Vaillant [F.23] fault codewhat it means and how to fix it

Vaillant describe F.23 as water shortage, with the temperature difference between the flow and return sensors being too large. Understanding what the appliance is actually measuring makes this code much easier to interpret. Water leaving the heat exchanger and water returning to it should be reasonably close together, because water moving at a healthy rate picks up heat gradually as it passes through. When flow slows down, each unit of water sits in the exchanger longer and gets much hotter, so the split between the two sensors widens sharply. The appliance watches that gap and stops when it exceeds what it considers safe. Vaillant's own documentation lists a blocked or defective pump, insufficient pump output, and flow and return sensors that have been interchanged. That last one matters: if F.23 appears immediately after somebody has worked on the appliance, the sensors being swapped over is a genuinely likely explanation and is quick to correct.

1. 1

   Check the system pressure is 1.0 to 1.5 bar cold, and top up through the filling loop if it is not. Low pressure and poor flow travel together.

2. 2

   Open every thermostatic radiator valve fully so the pump has a proper path to circulate through.

3. 3

   Bleed the radiators, starting downstairs and finishing with the highest one in the property, then top the pressure back up.

4. 4

   Think back to whether anyone has worked on the boiler recently. If F.23 started immediately afterwards, mention the possibility of swapped flow and return sensors when you call — it is a five-minute fix if that is what it is.

5. 5

   Reset once with the reset button held for three seconds. If it returns, book an engineer rather than resetting repeatedly.