Why scale matters more on tankless units

Water in Fullerton is a blend of local groundwater and imported water, and it carries enough dissolved calcium and magnesium to be considered hard. When that water is heated quickly, some of the minerals come out of solution and stick to hot surfaces.

In a tank, much of that material settles at the bottom. In a tankless heater, it builds up on the walls of a compact heat exchanger where water passes at speed. A thin layer reduces how well heat transfers; a thick layer restricts flow, makes the unit work harder and can trigger error codes or overheating protection.

Signs scale is building up

  • Water temperature swings during a shower even though the setting has not changed
  • Hot water takes longer to arrive or never gets as hot as it used to
  • The unit shows an error or maintenance code on its display
  • Reduced hot water flow at fixtures while cold water flow is normal
  • Unusual noises from the unit while it is firing
  • White mineral deposits around fixtures and showerheads in the rest of the house

How descaling usually works

Always follow the manufacturer's manual for your model. This is the general sequence a plumber follows.

  1. Shut off power and gas.The unit is switched off and the gas valve to it closed before any water work begins.
  2. Close the isolation valves.Service valves on the cold inlet and hot outlet isolate the heater from the house plumbing.
  3. Circulate a descaling solution.A small pump circulates a manufacturer-approved solution through the heat exchanger using the service ports on the isolation valves.
  4. Flush with fresh water.The solution is drained and the exchanger flushed with clean water.
  5. Clean the inlet screen.The small filter screen on the cold inlet is removed and cleaned of debris.
  6. Restore and test.Valves reopened, air purged, power and gas restored, and the unit tested for temperature and error codes.

At a glance

What builds up
Calcium and magnesium scale from hard water
Where
Inside the heat exchanger and at the inlet screen
How often
Per the manufacturer's schedule, adjusted for water hardness and use
Needed for service
Isolation valves with service ports on the heater lines
Possible add-on
A softener or scale-reduction system, if your plumber recommends one

Questions to ask before service

  1. What descaling interval does the manufacturer recommend for our water?
  2. Are my isolation valves in good condition, and do they have service ports?
  3. Is the error code on my unit related to scale or to something else such as venting or gas supply?
  4. Would a scale-reduction system help in our case, and what upkeep does it need?
  5. Will you clean the inlet filter and check the venting while you are here?

Common questions

Can I descale a tankless heater myself?

Some owners do, using the manufacturer's instructions and a descaling kit, as long as the unit has proper isolation valves. If you are not comfortable working near gas and electrical connections, have a licensed plumber do it.

Does skipping descaling void the warranty?

Warranty terms vary by manufacturer and often address scale and water quality. Read your warranty documents or ask the manufacturer directly.

Will a water softener stop all scale?

A properly maintained softener reduces hardness significantly. Ask your plumber how it would interact with your heater and your household's needs.

General information for Golden Hills, not a quote or professional advice. In an emergency, call 911.