Vaillant F23 fault: water not moving through the boiler

Why the boiler cares about flow and return

Your boiler heats water and a pump pushes it round the radiators, or through the hot water cylinder, and back again. Vaillant boilers have a temperature sensor on the pipe going out, the flow, and another on the pipe coming back, the return. In normal running the return is cooler than the flow, but only by a certain amount.

If water moves too slowly, the small amount passing through the heat exchanger gets very hot while the return stays cool. The gap between the two readings becomes too large and the boiler stops before it overheats. That is F.23. It does not mean the burner or the gas supply has failed. It means the water side of the system is not doing its part.

Check 1: the pressure gauge

Look at the pressure reading on the boiler display or gauge with the heating off and the system cool. On most sealed systems it should sit somewhere around 1 to 1.5 bar, but your user guide gives the figure for your model. If it is well below that, the pump may not be able to move water properly and F.23 or F.22 can follow.

You can top up with the filling loop if your user guide explains how and you are comfortable doing it. Go slowly and stop at the recommended pressure. Pressure does not disappear on its own, though. If you have had to top up more than once in a few weeks, there is a leak or a faulty part somewhere, and our guide to a boiler losing pressure covers where the water usually goes.

Check 2: air in the radiators

Air collects at high points in the system, especially after a leak, a radiator change or a top-up. Enough of it can stop water circulating and leave the pump spinning in a pocket of air. Signs are radiators that are cold at the top, gurgling or rushing sounds in the pipes, and pressure that drops when you bleed.

Bleeding the radiators is a reasonable job to do yourself with the heating off. Our guide on how to bleed a radiator walks through it. Check the pressure afterwards, because letting air out also lets pressure out, and top up if needed before you reset the boiler.

Check 3: has anything just changed?

F.23 straight after work on the system is a strong clue. If a radiator has just been replaced, the system drained, the boiler swapped or the pump changed, ask the person who did the work whether any valves under the boiler or on the pump were left closed, and whether the system was properly vented afterwards. On a new installation, Vaillant also lists flow and return sensors connected the wrong way round as a cause, which is an installer's check, not yours.

Also note whether the fault appears only on heating, only on hot water, or on both. On a system boiler with a cylinder, a sticking zone or diverter valve can restrict the flow on one circuit and not the other, which helps the engineer narrow it down.

Should you reset it?

Once the pressure is right and any obvious air has been bled out, one reset is reasonable. If the boiler then runs normally and the radiators heat evenly, keep an eye on the pressure over the next week.

If F.23 comes back, leave it. Resetting a boiler that cannot move water only makes it heat the same small amount of water over and over. Our guide on how to reset a boiler explains when a reset helps, and boiler lockout keeps returning explains what a repeating fault usually means.

What an engineer will look at

When the simple checks do not clear it, the causes are on the water side of the boiler and system. An engineer will usually work through these in order.

• The pump: whether it is running, seized, running slowly or set to too low a speed. A pump that hums but does not move water is a common finding on older systems.
• Air trapped in the pump or the boiler itself, which needs venting at the boiler rather than at the radiators.
• Sludge and magnetite: black iron oxide from corroding radiators can narrow pipes, block the plate heat exchanger and clog a magnetic filter. A system flush may be recommended if the water comes out dark.
• Closed or partly closed valves on the flow and return, or a blocked filter or strainer.
• The flow and return temperature sensors, including whether they are fitted to the right pipes and reading correctly.

Stopping F23 coming back

If the cause turns out to be sludge, fixing the pump alone may only buy time. Dirty system water is what wears pumps out and blocks heat exchangers, so ask the engineer to look at the colour of the water and the condition of any magnetic filter. Our power flushing service cleans the system when the build-up is heavy, and our guide to central heating inhibitor explains how the water is protected afterwards.

An annual service is also the natural time for the engineer to check the pump speed, clean the filter and confirm the pressure holds. Repeated circulation faults on an old boiler can be a sign that it is time to compare repair with replacement, and our guide on whether to repair or replace a boiler helps with that decision.

Getting a Vaillant F23 fixed

Loyal Plumbers works across London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Gas work is carried out by Gas Safe registered engineers. When you call, tell us the code, the boiler model, the pressure reading, whether you have bled any radiators and whether any work has been done on the system recently.

Our call-out charge is £60. The engineer will explain what they have found and what the repair involves before starting. See our boiler repair and servicing page for more on how we deal with breakdowns.