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WaterHeaterCalc

Descaling

How Often to Descale a Tankless Water Heater at 25 Grains per Gallon and 140 F

Researched from the DOE test procedures, NFPA 54 and NEC code tables, and manufacturer specifications. Updated .

Quick answer

At 25 grains per gallon (428 parts per million) with a 140 F setpoint, descale a tankless water heater roughly every 3 months. That water is classed as severe. The higher setpoint shortens the interval, because calcium carbonate deposits faster on a hotter surface.

Every tankless water heater owner descales, and the manufacturers all say annually and then say more often in hard water without defining hard. This page fills that gap.

At 25 grains per gallon, which is 428 parts per million and counts as severe water, a 140 F unit wants flushing about every 3 months. That is a planning convention rather than a measured figure, and it is labeled as one throughout.

Descaling interval against hardness

Descaling interval at a 140 F setpoint
HardnessIn ppmClassificationDescale every
3 gpg51 ppmSoft18 months
5 gpg86 ppmModerately hard14 months
7 gpg120 ppmModerately hard14 months
10 gpg171 ppmHard9 months
12 gpg205 ppmVery hard7 months
15 gpg257 ppmVery hard7 months
20 gpg342 ppmExtremely hard5 months
25 gpg428 ppmSevere3 months

Convention Source: A planning table. No test procedure publishes a descaling interval, and this is built on the published fact that calcium carbonate deposition rises with both hardness and surface temperature. Your water utility publishes its hardness in its annual report, and a test strip costs a couple of dollars. Guessing the hardness makes the interval meaningless.

What the 140 F setpoint does to it

Scale deposits faster on a hotter heat exchanger surface, which is why a 140 F unit in this water wants flushing at 3 months rather than the 4 months a 120 F unit would.

That is a real argument for storing at a lower setpoint, but it runs directly into the Legionella argument for storing hotter. The 120 against 140 comparison lays out both sides honestly rather than picking one.

What scale actually costs before it causes a fault

Efficiency, silently. A scaled heat exchanger transfers less heat for the same gas, so the unit burns longer for the same hot water, and the bill goes up months before anything faults. Then flow sensors and the exchanger begin to misbehave, usually showing as temperature swings or an error code.

By that point the repair costs far more than the decade of flushes that would have prevented it, which is the entire economic argument for a 3 month interval and a set of isolation valves.

Whether to treat the water instead

That is a different question and a bigger one. A scale inhibitor upstream keeps hardness in suspension and slows deposition without removing it. A softener removes it and changes the water chemistry throughout the house, which has its own consequences including a much shorter magnesium anode life on any storage tank.

Choosing between them is water treatment rather than water heating, and HomeWaterCalc covers that side properly. What belongs here is how fast scale deposits on a heat exchanger and what to do about it, which is this page.

Flush kits, solution and service valves

Plain five percent white vinegar is what most manufacturers specify and it works. Never use concentrated vinegar above five percent: it is a corrosive hazard to the operator and to the heat exchanger.

Frequently asked questions

How often should I descale a tankless water heater at 25 grains per gallon?
About every 3 months at a 140 F setpoint. That water is severe at 428 parts per million. Deposition rises with both hardness and outlet temperature, so a hotter setpoint or heavier daily use shortens the interval further.
How do I find my water hardness?
Your water utility publishes it in its annual water quality report, usually in parts per million, which you divide by 17.1 to get grains per gallon. A test strip costs a couple of dollars and takes a minute if you are on a well or want to check it yourself. Guessing it makes every interval on this page meaningless.
What should I flush with?
Plain five percent white vinegar is what most manufacturers specify, and it costs a fraction of a branded descaler. Circulate about four gallons through the isolation valves for 45 to 60 minutes with a small submersible pump, then flush with clean water. Never use concentrated vinegar above five percent, which is corrosive to both you and the heat exchanger.
Does a tank water heater need descaling too?
Not in the same way. A tank accumulates sediment at the bottom rather than scaling a narrow heat exchanger, so the maintenance is an annual drain and flush rather than an acid circulation. Sediment at the bottom of a gas tank insulates the water from the burner and wastes fuel every day it stays there, which is the same economic argument in a different form.
What happens if I skip it?
Efficiency falls first and silently, because a scaled exchanger transfers less heat for the same gas. Then the unit starts swinging in temperature or throwing error codes as the flow sensors and the exchanger foul. The repair at that point costs far more than a decade of flushes, and some manufacturers treat a unit with no service history as outside warranty.

Water temperature is a genuine tradeoff, not a single right answer. Water at 140 F scalds a child in seconds and 120 F is the common scald safety setpoint, but Legionella grows in roughly the 77 to 113 F range, which is why storing hotter and mixing down at a thermostatic mixing valve is what the plumbing trade actually does. Households with young children, older adults or anyone with reduced sensation have a different risk balance, and that is a decision for the household rather than for a calculator.