How to Descale a Tankless Water Heater (and When a Softener Means You Can Stop)

A tankless water heater on hard water needs descaling, usually once or twice a year, because calcium and magnesium build up inside its heat exchanger faster than anywhere else in the house. The job takes about an hour and $50–$100 of equipment the first time. Here's how to do it, how often, and what changes once the water feeding the heater is softened.

Why tankless heaters scale up

A tankless unit heats water on demand by forcing it through a heat exchanger with narrow passages at 120–140°F. Hardness minerals come out of solution fastest at exactly those temperatures, and they stick to the hottest surface in the system. As the scale layer grows, the heater has to fire harder to hit the set temperature, flow drops, the unit cycles or throws a fault code, and eventually the exchanger fails. Every major tankless manufacturer publishes a hardness limit in its manual and recommends periodic flushing above it.

Signs it's due: hot water that fluctuates or goes lukewarm during a shower, lower hot-water flow than when the heater was new, the unit taking longer to deliver hot water, a scale or maintenance code on the display, or a rumbling or whistling noise when it fires.

What you need

• Isolation valves with service ports on the cold inlet and hot outlet (most installers fit these; if yours didn't, have them added)
• A small submersible pump (1/6 hp is plenty)
• Two washing-machine-style hoses with ¾″ hose fittings
• A 5-gallon bucket
• About 4 gallons of undiluted white vinegar, or the descaling solution your heater's manufacturer specifies

Pre-packaged tankless flush kits bundle the pump, hoses and bucket together and cost about the same as buying the parts.

How to descale a tankless water heater, step by step

1. Cut the power at the breaker or unplug the unit. On gas units, also turn the gas valve off. Never run a descaling cycle with the burner able to fire.
2. Close both isolation valves (cold in and hot out) so the heater is cut off from the house plumbing.
3. Relieve pressure by slowly opening the hot-side service port with a bucket underneath; some water will come out.
4. Connect the hoses. Pump outlet to the cold-side service port; a second hose from the hot-side service port back into the bucket.
5. Fill the bucket with the vinegar or descaling solution and put the pump in it.
6. Open both service ports and switch on the pump. The solution now circulates through the heat exchanger and back into the bucket.
7. Circulate for 45–60 minutes. On heavily scaled units the solution may foam or discolor; that's the scale dissolving.
8. Flush with clean water. Switch off the pump, empty the bucket, close the cold service port and open the cold isolation valve so fresh water pushes the solution out through the hot hose for about 5 minutes.
9. Clean the inlet screen. Close the cold valve, remove the small filter screen at the cold inlet, rinse out any grit or scale, and refit it.
10. Restore service. Close and cap both service ports, open both isolation valves, open a hot tap to purge air, then restore gas and power.

How often to descale

Your city's annual water quality report lists hardness; on a well you'll need a test. To convert: 1 grain per gallon equals 17.1 mg/L (ppm).

How to stop descaling every few months

Descaling treats the symptom. On hard water the scale is back within months, and every cycle of buildup costs efficiency and heat-exchanger life in between flushes. The permanent fix is to remove the hardness before it reaches the heater:

• A salt-based water softener on the main line removes calcium and magnesium entirely, so scale can't form. It's the option heater manuals mean when they specify softened water, and the only one that works on well water or hardness above about 10 gpg. A metered Clack WS1 (from $1,319) or Fleck 5600 SXT (from $879) covers most homes.
• A salt-free conditioner keeps minerals from sticking to hot surfaces without removing them. It needs no drain, salt or power, and suits city water under about 10 gpg where a softener can't be installed. See the Filtersorb salt-free conditioner (from $899) or NuvoH2O Home (from $621).

Hardness thresholds by heater, salt vs. salt-free for this use, and sizing for a tankless heater's peak flow are all on our water softeners for tankless water heaters page.

Quick answers

Can I use vinegar to descale a tankless water heater? Yes; undiluted white vinegar is what most manufacturers allow. Some specify their own descaler; use that if your manual says so.

How long does descaling take? About an hour: 10 minutes to set up, 45–60 minutes of circulation, 10 minutes to flush and restore.

Can I skip descaling if I have a softener? Mostly. Flush every 2–3 years as a check and clean the inlet screen; you'll usually find little or no scale.

Does descaling void the warranty? No. Regular flushing is what the manuals require on hard water. Skipping it, or running on very hard water with no treatment, is what puts warranty coverage at risk.

Questions on hardness or which softener fits your heater: call 800-374-2143.