Common Vaillant Boiler Problems and What Usually Causes Them

The most common Vaillant boiler problems are low pressure (F22), failure to light (F28), flame loss (F29), pump or pressure sensor faults (F75), and heating or hot water failing while the other still works. Noise and a boiler that keeps cutting in and out come close behind. Most are routine repairs rather than signs the boiler is finished, and the fault code on the display, together with when it appears, tells you which problem you have. If the display shows a flashing radiator or tap symbol rather than an F code, see what a flashing radiator symbol on a Vaillant boiler means.

How Do You Read a Vaillant Fault Code?

Vaillant ecoTEC boilers show faults as F followed by a number, often with a dot, such as F.22. The code tells you which safety check tripped, which is not always the same thing as the part that failed.

Codes that start with S are status codes. They describe what the boiler is doing at that moment, such as waiting for a heat demand or running the fan, and are not faults. When an F code appears, write it down along with the model name on the front panel before you reset anything, because the code often disappears after a reset and it is the most useful thing an engineer can have.

Low Pressure and the F22 Code

Low pressure is the most common Vaillant problem. When the system pressure drops too far, the boiler shows F22 and stops to protect itself.

Most Vaillant systems should read around 1 to 1.5 bar when cold. You can top the pressure up yourself with the filling loop, following the manual. If it holds for months, that was probably normal loss over time. If it drops again within days or weeks, water is leaving the system through a leak, a pressure relief valve that is letting by, or an expansion vessel that has lost its air charge. Our page on the Vaillant F22 low pressure fault covers the checks in more detail, and why a Vaillant boiler keeps losing pressure goes through the three usual causes. Topping up week after week hides the problem and brings fresh oxygen-rich water into the system, which speeds up corrosion.

Ignition and Flame Faults: F28 and F29

F28 means the boiler tried to light and failed. F29 means it lit and then lost the flame. The causes overlap, and both are lockouts the boiler uses to avoid feeding gas to a burner that is not lit.

Some causes sit outside the boiler and are safe to check. Make sure other gas appliances such as the hob work, check a prepayment meter has credit, and in freezing weather look at the condensate pipe where it leaves the house, because a frozen pipe is a very common winter cause of F28 on condensing boilers. Inside the boiler the usual suspects are the ignition and flame sensing electrodes, a blocked condensate trap, the gas valve and the fan. Those are all jobs for a Gas Safe registered engineer. The Vaillant F28 fault code page lists the checks you can make first.

Pump and Circulation Faults: F75 and F83

F75 means the boiler did not see the expected pressure rise when the pump started. F83 means it did not see the expected temperature rise when the burner fired. Both usually come back to water not moving through the boiler properly. Our guide to the Vaillant F83 fault covers that code in more detail.

The common causes are low pressure, air in the system, a pump that has stuck or worn out, and sludge restricting the heat exchanger or magnetic filter. F75 can also be a faulty pressure sensor, and F83 a flow or return temperature sensor that has lost contact with its pipe. These codes often show up in autumn when the heating runs for the first time in months. Check the pressure, bleed any radiators that are gurgling or cold at the top, and reset once. If the code returns, the pump, sensors and filter need checking by an engineer.

Hot Water Works but the Heating Does Not, or the Other Way Round

On a Vaillant combi, one service working while the other does not usually points to the diverter valve, which switches the boiler between heating and hot water. The controls are the other common cause when only the heating has stopped.

If the hot water is fine but the radiators stay cold, check the thermostat, programmer and any wireless receiver first, because a boiler that is never asked for heat will not fire for it. If the controls are calling for heat and nothing happens, the diverter valve is the prime suspect. Our guide to what a diverter valve does and why it fails explains the symptoms. If the heating works but hot water is weak, lukewarm or swings between hot and cold, the plate heat exchanger or flow sensor is more likely. London's hard water makes scale on the plate heat exchanger a regular cause.

Noise, Kettling and a Boiler That Keeps Cutting Out

A Vaillant that bangs, rumbles or whistles, or fires up and stops every few minutes, is usually dealing with restricted flow or a boiler that is not set up for the house. Neither is usually dangerous, but both wear parts faster.

Banging and rumbling, often called kettling, comes from water boiling in pockets inside a heat exchanger narrowed by sludge or scale. A weak pump makes it worse. Short cycling, where the boiler lights and shuts off repeatedly, can come from a heating output set higher than the house needs, a thermostat in the wrong place, or poor circulation. An engineer can set the heating output to suit the house and check the flow. If sludge is behind it, cleaning the system and fitting or clearing a magnetic filter usually helps more than replacing parts.

Is a Vaillant Boiler Worth Repairing?

Usually, yes. Most Vaillant faults come down to a widely available part such as an electrode, pump, sensor or diverter valve. The decision gets closer when the boiler is old, the main heat exchanger is failing, or several faults stack up at once.

Before paying for parts, ask the engineer what failed, why it failed, and whether the cause, such as sludge, will damage the new part too. A straight answer on repair versus replacement is part of a proper diagnosis. For Vaillant faults in London, our boiler repair service starts by finding the cause. Have the model name and any fault code ready when you book.