Vaillant ecoTEC Plus 831 Error F75 Diagnosis: Solution for Lack of Hot Water

⚠️ WARNING / SERIOUS RISK. This appliance operates with critically dangerous components (High Voltage, Temperatures, or Gas). Improvised DIY repairs of critical devices can be fatal or prohibited by law. Physical intervention requires qualified specialized technicians to issue compliant safety certificates. ReeFix provides this diagnosis EXCLUSIVELY for educational and informational purposes.

QUICK TEST

Error F75 on your Vaillant ecoTEC Plus 831 indicates that the boiler does not detect a pressure change when the circulation pump should start, causing it to block and prevent hot water production. Perform these quick checks to understand the situation:

1. Check System Pressure: Check the boiler's pressure gauge. The pressure should be between 1.0 and 1.5 bar (when cold). If it's below 1.0 bar, try to restore the pressure using the filling loop.
2. Listen to the Pump: When the boiler attempts to start (or resets the error), try to listen if you hear the circulation pump starting. You should perceive a slight hum or the sound of water circulating. If you hear nothing, or hear an abnormal noise (e.g., screeching, blockage), it could be a pump issue.
3. Bleed Air: Do you hear gurgling in the radiators or pipes? This could indicate air in the system. Try bleeding the radiators with a Radiator bleed key.

DECISION FORK

• If the pressure was low and you were able to restore it (1.0-1.5 bar) and error F75 disappears: The problem was air or low pressure. Monitor the system. If the error returns frequently, there is a leak and a technician is needed.
• If the pressure is correct, but you don't hear the pump or the error persists after bleeding: The problem is more serious and requires a technician's intervention. Go to the "CONFIRMED DIAGNOSIS" section to understand the most probable causes and what to communicate to the professional.
• If you don't have the correct pressure and cannot restore it: Contact a technician. There might be a fault with the filling loop or a significant leak.

CONFIRMED DIAGNOSIS

If the quick tests haven't resolved the issue, the most probable causes of error F75 are:

1. Blocked or Faulty Circulation Pump (Probability: 55-65%)

   • Key signals: The boiler attempts to start but you hear no noise from the pump, or you hear a weak hum without water circulation. Radiators remain cold even if the boiler turns on for a short period.
   • Why it happens: The pump is an electromechanical component that pushes water through the system. Over time, it can wear out, become blocked by debris, or fail electrically, failing to generate the necessary flow and thus the expected pressure change.
   • Often happens when: The boiler is not used for long periods or the system has not been cleaned regularly, favoring the accumulation of dirt that blocks the impeller.
   • Estimated cost (replacement part): Medium-high.
   • Replacement part: Vaillant ecoTEC Plus 831 Circulation Pump

2. Malfunctioning Water Pressure Sensor (Probability: 25-30%)

   • Key signals: The pump seems to be working correctly (you hear the noise), but error F75 persists. The pressure reading on the boiler display might be inconsistent or not change at all when the pump starts.
   • Why it happens: The sensor detects pressure variations and communicates them to the electronic board. If it's faulty, it sends incorrect data or no data, making the boiler believe that the pump is not creating the required pressure differential.
   • Counter-example: If the pump makes no noise or doesn't start, it's less likely to be just the sensor.
   • Estimated cost (replacement part): Medium.
   • Replacement part: [Vaillant ecoTEC Plus 831 Water Pressure Sensor](https://www.ebay.it/sch/i.html?_nkw=Vaillant+ecoTEC+Plus 831+pressure+sensor&mkcid=1&mkrid=724-53478-19255-0&siteid=101&campid=5339148621&toolid=10001&mkevt=1)

3. Air in the System or Insufficient Pressure (Probability: 5-10%)

   • Key signals: Frequent gurgling, radiators with cold spots, need to frequently refill pressure. Error F75 can also appear with correct pressure if there is a lot of air trapped near the pump or sensor.
   • Typical user error: Not regularly bleeding radiators or not checking system pressure.
   • Safety notes: Do not attempt to disassemble or repair internal boiler components if you are not qualified. Handling high pressure and gas is extremely dangerous.

4. Other Minor or Concomitant Causes (Probability: <5%)

   • Obstructions in the heating circuit (e.g., sludge in the heat exchanger), wiring problems to the pump or sensor, or a rare fault with the main electronic board. These require an in-depth diagnosis by a technician.

Output for technician: The Vaillant ecoTEC Plus 831 boiler shows error F75, with no hot water. Quick tests indicate that the pressure is stable but the pump does not seem to start correctly or the pressure sensor does not detect the differential. It is recommended to check the operation of the Vaillant ecoTEC Plus 831 Circulation Pump and the functionality of the Vaillant ecoTEC Plus 831 Water Pressure Sensor with a Digital Multimeter and an external heating system pressure gauge.

OPERATIONAL DECISION: If the pressure is correct and the pump does not start or error F75 persists -> contact a technician for diagnosis and eventual replacement of the pump or pressure sensor; otherwise, if the problem is resolved by a simple pressure reset, monitor the system for any leaks.