Tankless Water Heater Descaling: $300, How Often, and the Codes That Mean Scale

What descaling does

A tankless heats water as it runs through a heat exchanger. Hard water leaves scale on the inside of it, the unit works harder for the same hot water, and eventually it throws a code or shuts down. Descaling dissolves the scale.

Descaling solution pumped through the heat exchanger at the service valves
Rinsed out with fresh water afterward
Cold water inlet filter pulled and cleaned
Air intake filter cleaned where there is one, and the vent checked
Fired back up and checked at a hot tap

A SwiftPro plumber working on the water and gas connections under a Navien tankless water heater in a Northern Virginia basement — A SwiftPro plumber on the water and gas connections under a Navien tankless water heater in a Northern Virginia basement.

How often, by your water

Navien's manual sets the line at 4 grains per gallon: at or over it, with no softener or scale filter, descale once a year.

Utility	Hardness	Yearly descale?
Arlington County (Washington Aqueduct water)	8.3 grains per gallon	Yes, yearly
City of Manassas Utilities	2.7 grains per gallon	Follow the unit’s reminder
Fairfax Water	3 to 10.8 grains per gallon	Yes, yearly
Fairfax Water (Washington Aqueduct water)	5.6 to 10.9 grains per gallon	Yes, yearly
Loudoun Water	4.1 to 10.8 grains per gallon	Yes, yearly
Prince William Water	2.6 to 10.5 grains per gallon	Yes, yearly
Town of Herndon (Fairfax Water's water)	4.1 to 10.8 grains per gallon	Yes, yearly
Town of Leesburg Utilities	5.8 to 8.8 grains per gallon	Yes, yearly
Town of Vienna (Fairfax Water and Washington Aqueduct water)	5 to 10 grains per gallon	Yes, yearly
Virginia American Water	4.6 to 11.2 grains per gallon	Yes, yearly

Hardness from each utility's own 2025 report; Loudoun Water's from Fairfax Water's Potomac plant, where its water quality page points. Navien NPE-A2/S2 installation manual, water hardness table. See your town on the water heater flush guide.

Codes that mean scale

Brand	Code	What it means
Rinnai	LC	Scale is building up and the unit needs a flush.
Rheem	1L (A01L on IKONIC models)	Lime has built up in the heat exchanger.
Noritz	C1	The descaling reminder has come due.
Navien	E016 or E030	Overheating. Flushing the heat exchanger is one of the fixes Navien lists.

An ignition or fan code is something else, and a descale won't change it.

How to descale a tankless yourself

Turn off the power to the unit, and the gas valve on a gas model.
Close the hot and cold service valves under the unit. Most installs have a pair with small drain ports.
Hook a small submersible pump and hoses to the ports so the solution runs from a bucket, through the heat exchanger, and back into the bucket.
Fill the bucket with white vinegar or a descaler made for tankless water heaters and let the pump circulate it, about an hour, or as long as your unit's manual says.
Drain the solution, then run fresh water through to rinse it out.
Pull and clean the cold water inlet filter, open the service valves, and turn the unit back on. Check it at a hot tap.

It only works with service valves. No valves with ports under the unit means no clean way to circulate the solution. We can add a set, priced before we start.

A SwiftPro plumber's gloved hand on the service valves under a Navien tankless water heater in Northern Virginia — A SwiftPro plumber's hand on the service valves under a Navien tankless water heater.

What the makers say

Brand	Their manual
Navien	A flush once a year wherever the water runs 4 grains per gallon or harder with no softener on the line, which is nearly all of Northern Virginia. Manual
Rinnai	Once a year in a home wherever the water runs 4 grains per gallon or harder, and its manual says skipping the flush will damage the heat exchanger. An LC code on the display means it is scaling up and must be flushed. Manual
Noritz	Once a year from 3 grains per gallon up with no softener. Its C1 code is a descaling reminder, and Noritz will not cover the damage if the reminder came up and nobody flushed it. Manual
Rheem	Flush the heat exchanger once a year, the manual says, or risk higher running costs and early failure. An A01L code on the display means lime has built up inside. Manual
Takagi	Its standard warranty does not cover damage from water harder than 7 grains per gallon in a single-family home (X3 models are the exception). Every utility on the chart below tops 7 at some point in the year except the City of Manassas. Manual

More on keeping a tankless going in tankless water heater maintenance in Northern Virginia, and the visit itself on SwiftPro's flush and descaling page.

Questions people ask

How much does tankless water heater descaling cost?

$300 at SwiftPro, regularly $350, for one tankless water heater, any brand, with the plumbing safety check included. You pay after the visit.

How often should a tankless water heater be descaled?

Once a year wherever the water runs 4 grains per gallon or harder without a softener, per Navien's manual. Nearly every utility in Northern Virginia runs harder than that. The City of Manassas, at 2.7, is the exception.

Why is descaling more than a tank flush?

It's a different job. The unit is isolated at its service valves, descaling solution is pumped through the heat exchanger, then it's rinsed and the inlet filter cleaned. That takes a pump, hoses and the solution.

What if my tankless has no service valves?

Most installs have them. If yours doesn't, we price adding a set before we start and you decide. They make every descale after that easier.

Does skipping descaling void the warranty?

It can leave you without coverage for the damage it causes. Navien, Rinnai and Rheem exclude damage from lime and scale, and Noritz won't cover a unit whose descaling reminder came up and was never done.

Get it flushed for $99.

Normally $199, with the plumbing safety check included. Tankless descaling is $300, regularly $350. Pick a time online, and you pay after the visit.

Pick a time Call (571) 396-0299