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

Vaillant describe F.23 as a safety shutdown, "temperature spread too great", measured between the flow and return sensors. It is worth being precise about that, because "water shortage" gets attached to this code all over the internet, and the history explains why. The servicing instructions for the 2005 to 2012 ecoTEC plus and pro head both F.23 and F.24 "water shortage" and then give the measurement — for F.23, "temperature difference between flow and return NTC too large"; the ecoMAX before them wrote "water level low" and the turboMAX plus "dry fire protection", each with the same spread condition in brackets.

From the 2012 ecoTEC plus onwards Vaillant dropped the heading and print only the measured condition, and they now use "water shortage" for the status code S.53 and "water deficiency" for F.22. So on every generation F.23 is about the gap between two temperatures; the older manuals simply filed that gap under the heading of what usually causes it. 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 press your range uses — see the reset line on the Vaillant hub, because the ecoTEC plus wants a one-second press and the ecoTEC sustain a hold of more than three seconds. If it returns, book an engineer rather than resetting repeatedly.