How to descale a tankless water heater

Tankless water heaters heat water as it passes through a narrow heat exchanger. Minerals in the water leave scale inside it, which reduces flow, makes the unit work harder and can trigger error codes or shut the unit down.

Descaling, also called flushing, dissolves that scale. It takes about an hour and a half with a simple kit.

What you need

• A small submersible pump
• Two washing machine style hoses
• A 5 gallon bucket
• About 2 to 4 gallons of white vinegar, or a descaling solution approved by your heater's manufacturer

Most tankless installs have isolation valves with service ports, often sold as a valve kit. If yours does not, a plumber can add them, and it makes every future descale easier.

Step-by-step

1. Turn off the power to the unit, then close the gas valve to the heater.
2. Close the hot and cold isolation valves so water no longer flows through the heater from the house.
3. Remove the caps from the service ports. Put a bucket under them, since some water will come out.
4. Connect one hose from the pump outlet to the cold side service port. Connect the second hose to the hot side service port and put its open end in the bucket.
5. Pour the vinegar or descaler into the bucket and set the pump in it.
6. Open both service port valves and run the pump for about 45 to 60 minutes so the solution circulates through the heater and back into the bucket.
7. Turn off the pump, empty the bucket and flush the heater with clean water. Close the hot service valve, open the cold isolation valve and let fresh water run out through the hot side hose for about five minutes.
8. Close the service valves, remove the hoses and replace the caps.
9. Clean the inlet water filter screen if your model has one, following the manual.
10. Open the hot and cold isolation valves, open a hot water tap to purge air, then turn the gas and power back on.

How often to descale

Once a year works for most homes. With hard water, every six months is safer. Some units tell you when it is time: Rinnai heaters, for example, can show an LC code when they detect scale buildup.

Signs your tankless heater needs descaling

• Hot water temperature swings or runs lukewarm
• Lower flow at hot water taps than before
• A louder unit or new error codes

Prevent scale from building up

A water softener or a scale-prevention filter on the cold supply reduces how quickly scale forms. Setting the temperature no higher than you need also helps, since hotter water drops minerals faster.