Descaling a Tankless Water Heater When Your Water Comes From Brighton or Thornton
Tankless heaters wear out their heat exchangers through scale more than through age. How hard your water is decides how often to flush, and the published reports give a starting point.
Call (720) 794-0054What scale does inside a tankless unit
A tankless heater heats water as it flows through a narrow exchanger. Minerals that stay dissolved in cold water come out of solution when heated and stick to the metal. The layer is thin at first. It slows heat transfer, so the unit works harder to deliver the same temperature.
Symptoms show up in stages. Output drops, error codes appear and flow falls. Left alone, the exchanger can overheat and fail. Descaling clears the buildup before that point.
What the water reports say
Thornton's 2025 water quality report lists hardness from 152 to 356 milligrams per liter, with an average around 230, or about 13.5 grains per gallon. The report notes that its treatment does not remove hardness. Hard water of that level is in the range where many manufacturers call for yearly flushing, though you should follow your own manual.
The City of Brighton's 2023 report describes shallow wells, a reverse osmosis and greensand treatment plant, and purchased water from Thornton and Denver Water. It does not list a hardness figure. Your tap may carry a blend, so the number can differ from Thornton's.
The most reliable number is one measured at your own tap. A test strip costs little. A lab test gives more detail.
Match the flush schedule to the water
A rough planning guide
| Hardness at the tap | Typical approach | Notes |
|---|---|---|
| Soft to slightly hard | Flush on the schedule in the manual | Often yearly or longer |
| Moderately hard | Flush yearly | Check the inlet screen too |
| Hard to very hard | Flush yearly or more often | Consider a softener |
| Unknown | Test first, flush yearly in the meantime | Keep a log |
Always defer to the manufacturer. Some warranties require documented maintenance. Keep a record of each flush with the date and the product used.
Tankless due for a flush?
Call and tell the dispatcher the brand and whether it has service valves.
How a descaling flush works
- Isolate the unitClose the gas and water valves and relieve pressure.
- Connect hoses to the service valvesA small pump pushes the solution through the exchanger, which is why service valves matter.
- Circulate descaling solutionA mild acid, often food-grade vinegar or a manufacturer-approved product, runs through for the time the manual calls for.
- Rinse thoroughlyClean water flushes out the solution and loosened scale.
- Clean the inlet screenDebris here reduces flow and can mimic scale problems.
- Restart and testCheck temperature and flow at a fixture.
If your unit has no service valves, ask a plumber to install them. It is a small upgrade and makes every later flush simple. More detail is on the tankless service page.
Signs you are overdue
- Hot water takes longer to reach the temperature you set.
- The unit shows an error code that mentions flow, temperature or the exchanger.
- Output falls when two fixtures are open.
- You hear popping, rumbling or kettling sounds.
- The unit shuts down mid-shower.
- White flakes show up in aerators or on shower heads.
A softener changes the math
A water softener removes the hardness minerals before they reach the heater. That cuts scale at the source. Even with a softener, annual checks make sense, since a softener can run out of salt or fail without warning. The softener service page explains sizing and bypass valves. A softener also protects fixtures, dishwashers and tanks, not just the tankless unit.
What not to do
- Do not use a strong acid that the manufacturer does not approve.
- Do not flush without closing the gas valve first.
- Do not skip the rinse step.
- Do not ignore an error code and keep running the unit.
- Do not assume a new unit is immune. Scale starts on day one.
If a unit has already failed, ask whether the exchanger can be cleaned or whether replacement makes sense. See water heater replacement if a swap is on the table.
Pairing descaling with a general check
A flush visit is a chance to check the vent, the gas connection and the condensate drain if the unit is a condensing model. A blocked vent or a clogged condensate line causes shutdowns that look like scale problems. Ask the plumber to read the stored error history before starting. Patterns in the codes can show a problem that scale alone does not explain. If your unit starts failing in unexpected ways, water heater repair covers the diagnostic path.
What the flush water can tell you
The rinse water after a flush is a free report on your heat exchanger. Clear water with a few flakes suggests light scale. Cloudy or milky water means the solution lifted a lot of buildup, and a second pass may be worth planning.
Catch some of the first runoff in a clear container and look at it in daylight. Write what you see in the maintenance log. Over a few years, the log shows whether your water is hard enough to justify a softener, or whether a yearly flush is keeping up.
If the unit still shows flow or temperature codes after a flush, the cause may not be scale. Blocked vents, clogged inlet screens and gas supply limits produce similar symptoms. The tankless water heater service page covers those and explains what a full service visit checks.
Keeping a maintenance log
Write down every flush: the date, the solution used, the circulation time and what the rinse water looked like. Cloudy or flaky rinse water shows how much scale came off. A log also helps if a warranty claim ever needs proof of maintenance.
Store the log near the unit or in a phone note. Add the model and serial number, and the date the unit was installed. If a different technician services the unit later, the history saves time. If a unit already shows heavy scale, plan a second flush a few weeks after the first. Stubborn deposits sometimes need more than one pass.
Sources
Local facts on this page come from public documents.
Quick answers
The city's 2023 report does not list a hardness number. Test your tap. Thornton's 2025 report lists 152 to 356 milligrams per liter.
Many owners do, with service valves, a pump and the right solution. The manual describes the steps.
Follow the manufacturer. Many approve white vinegar or a specific descaling product.
The manual states the circulation time. Setup and rinsing add to it.
It reduces buildup, but periodic checks are still wise.
Keep the unit running at full output
Call to book a flush, add service valves or plan a softener.