Every gallon of hard water that passes through a tankless heater leaves a trace of calcium and magnesium behind on the hottest surfaces. Over months, those traces become a crust. Descaling dissolves it before it restricts flow, trips shutdowns or damages the heat exchanger. It's the single most important maintenance task a tankless unit has.
How scale forms inside a tankless unit
A tankless heat exchanger is a network of narrow tubes or channels with a powerful burner or element beneath. When water flows, heat passes through the metal into the water almost instantly. Dissolved minerals are less soluble at high temperatures, so they come out of solution right at the metal surface, exactly where heat needs to pass. The crust grows thicker with each use. Because the passages are narrow, even a modest layer reduces flow and insulates the metal, which then runs hotter, which speeds up scaling further. It's a self-reinforcing cycle.
Signs your unit needs descaling
- A maintenance or scale-related code on the display
- Water that doesn't reach its usual temperature
- Temperature swings during a steady shower
- Noticeably reduced hot water flow
- Shutdowns during long showers or when two fixtures run
- Rushing, ticking or kettling sounds from the unit
Descaling on a schedule, before these appear, is better still.
California water and descaling
Water hardness varies enormously across the state, and even between neighboring districts where supplies are blended. As a broad pattern, much of the inland south, including large parts of the Inland Empire, and most of the Central Valley have heavily mineralized water, so tankless units there need descaling more often. Some coastal and northern districts are softer and can go much longer. Private wells in the foothills and the north vary from one well to the next.
We don't publish hardness figures, because they differ by district and change as supplies change. Your water provider can tell you how hard your supply is, and a technician can tell you what the scale inside your unit says about it.
Winter makes scale matter more. Colder inlet water in the Bay Area, the valley and the foothills already pushes units harder; a scaled heat exchanger loses what little margin remains, which is why so many descaling calls come in the first cold weeks of the year.
The first visit tells us a lot about your water.
How a descaling visit works
The technician turns off the unit's gas and power and closes the isolation valves on the hot and cold lines. They connect a small circulating pump and hoses to the service ports on those valves, with the pump sitting in a bucket of descaling solution. The solution circulates through the heat exchanger for a set period, dissolving the scale; the color of the returning solution often tells its own story. Then the solution is drained, fresh water is flushed through thoroughly, and the inlet filter is removed, cleaned and replaced.
Finally the valves are reopened, air is purged, the unit is powered and fired, and the technician checks temperature and flow at the taps and clears any maintenance code. Many technicians combine the visit with a check of the venting, combustion air, condensate system and burner, since the unit is already being serviced.
Why isolation valves matter
Isolation valves are a pair of valves on the unit's water connections, each with a small service port. They let the technician isolate the heater from the house plumbing and circulate solution through it without cutting pipes. Most professionally installed tankless units have them. If yours doesn't, the first descale takes longer because the plumbing has to be opened, and it's the right moment to fit a set so every future descale is quick.
Reducing how often you need it
Descaling treats the result; reducing scale formation stretches the interval. A water softener greatly reduces scale if it's maintained. Other scale-control treatments exist, with varying claims, and a water treatment specialist can explain what suits your supply. Keeping the unit's temperature setting no higher than you need also helps, because scaling speeds up at higher temperatures. None of these replace periodic checks entirely in hard water.
What descaling can't fix
Descaling removes mineral build-up; it doesn't repair damage the build-up has already caused. A heat exchanger that has been overheating for a long time can develop pinhole leaks or fatigue, and a unit that has shut down repeatedly may have stressed sensors or components along the way. Descaling also can't make an undersized unit bigger or fix a gas supply that can't keep up at full fire. If the unit still underperforms after a thorough descale, the technician will look at those possibilities next and explain what they find. That's one more reason to descale on a schedule rather than waiting for symptoms.
Why it isn't a DIY job
Leave it to the technician
- Shutting off and restoring the unit's gas and electrical supply
- Operating isolation valves and service ports under pressure
- Handling and disposing of descaling solution
- Opening the unit's cover or cleaning internal components
Wrong solutions, trapped air or valves left partly closed can damage a heat exchanger that costs far more than the visit.
What affects the cost
| Factor | Effect |
|---|---|
| Isolation valves present | Makes the visit quick and straightforward. |
| No valves | More labor the first time; fitting valves adds parts. |
| Scale severity | Heavily scaled units may need a longer circulation or a second pass. |
| Combined service | Adding combustion, venting and condensate checks is efficient. |
| Outdoor or awkward mounting | Access affects time. |
How long it takes
A descale on a unit with isolation valves is typically a single, fairly short visit. Units without valves, heavily scaled exchangers or combined servicing take longer. If the technician finds scale has already damaged something, they'll explain and price the repair before going further.
If temperatures swing or showers cut out, scale is the first suspect.
Tankless descaling questions
Descaling circulates a mild acidic solution through the unit's heat exchanger to dissolve mineral scale that hard water has left behind. A pump pushes the solution in through one isolation valve and out through the other for a period, then the unit is flushed with clean water and returned to service. The inlet filter is usually cleaned at the same time.
There's no universal interval; it depends on how hard your water is, how much hot water the household uses, the temperature setting and the manufacturer's guidance. In hard-water areas many owners descale on a regular schedule, and some units show a maintenance reminder. After the first visit, a technician can recommend a sensible frequency based on how much scale they found.
Scale keeps building. Flow drops, temperatures fluctuate, the unit overheats and shuts down, error codes appear, and eventually the heat exchanger can be damaged by sustained overheating. A heat exchanger replacement costs far more than years of routine descaling, and scale-related damage may raise questions under a manufacturer's warranty terms; check yours.
Yes, but it takes more work, because the technician has to open the plumbing to connect the pump. Many will fit isolation valves during that first visit so future descaling is quicker and less costly. It's one of the most useful small upgrades on a tankless installation in hard water.
A properly maintained softener greatly reduces scale, so descaling can be needed much less often. It doesn't necessarily eliminate it, and softeners need their own upkeep. Other scale-control treatments exist too. Your technician can talk through what makes sense for your water.
We don't give DIY instructions for it. Descaling involves isolating the unit's gas and power, working with valves under pressure, and handling descaling solution, and a mistake can damage the heat exchanger or cause leaks. A technician does it quickly and checks the unit's operation afterward.
Common signs are a maintenance or scale-related code on the display, water that won't get as hot as it used to, temperature swings during a steady shower, reduced flow, shutdowns during long showers, and unusual rushing or kettling sounds. Any of these on a unit that hasn't been descaled recently is worth a call.
The descaling itself is similar, but condensing units also have a condensate system and sometimes a neutralizer that the technician checks at the same visit. Venting and combustion checks are often combined with descaling too, since the unit is already being serviced.
No. Descaling is done through the water connections with the unit in place on the wall. If the technician notices anything about the mounting or connections while they're working, they'll mention it. Isolation valves, if the unit doesn't have them yet, can be added so future descaling is quicker.