Vaillant boiler fault codes explained: F22, F28, F29, F75 and what to do
- Checked September 2026
- 5 min read
- By PROS AI editorial team
Quick answer
The short version, before the detail.
Vaillant's F codes split into two families: F22 and F75 are about water pressure and circulation, meaning the gauge has dropped below 1 bar or the boiler cannot see the pressure change it expects when the pump starts. F28 and F29 are about the flame: the burner failed to light, or it lit and then went out. A homeowner may repressurise to 1 to 1.5 bar, check the gas supply, thaw a frozen condensate pipe and reset once. Everything else is Gas Safe work.
What is the likely cause?
One code per item, most common first, in Vaillant's own wording from its fault code list.
F22 (low water pressure): Vaillant defines F22 as "Low water pressure in the system".
The usual cause is pressure that has fallen below the 1 to 1.5 bar range after bleeding radiators or through a slow leak; less often the sensor, its wiring or the expansion vessel. The one safe thing: if the gauge reads below 1 bar, use the filling loop and "slowly increase the water pressure to be between 1 to 1.5 bar", then reset. If the gauge already reads above 1 bar and F22 is still showing, the sensor is the problem and that is an engineer's job.
F28 (no flame at ignition): "The flame signal was not detected during the ignition phase." Vaillant names three causes: a frozen condensate pipe, the most common winter reason; a gas supply problem, meaning no gas, low pressure or a faulty gas valve; and an ignition fault.
The one safe thing: check another gas appliance lights and a prepayment meter has credit, and in cold weather look for ice on the outside condensate pipe, thaw it with warm water and reset once. If gas and condensate are fine, Vaillant says plainly that "this is a case for a Vaillant engineer".
F29 (flame lost while running): "The ignition following a flame loss during operation was unsuccessful." The flame went out while the boiler was running and the re-light failed.
Vaillant's causes are a weak or interrupted gas supply, a blocked flue, poor electrical earthing and a blocked condensate pipe. The one safe thing is the same as F28: gas, condensate, one reset. Vaillant's winter page says a frozen condensate pipe will show "an F28 or F29 code".
F75 (no pressure change when the pump starts): "The pressure change is too low when the heat pump is started up", the heat pump here being the boiler's own circulating pump.
Vaillant lists system pressure below 1 to 1.5 bar, air in the system, a faulty pressure sensor, a defective pump or a broken cable. The one safe thing: if the gauge is below 1 bar, repressurise to 1 to 1.5 bar and reset once. If the gauge is already above 1 bar, the sensor or the pump is the fault: call an engineer.
If F22 or F75 comes back within days of a top-up, water is leaving the system; see boiler losing pressure.
What can I safely check myself?
Everything here is done from the display, the gauge, the filling loop and the outside wall. Vaillant's own advice is that "opening the boiler casing can be dangerous"; leave it shut.
- Note the code and the gauge reading; the engineer will want both.
- Reset once. Vaillant says its boilers need the reset button pressed for about three seconds; hold it no longer than ten. Some boilers "may even require two attempts". If the code returns after that, stop; regular resets mean a fault that needs finding.
- Pressure, for F22 and F75. Cold, the gauge should sit at 1 to 1.5 bar. Open the filling loop slowly, watch the needle rise to about 1.2 bar, then close both valves fully. Vaillant warns that if the pressure "drops as soon as you close the valve" there is a significant leak, and you should "not keep trying to repressurise the system".
- Gas, for F28 and F29. Light a hob or check another gas appliance, top up a prepayment gas meter, and check the gas cock at the meter is open.
- Condensate pipe, for F28 and F29 in cold weather. Find the white plastic pipe outside and pour warm water along it from a jug, or hold a hot water bottle against it; Vaillant's words are "do not use boiling water from a kettle as this can damage or crack the pipe". From the ground only, then reset. The full method is in frozen condensate pipe.
- Controls and power. If the display is dark rather than showing a code, check the fused spur, the timer and the thermostat, as in boiler not firing up after summer.
When do I stop and call an engineer?
- If you smell gas, leave the property, do not switch anything on or off, and call the National Gas emergency line on 0800 111 999 from outside.
- If water is dripping near the fused spur, sockets or the consumer unit, switch off at the main switch only if it is dry and safe to reach.
- Stop if the same code returns after one reset, if F22 or F75 shows while the gauge already reads above 1 bar, if the pressure falls again as soon as the filling loop is closed, or if F28 or F29 appears with the gas on and the condensate pipe clear. Those point to a sensor, the pump, the gas valve, the electrodes or the flue.
- Vaillant's own line is that if the boiler "doesn't turn on after resetting, or the error code is still in place, you will need to contact a Gas Safe registered engineer". Check the seven-digit number on the engineer's ID card at gassaferegister.co.uk, or text Gas and the number to 85080.
What does it cost in London?
A weekday call-out with diagnosis in London is £60 to £125, often waived if the repair goes ahead, and labour is £80 to £140 an hour. A pressure fault, meaning the expansion vessel or a valve behind F22 and F75, is £120 to £280 with the part, and London firms call it the most common winter fault. A pump is £180 to £420, a fan £200 to £420 and a circuit board £290 to £650; most repairs, including the ignition parts behind F28 and F29, land between £150 and £400. Out of hours the first hour is £140 to £260.