Bosch Water Heater Troubleshooting by Model Type

The Short Answer: Start by identifying which Bosch you have. Therm and Greentherm units are gas tankless, Tronic and Powerstar units are electric tankless, and Tronic “T” models are small electric tanks. On every type, Bosch’s first checks are power, a clogged inlet filter, too little flow to switch the heater on, and swapped hot and cold connections. Gas and electrical faults beyond that need a technician.

Which Bosch do you have?

The rating label on the side or front of the cover gives the model. The type decides what can go wrong.

Gas tankless (Therm and Greentherm)

Bosch’s Therm 830 ES troubleshooting page says the heater needs at least 0.5 gallons per minute to switch on. In practice, a quart container should fill in 25 seconds or less at the tap. If the burner won’t ignite, Bosch’s checks are power to the unit, correct water inlet plumbing, enough flow and a clean inlet filter screen. For water that is not hot enough, it adds the temperature setting, gas pressure and gas type. Gas pressure and gas type are technician checks.

What you can check:

• That the unit has power and the display (if fitted) is lit.
• That the water shutoff valves at the heater are fully open.
• Flow at the tap you’re using. Low-flow aerators and shower heads may not reach the activation flow on their own.
• Whether only one fixture is affected. If it is, a mixing valve in that fixture may be the cause rather than the heater. See no hot water to the kitchen sink.
• The inlet filter screen, if your owner’s manual assigns that cleaning to the owner. Turn the water off first.

Codes. Bosch defines codes by model generation. Its service bulletin for the 250SX, 635ES and 2400E families defines EA as “burner flame not recognized by control unit with water flow.” The technician’s checks it lists cover the spark electrode connections, a correctly wired and grounded outlet, a closed gas valve, the wrong gas type, and air in the gas line after a new installation. If your model isn’t covered by that bulletin, look up the code in your own manual instead of a general list.

Electric tankless and mini-tank (Tronic, Powerstar)

Bosch’s electric water heater FAQ lists the common reasons an electric tankless won’t turn on: no power or incorrect wiring, a clogged inlet filter, low water pressure, reversed hot and cold connections, and a tripped thermal cutout. For lukewarm water it lists flow too high for the model, a low temperature setting, cold incoming water in winter, and supply voltage below the rating. Bosch notes that a 240-volt heater running on 208 volts loses about 25% of its heating ability.

Larger units use several double-pole breakers. Check that every breaker labeled for the heater is on. You can reset a tripped breaker once. If it trips again, leave it off. Bosch’s FAQ describes the thermal cutout reset on its electric tankless models, and that reset is behind the cover. Leave it to an electrician or technician, who should also find out why it tripped. See tankless water heater keeps tripping breaker for the breaker side.

On Tronic mini-tanks, the high-limit reset sits on the thermostat. Bosch’s FAQ tells owners to disconnect power before pressing it. A high limit that trips again points to a thermostat or element fault. Never power a mini-tank that hasn’t been filled.

Leaks

On a tankless unit, a drip from a fitting or the relief valve can often be repaired. A leaking heat exchanger usually means replacing the unit or a major part. On a Tronic mini-tank, water from the tank itself means replacing the heater. Bosch also warns that mini-tank anodes need regular inspection to avoid early tank failure.

Frequently Asked Questions

Q: My Bosch tankless clicks but never lights. Can I light it manually? A: No. Therm and Greentherm models use electronic ignition and have no manual lighting procedure. See manually lighting a heater with electronic ignition.

Q: Why does my Bosch electric tankless go lukewarm in winter? A: The incoming water is colder, so the heater has to raise the temperature further at the same flow. Bosch lists cold inlet water and high flow as causes of low temperature. Using a lower flow usually brings the temperature back up.

Q: Where are older Bosch models covered? A: For AquaStar units, see AquaStar tankless troubleshooting. For the Powerstar electric line, see Powerstar tankless troubleshooting.