when the deposit is thick, the unit has already thrown a scale code, or the water is very hard and you want to be sure the hour actually finishes the job. Several heater makers sell their own branded solution. Mix them strictly to the label, because a stronger solution is not a better one.

The rule we would follow is simple. On a heater that is flushed on schedule, use vinegar. On a heater that has gone two or more years without a flush, is showing a scale code, or lives on very hard well water, use a purpose-made descaler, and consider running it twice. Either way, use something sold for potable water equipment; never use muriatic acid, toilet-bowl cleaners or general-purpose lime removers that are not labelled for water heaters, because they can damage the exchanger and void the warranty.

How long to circulate

Sixty minutes is the floor that Rinnai and Noritz both publish, and it is enough for a heater on an annual schedule. If the solution in the bucket is still fizzing or turning cloudy when the hour is up, the acid is still working on scale and is getting used up. Swap in fresh solution and give it another 30 to 60 minutes. Running past that rarely helps; if a heater still shows scale symptoms after two full cycles with a commercial descaler, the deposit is beyond what a flush will clear and it is time for a professional service call.

Do not reuse spent vinegar from one year to the next; Rinnai says to discard it after every flush. Once it has neutralized a load of scale it has little acid left, which is why the second batch in a long flush often does more than the first one did.

5. How Often to Flush: Go by Grains per Gallon

The right interval depends on your water hardness, and you should look it up rather than guess. If you are on city water, your utility publishes it in the annual water quality report, often in milligrams per liter of calcium carbonate. Divide by 17.1 to get grains per gallon. On a private well, a test strip or a lab test will give you the number. The US Geological Survey classes water under 60 mg/L as soft, 61 to 120 as moderately hard, 121 to 180 as hard, and anything above 180 as very hard.

The upper rows matter for warranty reasons as well as performance. Rinnai's published water quality limit for its heaters puts total hardness at up to 200 mg/L, which is roughly 12 grains per gallon, and manufacturers generally exclude scale damage from heat exchanger coverage. Stiebel Eltron gives a similar schedule to ours, recommending a descale once a year in moderately hard water and every six to nine months in very hard water.

Usage moves the number too. A household of five that runs the heater hard all day pushes far more water through the exchanger than a retired couple, and deposits more scale. Hotter setpoints accelerate scaling as well, so a heater set to 140 degrees scales faster than one at 120. If you are on the boundary between two rows, the larger household or higher setpoint should take the shorter interval.

Error codes that mean scale

Some heaters will tell you. Rinnai displays an LC code, short for lime scale, and its installation manuals title the procedure Flushing the Heat Exchanger, Error Code LC or 00, with 00 used on some older models. The unit keeps running when LC first appears, which is exactly why people ignore it; flush it promptly, because the code comes back and the problem gets worse. Navien does not have a dedicated scale code, but an E016 heat exchanger overheat on a unit with good flow and no venting problem is very often scale. Rheem, Noritz and others vary by series, so check the code table in your manual rather than relying on a list online.

Without a code, the symptoms are consistent: hot water that fluctuates or goes lukewarm at higher flow, hot water taking longer to arrive, reduced pressure on hot taps only, and the burner or fan running harder and louder than it used to. Any of those on a heater more than a year past its last flush is a strong hint.

If you are flushing every six months, the honest fix is upstream. A water softener removes the calcium and magnesium that form scale in the first place, and on very hard water it is the single best thing you can do for a tankless heater; our water softener picks are tested for exactly this. A whole-house sediment or scale-inhibiting filter is a lighter-touch option that also protects the inlet screen, covered in our guide to the best water filtration systems for home.

6. Gas vs Electric: What Changes

The pump loop is identical on gas and electric units. What changes is how you shut the heater down, what else you inspect while you are there, and the one mistake on electric units that destroys heating elements in seconds.

Gas units

Close the gas valve on the supply line before anything else, and follow the manual on whether to unplug the unit. A condensing gas heater such as the Rinnai RX199iN or Navien NPE-240A2 also has a condensate drain, and on most installations a neutralizer cartridge that raises the pH of the acidic condensate before it enters the drain. While the heater is off, check that the drain line is clear and running freely, look at the neutralizer media, and replace it if it has dissolved down or the manufacturer's interval has passed. A blocked condensate line will shut a condensing unit down, usually on the coldest night of the year.

Gas units also have venting to check. Look at the exhaust and intake terminations outside for nests, leaves, ice or snow drift, and inspect the visible vent pipe for separated joints. Leave the burner, flame rod and gas pressure to a qualified technician every few years; that is not owner work.

Electric units

Switch off every breaker feeding the heater; a 27 kW whole-house unit is typically on three double-pole breakers, and missing one leaves live elements. The descaling itself is the same. The critical difference comes at the end: before you restore power, open a hot tap and run water through the heater until all the air is purged and the flow is steady. A heating element energized with air around it, rather than water, overheats almost instantly and can burn out. Electric tankless manufacturers are explicit about this, and dry-firing is a failure warranties do not cover.

Electric heaters often show scale differently, too. Instead of a code, you tend to get temperature swings and reduced output as deposits build on the elements. Some designs, such as the bare-wire elements in Stiebel Eltron's Tempra line, are less prone to scaling because the element mass is low and there is little residual heat after flow stops. Less prone is not immune; Stiebel itself recommends descaling on a six to twelve month schedule depending on hardness.

One more difference worth knowing: an electric unit has no venting, condensate or combustion parts, so its annual maintenance list is shorter. That is a genuine point in its favour, which we weigh alongside its drawbacks in our breakdown of tankless water heater pros and cons.

7. The Full Tankless Water Heater Maintenance Checklist

Flushing is the big one, but tankless water heater maintenance covers a handful of other checks that take minutes and catch problems early. Do them on the same day as the flush so nothing gets forgotten.

Pressure relief valve. Lift the lever on the relief valve briefly with a bucket under the discharge pipe. Water should flow freely and stop cleanly when you release it. A valve that drips afterward or does not discharge at all needs replacing, and that is a cheap part guarding against an expensive failure.

Inlet screen on well water. If you are on a well, the inlet screen can clog in months rather than years, especially after pump work stirs up sediment. Check it quarterly until you know how quickly your water loads it. A whole-house sediment filter ahead of the heater is a far better long-term answer than scrubbing the screen every few weeks.

Freeze protection. Units in garages, attics or on exterior walls rely on electric freeze protection that only works with power. If you expect an extended outage in a hard freeze, shut off the water and drain the heater using the isolation valve service ports, the same ones you flush through. A frozen heat exchanger usually splits, and that is not covered.

Error history and settings. Many units keep a log of recent codes you can read from the controller or app. A cluster of the same code since last year is worth investigating even if the heater seems fine. While you are there, confirm the setpoint is still at 120 degrees unless you have a reason for higher; a hotter setpoint wastes energy and scales the exchanger faster.

If your heater is a few years from the end of its life and the checklist keeps turning up problems, it may be time to look at a replacement rather than another service call. Our tested tankless water heater picks cover the units we would buy now, and our tankless sizing guide will make sure the next one actually keeps up in winter. For the full installed picture, see our guide to what a tankless water heater costs.