How do you descale a tankless water heater, and how often?

Why tankless units need descaling

Hard water carries dissolved minerals. When it is heated, some of those minerals drop out as scale. In a tank, most of it settles on the bottom. In a tankless unit, it coats the narrow passages of the heat exchanger. DOE's Building America guide notes that scale restricts flow through a tankless heat exchanger and can shorten the unit's life^[6].

The makers build alarms for it:

Brand	Scale or service code	What it means
Rinnai	LC, SS	LC: scale is building up and the unit must be flushed. SS: a time-based "service soon" reminder[2]
Rheem and Ruud	1L	Lime detected in the heat exchanger; drain and flush to prevent permanent damage[1]
Navien	E760	Flushing or service reminder[3]

Ignore them long enough and the unit starts tripping overheat codes. All three makers exclude scale damage from their warranties^[2][7][8].

How often to descale

It depends on your water hardness. Rinnai's SENSEI manual gives the clearest table^[2]:

Water hardness	Grains per gallon	Rinnai's guidance
Soft, 0 to 60 mg/L	0 to 4	No treatment listed
Slightly to moderately hard, 61 to 160 mg/L	4 to 9	Flush residential units once a year (commercial twice); softener or scale control recommended
Very hard, 161 to 200 mg/L	9 to 12	Softener or scale control strongly recommended
Extremely hard, over 200 mg/L	12+	Softener or scale control required

Rinnai also notes that scale builds faster when the unit is set to a high temperature^[2]. Your water utility's annual water quality report usually lists hardness. See our water hardness by city page and the hard water maintenance plan.

What the makers' procedures have in common

Every procedure below assumes the unit has isolation (service) valves on the hot and cold lines. Rinnai strongly recommends them for exactly this job^[2]. If your unit has none, a plumber needs to add them before a flush is practical.

Step	Rheem SVC820[1]	Rinnai SENSEI RX[2]	Navien NPE-2[3]
Shut down	Gas off and stays off; hot and cold supply off; power disconnected	Gas off; controller off; hot and cold shutoff valves closed	Power off and disconnected; hot and cold valves closed
Equipment	Submersible utility pump (Rheem flush kit RTG20124), hoses, 5-gallon bucket	Pump, hoses, pail	5-gallon or larger bucket, 3 hoses, circulation pump
Solution	2 gallons food grade white vinegar	4 gallons undiluted food grade white vinegar	Cleaning solution diluted with water
Circulate	45 to 60 minutes	At least 1 hour, about 4 gallons per minute	At least 45 minutes
Rinse	Fresh water through the unit about 5 minutes	Fresh water for 5 minutes	Fresh water for 5 minutes
Finish	Clean the inlet water filter	Clean the in-line inlet filter	Clean the cold water inlet filter

The general sequence: isolate the unit, connect the pump outlet hose to the cold side service valve and a drain hose to the hot side, put both hose ends in the bucket of solution, circulate, then rinse with house water to a drain, clean the inlet filter, close the service valves, open the main valves and restore gas or power. Follow your own model's manual exactly; valve labels and order differ.

Brand details that matter

• Rinnai SENSEI units have a descaling mode set with buttons on the circuit board behind the front panel. The display counts down 60 minutes, then shows a rinse prompt^[2]. That step involves opening the cabinet, so many owners leave SENSEI flushes to a pro. Rinnai's general flushing guide for owners says to use undiluted white vinegar and never chemical solutions^[4].
• Rheem says heavy scale may need the flush repeated^[1]. The 1L code is cleared afterward by holding the MIN and MAX buttons together for 10 seconds^[1].
• Navien calls the flush "a somewhat complicated procedure," says to contact an authorized technician or licensed professional if you are unsure of any step, and warns that improper maintenance can void the warranty^[3].

Should you do it yourself?

Rinnai and Rheem publish owner-facing flush steps, so a careful homeowner can do it on many units^[1][4]. It makes sense when:

• Isolation valves with hose ports are already installed.
• You can shut off gas or power at the unit, and the job does not require opening the cabinet.
• You have the right pump, hoses and the solution your maker names.

Hire a pro when the unit lacks service valves, the procedure goes behind the cover, the unit is still throwing overheat codes after a flush, or you would rather not handle valves and a pump around a gas appliance. HomeGuide's national range for a professional flush and descale is $75 to $300 (August 2025)^[5]. A pro visit can also cover the inlet and air filters, vent terminals and condensate line in the same trip. See tankless descaling service cost.

Mistakes to avoid

• Skipping the rinse. Vinegar left in the lines ends up in your hot water.
• Opening the hot side valve too early. Every procedure rinses through to a drain first.
• Forgetting the inlet filter. Loosened scale collects there.
• Using an unapproved chemical. Use what the maker names.
• Restoring gas before water flows normally. Bring water back first, then gas or power.

When to call a pro

• A scale or service code (LC, 1L, E760) and no isolation valves on the unit.
• An overheat code that returns after a flush.
• Any flush that requires opening the front cover or using board buttons.
• Very hard water, to size a softener or scale control system along with the flush.