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How to descale (flush) a tankless water heater: step-by-step

A tankless water heater heats water as it flows through a small heat exchanger. Minerals in hard water can coat the inside of that heat exchanger as scale, which makes it harder for heat to get into the water. You can't drain a tankless unit like a tank heater. Instead you descale it by pumping a mild acid solution through it in a loop. It's a common homeowner job if your unit has service valves, and most manufacturers describe the procedure in the owner's manual.

Safety first. Turn off the power (and the gas on gas models) before you start, and keep the electrical cord, pump, and any outlet away from splashing water. Use only a descaling solution your manufacturer allows: some chemicals can damage the heat exchanger or void the warranty. If your unit has no service valves, see the section below before you cut into any pipe.

Signs your tankless heater needs descaling

Water doesn't get as hot as it used to, or temperature swings during a shower.
Lower hot water flow than before.
The unit shows a scale, flow, or overheat error code. Look the code up in your manual: many brands have a specific code that means "descale me."
A rumbling or kettle-like sound from the unit while it runs.
You have hard water and can't remember the last time it was done.

What you'll need

A small submersible pump (sump-style utility pump or a pump sold in a tankless flush kit)
Two short washing-machine-style hoses that fit the service valve ports
A clean bucket, roughly 3 to 5 gallons
Undiluted white vinegar or a descaling solution approved by your heater's manufacturer, in the amount your manual calls for
Towels, gloves, and eye protection

Step-by-step

1. Turn the unit off

Unplug the heater or switch off its breaker. On a gas model, also close the gas shutoff valve to the unit. Let it sit a few minutes so the water inside cools.

2. Close the service valves

Most installations have a pair of isolation valves under the unit: one on the cold inlet and one on the hot outlet, each with a small side port that has a cap. Close both main valves (handles crosswise to the pipe). This isolates the heater from the house plumbing so your cleaning loop only runs through the unit.

3. Relieve pressure and connect the hoses

Put a towel under the valves. Slowly remove the caps on the side ports, since a little trapped water will come out. Connect one hose to the cold-side port and run it to the pump's outlet. Connect the other hose to the hot-side port and drop its free end into the bucket. This makes a loop: bucket, pump, cold side, through the heat exchanger, out the hot side, back to the bucket.

4. Add the solution and open the ports

Pour the vinegar or descaler into the bucket and set the pump in it. Open both side port valves (not the main valves). Check that the hot-side hose is secured in the bucket so it can't flip out.

5. Circulate

Plug in the pump and let it circulate for the time your manual specifies. Many manufacturers call for about an hour, longer for heavy buildup. You may see the solution get cloudy or foamy as it dissolves scale. Stay nearby and watch for leaks at the hose connections.

6. Rinse with fresh water

Unplug the pump and dump the used solution (it's mostly spent vinegar, but follow your descaler's disposal directions). Leave the hot-side hose draining into the empty bucket or a drain. Close the cold-side port, open the main cold inlet valve, and let fresh water run through the unit and out the hose for several minutes to rinse out the solution. Then close the cold inlet again.

7. Clean the inlet filter

Most tankless units have a small mesh screen on the cold water inlet, either in the unit's inlet fitting or in the isolation valve. With the water shut off, unscrew it, rinse out any grit or flakes, and reinstall it. A clogged screen alone can cause low flow and error codes.

8. Put it back in service

Close both side ports, remove the hoses, and put the caps back on. Open the main hot and cold valves. Open a hot faucet in the house and let it run until it flows steadily with no sputtering air. Then restore power (and turn the gas back on for gas units) and confirm the unit fires and makes hot water. Check the valves and caps for drips.

How often to descale

Follow your owner's manual first. Recommendations vary by brand and by how hard your water is. Many manufacturers suggest at least once a year, and more often with hard water. If you need to do it frequently, a water softener or a scale-reduction filter made for tankless heaters can stretch the time between flushes. Some units also have a maintenance reminder or scale-detection code that tells you when it's due.

If your unit has no service valves

Older or bare-bones installs sometimes lack isolation valves with service ports, which makes descaling much harder. Adding a tankless service valve kit is a worthwhile one-time upgrade: it means cutting and soldering, pressing, or threading new valves into the water lines under the unit. On a gas unit that work sits right next to the gas line and venting, so if you're not comfortable with it, it's a quick job for a plumber, and every future flush gets easy.

When descaling won't fix it

No hot water at all or the unit won't fire: check power, gas supply, and error codes first. See water heater not working.
Hot water goes cold at low flow, like a trickling faucet: many tankless units need a minimum flow rate to fire. That's normal behavior, not scale.
Water dripping from inside the cabinet or a corroded heat exchanger: that usually needs a service visit or replacement. See water heater leaking.
Error codes that come back right after a flush can point to a failing sensor, venting, or gas supply problem rather than scale.

When to call a plumber

You need service valves installed.
You smell gas or need to work on the gas line or venting.
The unit still throws error codes or underheats after a proper descale and filter cleaning.
The heater is under warranty and you're unsure whether DIY maintenance or a particular chemical is allowed.

Tip: Write the date you descaled on a piece of tape on the side of the unit. It makes it easy to keep on schedule, and it's useful if you ever need a warranty claim.