Vaillant F23 Fault Code: Flow and Return Too Far Apart

Why does a Vaillant show F23?

The boiler has two temperature sensors, one on the flow pipe and one on the return. With water circulating well, the gap between them stays within a set range. When circulation slows, the water sits in the heat exchanger for longer, picks up more heat, and the gap widens. Past the limit, the boiler shows F23 and stops.

Vaillant's causes for F23 fall into three groups:

• Not enough water moving: air or low water in the heating circuit, sludge restricting flow, a diverter valve that is stuck or faulty, or an internal expansion vessel that has failed.
• The pump: its plug or cable, a seized or weak pump, or pump settings too low for the system.
• The sensors: a faulty flow or return sensor, a fault in its wiring, or the two sensors swapped over, which only happens after someone has worked on the boiler.

What should I check first?

Work through these in order, because each one can cause the next. First the pressure: with the heating cold, the gauge should read between 1 and 1.5 bar. Low water is one of the causes Vaillant lists, so a low reading is the first suspect. Top up if it is low.

Then air. Feel the radiators with the heating on. One that is cold at the top and warm at the bottom has air in it; bleed it with the heating off and a cloth ready, then check the pressure again, as bleeding lowers it. See bleeding radiators with the heating on or off for which way round to do it.

Finally the radiator valves. If several radiators were isolated for decorating, or a new radiator was fitted and never opened fully, the boiler can run short of places to push water.

Then press reset once. If the boiler fires and runs, watch it for a day. If F23 comes back, stop resetting.

The pump, sensors and expansion vessel are all inside the casing. None of them is a householder job, and freeing a stuck pump is not something to try with the casing off.

When F23 needs an engineer

Vaillant's own advice for F23 is to contact a heating engineer, because the code is about how the boiler works with the rest of the heating system. A boiler that locks out with F23 again after you have dealt with pressure and air usually has a pump or blockage problem. Signs that point that way: a hum or buzz from the boiler with no heat, radiators barely warm even when the boiler briefly fires, or a history of black water when radiators are bled, which means sludge.

If F23 appeared right after a service or repair, say so when you book. Swapped sensor connections is one of the causes Vaillant lists, and it is quick to rule out.

What the repair usually involves

The engineer checks pressure and purges air using the boiler's check programme P.0, then tests the pump and its wiring, checks the expansion vessel charge, and tests both temperature sensors. If radiators come out black when bled, they may suggest a flush and a magnetic filter so the new pump does not clog.

Purging air or recharging the expansion vessel is a short job. A new pump is the costly outcome: a complete genuine pump for a recent ecoTEC plus is listed by parts suppliers at roughly £250 to £300 as a part alone, before labour.

Our visit starts with the £60 call-out, and the repair price is agreed with you on site before any work starts. To book, go to Vaillant boiler repair.