Buderus Indirect Water Heater Problems

The Short Answer: A Buderus indirect (Logalux and cousins) stores potable water and steals heat from the boiler through a coil. Cold showers with a hot boiler mean the DHW sensor, pump, or priority setting. A wet tank or rusty hot water from a pinholed coil is replacement. Do not treat it like a gas tank with a pilot light.

Search Intent & The Real Problem

Bosch/Buderus boilers are paired with a matching indirect. The house heats. The kitchen does not. Or the boiler short-cycles all afternoon on DHW. Installers often leave the tank sensor in the wrong well, invert the mixing valve, or never enable DHW on a newly programmed control.

What other guides get wrong

Generic troubleshooting pages often jump to a part or a reset before identifying the unit, fuel, and failure sequence. Start with the model label and the safest outside checks. If the fault involves gas, combustion, pressure, or 240 volts, stop at that boundary and call a licensed professional.

Step-by-Step Troubleshooting

Step 1: Identify tank vs boiler vs mixing valve

• What to look for: Rating plate (Logalux, S-series, etc.), a sensor well on the tank, a DHW pump or zone valve, a mixing valve on the domestic hot outlet. Photograph the piping before anyone “straightens it out.”
• Required tools: Phone camera, flashlight.

Step 2: Does the boiler display a DHW call?

• What to look for: A tap icon or DHW temperature on the Bosch/Buderus controller. No icon when you run a faucet (on tanks with a flow sensor) or when the tank is below setpoint (sensor tanks) means programming or a failed probe.
• Required tools: The boiler manual for that control (Logamatic, RC35, EMS, etc.).

Wrong sensor well = the control thinks the tank is always hot or always cold. I find probes shoved under insulation instead of into the well at least once a month.

Step 3: DHW pump / zone valve and coil delta-T

• What to look for: Pump spinning, union valves open, supply pipe hot, return cooler if the tank is taking heat. Both pipes hot and the tank cold = air or a closed coil. Both pipes cold during a call = pump, relay, or control output.
• Required tools: Contact thermometer.

Purging the hydronic side is a boiler procedure. Do not open domestic faucets to “bleed the coil.”

Step 4: Mixing valve and recirculation

• What to look for: Scald at one fixture, tepid at another; a recirc line that returns into the cold inlet and strips the tank; a mixing valve that failed closed on the hot port.
• Required tools: Thermometer at tank-out vs mixed-out.

Step 5: Anode, coil leak, vessel leak

• What to look for: Magnesium anode gone (rotten-egg hot water), coil leak (boiler losing pressure while domestic system gains pressure or discolors), foam jacket wet at the base. Coil-to-potable leaks are a health issue — shut the domestic and call it in.
• Required tools: Anode socket if the port is free, pressure gauges on the boiler.

A pinholed coil is not a “flush.” It is a tank.

When DIY Ends: Calling a Licensed Pro

Stop at boiler programming, hydronic purging, gas combustion, and any leak from the vessel or coil. You may verify valves are open, read the control, and replace a mixing valve on accessible copper if you are competent with potable water. EMS bus sensors, DHW priority parameters, and coil replacement are licensed boiler work. Bosch/Buderus warranties are allergic to DIY control jumps.

Frequently Asked Questions

Q: Why is my Buderus indirect not making hot water? A: The boiler is not getting a DHW call (sensor or programming), the DHW pump/zone is dead, or the coil is air-bound. House heat working does not prove the tank circuit.

Q: My boiler pressure drops and my hot water looks rusty. What failed? A: Suspect a leaking coil mixing hydronic water into the domestic tank. Shut the system down and hire a licensed tech. Do not keep filling the boiler.

Q: How often should a Buderus indirect be serviced? A: Check the anode annually in aggressive water, verify mixing-valve outlet temperature, and include the tank in the boiler’s yearly combustion service.