Bosch says descaling may be necessary periodically, where water is highly mineralized, and it sets the lowest single-family hardness limit of the brands covered on this site: 100 ppm. Above that, Bosch recommends a water conditioner or softener. Every Utah hardness figure on this page is above it, so a Bosch here needs treated water, a regular descale, or ideally both.
Signs your Bosch needs descaling
Bosch’s TWH-G2-13 bulletin names the symptoms of a scaled heat exchanger: lower flow rates, A7 and E9 codes and boiling sounds. In everyday terms, you might notice:
- A rumbling or boiling noise from the heater while hot water runs.
- Weaker hot water flow at showers and taps, even with the faucet wide open.
- A code on the display, most often E9 or A7, sometimes E1.
The heat exchanger is the coiled tube inside the heater that the burner warms. In hard water, calcium and magnesium stick to its inside walls as scale. Scale insulates the tube, so spots run hotter than normal. Water touching those hot spots can briefly boil, which makes the noise. The same hot spots trip the temperature sensors, which is where the codes come from.
Bosch’s rules on hardness, temperature and warranty
Bosch’s descaling bulletin for its Therm models (C1210ES, C1050ES, C950ES, 940ES and 830ES) lists a maximum total hardness of 100 ppm. It recommends a water conditioner or softener for anything above the listed levels.
Temperature matters too. Bosch says a softener is strongly recommended above 140 F outlet temperature, because higher set temperatures raise the risk of scale. If you keep a Bosch set hot for a large tub or a long shower line, that rule applies to you even more.
On the warranty, Bosch is short and clear: damage from scale/mineral deposits is not covered. Our warranty and scale guide covers how that compares with other brands and what records to keep.
Seeing a code? Call to book a descale
Bosch’s limit and Utah water
Bosch’s limit is in parts per million (ppm). Utah water reports usually use mg/L, and for drinking water the two are the same. The USGS calls water 61–120 mg/L “moderately hard”, so Bosch’s limit falls inside the moderately hard band. The Utah figures below are all higher.
- Saratoga Springs: the city’s water averages 142 mg/L.
- Orem, Provo and Lehi: treated surface water from the Central Utah Water Conservancy District’s Don A. Christiansen plant, which supplies part of each city’s water, measured 112–154 mg/L in Orem’s report. Orem’s system as a whole runs harder.
- Riverton: the city buys all its water from the Jordan Valley Water Conservancy District, whose 2025 report averaged 182.4 mg/L.
- Spanish Fork: the city listed 231–349 mg/L.
Every one of these is over Bosch’s limit. Our Utah hard water guide has more cities and the full USGS scale.
Bosch error codes linked to scale
| Code | Bosch’s meaning | Likely cause | Safe check for you | When to call |
|---|---|---|---|---|
| E9 | the temperature limiter has sensed over 220 F, which may indicate scale build-up in the heat exchanger | Scale causing hot spots, or low water flow | Make sure the isolation valves are fully open | Call; it is a safety shutdown |
| A7 | a problem related to the outlet hot water temperature sensor | Scale around the sensor, or a failing sensor | Note when it appears and whether flow has dropped | Call if it comes back |
| E1 | the hot water sensor is reading above 185 F, possibly due to scale build-up | Scale, or water that is already warm coming in | Check whether a solar system or another heater feeds this one | Call if it repeats |
These come from Bosch service bulletins for the 250SX, 635ES and 2400E models and from Bosch’s current troubleshooting page. Codes can differ on other models, so check your own manual. The error code hub compares Bosch with other brands.
E9: overheat at the temperature limiter
E9 means the temperature limiter has sensed over 220 F, which may indicate scale build-up in the heat exchanger. The temperature limiter (also called the ECO, for energy cut-off) is a safety switch on the heat exchanger. It shuts the burner off before the metal gets dangerously hot.
When scale coats the inside of the heat exchanger, water can’t pull heat away fast enough. The metal near the limiter overheats and E9 trips. Utah’s hard water speeds up that coating, and a high temperature setting makes it worse. Scale isn’t the only possible cause, so a technician should check it. Don’t keep resetting a heater that keeps showing E9.
A7: outlet temperature sensor
Bosch says A7 points to a problem related to the outlet hot water temperature sensor. The outlet sensor reads how hot the water is as it leaves. A7 can mean the sensor itself has failed. But Bosch’s TWH-G2-13 bulletin also lists A7 among the results of scale, possibly because scale changes how heat reaches the outlet. A technician can test the sensor and look for scale in the same visit.
E1: hot water sensor reading too high
On Bosch’s troubleshooting page, E1 means the hot water sensor is reading above 185 F, possibly due to scale build-up. The other cause Bosch names is pre-heated feed water, which can happen when a heater is fed by a solar system or another heater. If neither applies to your home, scale is the more likely culprit.
Seeing a code? Call to book a descale
How Bosch’s descaling procedure works
Bosch’s Therm descaling bulletin describes the process. This outline is here so you know what the job involves. It isn’t a full guide, and any gas, venting or electrical step belongs to a technician.
- Shut off the water. The first step is to shut off water with isolation valves, the shut-off valves the installer fitted on the hot and cold lines. Without them, a proper descale isn’t practical. Our isolation valve guide explains what to look for.
- Fill the bucket. Bosch recommends straight white vinegar. A commercial descaler also works if you follow its own dilution directions.
- Circulate. A pump runs the solution through the heat exchanger until the returning solution comes out clear. Bosch ties the time to the result, not the clock, so heavily scaled Utah heaters may take longer.
- Rinse and restore. The solution is cleared out with fresh water before the heater goes back into service.
Bosch doesn’t give a fixed interval, so a technician can judge the next date from what comes out of the heater.
Keeping scale off a Bosch
Bosch’s own advice is a water conditioner or softener above its limit, and a softener above its temperature threshold. In Utah that covers most homes. Treatment slows scale; it doesn’t replace descaling. Our scale prevention page compares the options.
Seeing E9? Call to book a descale. If it’s A7 or E1, or the unit is rumbling, a local tankless technician can descale it or diagnose a repair.