F23 tells you that your Vaillant has stopped itself because the water leaving the boiler is far hotter than the water coming back, which is a sign that circulation through the heat exchanger is too slow. Most of the causes sit inside the casing, but two quick checks can rule out the simple reasons first.
What the F23 code means
Every ecoTEC boiler has two temperature sensors on its water circuit. One sits on the flow pipe that carries heated water out to the radiators or cylinder, and the other on the return pipe that brings it back. In normal running the return is cooler than the flow by a steady margin, and the control board keeps an eye on that gap. Vaillant calls it the temperature spread.
When water moves too slowly, it spends longer in the heat exchanger and comes out much hotter than it went in, so the spread widens. Past a set limit the boiler shuts the burner down and locks out with F23 rather than carry on and risk overheating. Vaillant's ecoTEC plus installation and maintenance instructions (document 0020244997) give the display text as "Safety switch-off: Temp.spread too large" and the possible cause as "Water circulation too low".
Which Vaillant boilers use F23
F23 carries the same meaning in all four Vaillant installation manuals we checked:
| Vaillant document | Models covered | How F23 is worded |
|---|---|---|
| ecoTEC plus, 0020244997 | VU 126 to 386/5-5 system and regular boilers, VUW 196 to 306/5-5 combis (sold as the 825, 832, 835 and 838) | Safety switch-off, temperature spread too large |
| ecoTEC pro, 0020244995 | ecoTEC pro 24 and ecoTEC pro 28 combis | The same safety switch-off |
| ecoTEC plus, 0020238195 (2021) | VU 126 to 356/6-5 OVZ open-vent regular boilers | "Safety shutdown: Temperature spread too great (NTC1/NTC2)" |
| ecoTEC plus LPG, 0020239015 | VU 246/6-5 OVZ running on LPG | Safety switch-off, temperature difference too great |
Vaillant's own fault code web page describes this fault without giving the code number, so the manuals are the better reference.
The causes Vaillant lists, and how to tell them apart
A blocked or weak pump
The pump pushes water through the heat exchanger. If it has seized, is partly blocked or is running below the output the boiler needs, flow drops and the spread opens up. Radiators that warm slowly while the boiler is firing fit this cause. Sludge in an older system can be what is clogging the pump.
Air in the boiler
A pocket of air inside the boiler or pump stops water reaching part of the circuit. This often follows work on the heating, such as a drain down, a new radiator or a replacement part. Gurgling from the boiler is a common clue.
Flow and return sensors swapped
The ecoTEC plus LPG manual (document 0020239015) lists flow and return NTCs connected the wrong way round. With the plugs reversed, the boiler reads the two temperatures backwards. This is only likely if F23 began straight after the boiler was installed, repaired or serviced, and the person who did that work should put it right.
Low system pressure is not on Vaillant's F23 list (it is on the list for the related F24 code), but a system that is short of water traps air more easily, so the gauge is still worth a look.
| Cause | What you might notice | Who deals with it |
|---|---|---|
| Pump blocked or weak | Boiler fires, radiators stay lukewarm, humming | Gas Safe engineer |
| Air in the boiler | Gurgling, recent work on the system | You for radiators, engineer for the boiler itself |
| Sensors connected the wrong way round | F23 started right after an installation or repair | The engineer who did the work |
Checks you can make first
- Read the pressure gauge or display with the heating cold. Between 1 and 1.5 bar is the usual target. If it is lower, top up with the filling loop and close both valves afterwards. Our Vaillant boiler pressure guide shows where the filling loop sits on ecoTEC models, and very low pressure normally brings up F22 instead.
- Bleed any radiator that is cold at the top or gurgles, starting downstairs. Check the gauge again afterwards, because bleeding lets out water as well as air.
- Reset the boiler. In the 2021 open-vent and LPG ecoTEC plus manuals, Vaillant says to press and hold the reset button for more than three seconds. Allow a few minutes for the boiler to restart.
- If F23 comes back after two or three resets, stop there. Repeated resets make the boiler cycle without curing the flow problem.
Before you ring anyone, note whether F23 appears on heating, hot water or both, and any noises. If several faults are active, the 2021 manual says the display shows each one for two seconds in turn, so note every code you see.
What a Gas Safe engineer will check
The engineer will usually start with the fault memory, which holds the last 10 fault messages on the 2021 and LPG ecoTEC plus models. From there the work tends to run in this order:
- Venting air from the boiler and the pump.
- Checking the pump is turning freely and moving enough water, and replacing it if it is weak.
- Confirming the flow and return sensor plugs are in the right sockets and each sensor is seated firmly on its pipe.
- Looking at the system water for signs of sludge that could be restricting the pump.
A pump problem can also show as F75, so mention it if you have seen that code.
Do not remove the casing to vent the pump or reach the sensors yourself. Those parts sit next to gas and mains electrical components. If you ever smell gas, call the National Gas Emergency Service on 0800 111 999.
Typical repair costs in the UK
If bleeding radiators and a reset clear F23, it costs nothing. If not, these are the jobs that match the causes Vaillant lists:
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Venting the boiler or correcting sensor connections | Typically £100 to £180 |
| Pump replacement | Typically £150 to £400 |
| Powerflush, if sludge is blocking the pump | Typically £400 to £800 |
F23 and the codes it gets confused with
Several Vaillant codes draw on the same two temperatures. F24 is a switch-off for temperature rising too fast, and shares the pump and air causes but also lists low pressure and a non-return valve. F72 is a sensor fault, where the difference between the two readings points at a defective sensor rather than poor circulation. F22 is simply low water pressure.