Inside a tankless heater, water passes through narrow tubes in a heat exchanger that gets very hot for short bursts. Hard water leaves calcium and magnesium on those hot surfaces, and the layer builds with every use. Because the passages are small, even a thin coating restricts flow and slows heat transfer. The unit works harder, sensors read odd temperatures, and eventually it throws a fault code or shuts down.
Most manufacturers describe a descaling or flushing procedure in the owner's manual, and how often to do it depends on water hardness and how much hot water the household uses. Your water is hard here, so ask about a schedule rather than waiting for a symptom.
Signs descaling may be overdue
- An error or maintenance code that points to scale or overheating
- Hot water temperature that wanders even at steady flow
- Reduced hot water flow rate compared with when the unit was new
- The unit cycling off during long showers
- White crust at fixtures and showerheads elsewhere in the house
- No record of the unit ever being descaled
How descaling is usually done
This is the general shape of the job. Follow the manufacturer's procedure for your model.
- Shut down.Power off the unit and close its gas supply and water isolation valves.
- Connect a pump.Hoses go to the service ports on the isolation valves, with a small pump and a bucket of descaling solution.
- Circulate.The solution loops through the heat exchanger for the time the manufacturer specifies.
- Rinse.Fresh water flushes the solution out of the unit.
- Clean the inlet screen.The cold-water inlet filter is removed, rinsed and reinstalled.
- Restart and test.Valves reopen, the unit is powered and fired, and temperatures and codes are checked.
What affects the time and cost of descaling
- Access
- Garage, closet, attic or exterior wall mounting
- Valves
- Whether service valves are already installed
- Buildup
- How heavy the scale is and whether the inlet filter is clogged
- Extras
- Venting or combustion checks done on the same visit
Ask before the job
- Are you following this manufacturer's descaling procedure?
- What solution will you use, and how will it be disposed of?
- Will you clean the inlet filter and check the gas pressure and venting while you're there?
- How often do you recommend descaling for our household and water?
Common questions
Can I descale my own tankless heater?
Some owners do with a pump kit and the manufacturer's instructions. Shutting off gas, handling service valves and restarting the unit carefully all matter. If you're unsure, have a plumber do it.
Will a water softener remove the need to descale?
Softened water greatly reduces scale, but the manual may still call for periodic maintenance. Ask how softening affects your unit's schedule.
Can a badly scaled heat exchanger be saved?
Often descaling restores performance, but severe buildup or damage may not fully recover. A plumber can tell you whether the unit responded.
General information for Orange, not a quote or professional advice. In an emergency, call 911.