Buderus Indirect Water Heater Problems
On this page (6 sections)
The Short Answer: A Buderus indirect tank, such as a Logalux, has no burner of its own. Boiler water flows through a coil inside it to heat the domestic water. If the house heats but the hot water doesn’t, either the tank isn’t calling for heat (sensor, setting or wiring) or boiler water isn’t reaching the coil (pump, zone valve or trapped air). A leaking tank means replacement.
How the system makes hot water
When the tank’s sensor or thermostat reports that the water has cooled, the boiler control switches to hot-water mode. Many Buderus and other boiler controls give hot water priority, which means they pause space heating while the tank heats. A circulator pump or zone valve then sends hot boiler water through the coil until the tank is satisfied. A fault anywhere in that chain leaves the tank cold while the boiler itself works normally.
Narrow it down
| What you notice | Likely cause |
|---|---|
| House heats, hot water is cold, and the boiler never seems to switch to hot water | Tank sensor or thermostat not calling, a hot-water setting turned off or set low, or a wiring or control fault |
| Boiler runs for hot water, but the tank stays cool | Pump or zone valve to the coil not working, air in the coil, or a closed isolation valve |
| Hot water runs out quickly or recovers slowly | Low setting, boiler water temperature too low for hot-water mode, or a mixing valve set low |
| Scalding at some taps, lukewarm at others | Mixing valve fault or a crossover |
| Boiler keeps losing pressure, or hot water is discolored | Possible coil leak between the boiler water and the domestic water. Needs prompt service |
| Water under the tank, from the tank itself | Tank leak. The tank needs replacing |
| Relief valve on the tank discharging | Pressure or temperature problem. Never cap it |
What you can check
- The boiler control display. Check whether hot water production is switched on. Many controls show a tap symbol or a hot-water temperature. Note what it shows and any fault code.
- The tank temperature setting on the control or on the tank’s own thermostat. It may have been turned down or set to a holiday mode.
- Power to the boiler and any separate pump or zone control. Check that breakers or service switches are on.
- A mixing valve at the tank’s hot outlet. If it’s set low, the taps will be cool even when the tank is hot.
- Visible leaks at the tank’s connections, and the boiler pressure gauge if it has one. Steadily falling pressure points to a leak somewhere in the boiler circuit.
Purging air from the coil and working on the boiler circuit are service tasks. Don’t open boiler-side valves or bleed points unless the boiler’s owner manual tells you to.
What the technician will check
The technician will confirm that the control receives the tank’s call for heat, test the sensor against the manufacturer’s values, and check that the pump or zone valve gets power and actually moves water. They’ll purge air from the coil if needed and confirm the boiler’s hot-water settings. If the boiler loses pressure or the domestic water is discolored, they’ll test for a coil leak, which mixes boiler water with drinking water. They’ll also check the tank’s anode if your model has one, and the system’s thermal expansion control.
Repair or replace?
Sensors, pumps, zone valves and mixing valves are routine repairs. A leaking tank shell can’t be repaired. A leaking internal coil on a typical residential indirect tank usually means replacing the tank too. Buderus documentation for current and discontinued models is available from Bosch Home Comfort’s product information pages.
Frequently Asked Questions
Q: Why does the house get cold while the tank is heating? A: Many boiler controls give hot water priority, so space heating pauses while the tank recovers. Short pauses are normal. Very long ones suggest the tank is recovering slowly. Possible causes are air in the coil, a low boiler water temperature, or a heat exchanger restricted by scale.
Q: My hot water smells of sulfur. Is the tank failing? A: Not necessarily. Odors usually come from a reaction between the water and the anode, or from bacteria. See tankless water heater sulfur smell for the causes, which are similar for storage tanks, and have the anode checked.
Q: Is an indirect tank the same as a combi boiler? A: No. A combi heats tap water on demand through a plate heat exchanger. An indirect tank stores hot water heated by the boiler. For a combi, see Baxi combi no hot water, which explains how combis work.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- Bosch: product information and manuals (opens in a new tab)Find current or discontinued model documentation for gas or electric equipment.
- A. O. Smith technical bulletin 45: thermal expansion (opens in a new tab)Background on pressure rise in a closed water system. Pressure testing and repairs are professional work.
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