Close the heater's isolation valves, connect two washing-machine hoses to the service ports, and use a small submersible utility pump (1/6 to 1/4 hp) in a bucket to circulate about 2 to 4 gallons of white vinegar or the maker's descaler through the heat exchanger for 45 to 60 minutes. Then flush with clean water for 5 minutes, clean the inlet screen, and restore the valves. Most makers recommend descaling once a year, or every 6 months with hard water.
Why tankless heaters need flushing
Heating water makes calcium and magnesium drop out as scale, which coats the narrow passages of a tankless heat exchanger. Scale insulates the exchanger, so the burner or elements work harder, efficiency falls, and the heater may show error codes or overheat. Descaling dissolves that buildup before it causes damage, and most warranties require regular maintenance in hard-water areas.
What you need
- A small submersible utility pump, about 1/6 to 1/4 hp, with a 3/4-inch garden hose outlet.
- Two washing-machine hoses (3/4-inch female ends).
- A 5-gallon bucket.
- 2 to 4 gallons of food-grade white vinegar or the heater maker's descaling solution.
- Isolation valves on the hot and cold lines with service ports; most tankless installs include them. If yours does not, a plumber can add a set.
Pre-assembled flush kits bundle the pump, hoses and bucket. Any small submersible utility pump with a hose outlet does the same job.
Step by step
- Turn off power to the heater (and the gas valve for gas units).
- Close the hot and cold isolation valves to cut the heater off from the house.
- Connect hoses: one from the pump outlet to the cold-side service port; one from the hot-side service port back into the bucket.
- Open the service ports and relieve pressure into the bucket.
- Fill the bucket with 2 to 4 gallons of vinegar or descaler and submerge the pump.
- Circulate for 45 to 60 minutes (follow the maker's time if it differs). The solution may change color as scale dissolves.
- Rinse: remove the pump, empty the bucket, and open the cold isolation valve to flush clean water through to the bucket for about 5 minutes.
- Clean the inlet filter screen on the cold-water inlet.
- Close the service ports, open both isolation valves, open a hot tap to purge air, then restore power and gas.
How often to flush
| Water hardness | Grains per gallon | Descale |
|---|---|---|
| Soft | Under 3.5 | Every 1 to 2 years |
| Moderately hard | 3.5 to 7 | Once a year |
| Hard | 7 to 10.5 | Every 6 to 12 months |
| Very hard | Over 10.5 | Every 6 months, and consider a softener or scale-reduction filter |
Your water utility publishes hardness, or a cheap test strip will tell you. Related: utility pump vs transfer pump and hot water recirculation pumps.
