Vaillant F28 fault: what it means and what to check

Vaillant boilers are common across north and east London, from flats in Islington to semis in Enfield. Vaillant also features in local searches: “vaillant boiler repair london” and “vaillant boiler service london” both show steady demand in Google’s Keyword Planner. The fault code people search for most is F28.

What F28 means

Vaillant’s own help centre explains F28 as an ignition error. Its official meaning is that the flame signal was not detected during the ignition phase. In plain terms, the boiler tried to light the gas, did not see a flame, and shut itself off. This shut off is called a lockout. It is a safety feature.

F28 is a start up fault. It is different from a flame that goes out once the boiler is already running, which Vaillant boilers report with a separate code. Check your own manual, since codes and wording can vary between models.

Why a boiler fails to light

The causes fall into two groups. Some you can check yourself. Others need an engineer.

Cause	Can you check it?
Gas supply turned off at the emergency control valve	Yes
No credit on a prepayment gas meter	Yes
Gas off to the whole street or building	Yes, ask neighbours or check Cadent’s incident page
Frozen or blocked condensate pipe	Yes, if it runs outside and you can reach it safely
Low gas pressure at the boiler	No, an engineer measures it
Faulty ignition electrode or lead	No
Faulty flame sensing electrode	No
Faulty gas valve or circuit board	No
Blocked flue or air intake	No

Safe checks before you call

Never take the boiler casing off. These checks are safe.

1. Try another gas appliance. If you have a gas hob, does it light? If not, the problem is the supply, not the boiler.
2. Check the emergency control valve. This is the lever on the pipe next to your gas meter. When the handle is in line with the pipe, gas is on. In converted Victorian houses the meter can be under the stairs, in a cellar, or in a cupboard in a shared hall. Our guide to finding your emergency control valve shows where to look.
3. Check prepayment credit. Many rented flats have prepayment meters. If credit or emergency credit has run out, the gas stops.
4. Check for a supply problem. North London’s gas network is run by Cadent. Its website lists current incidents. If works in your street have cut supply, wait for it to be restored.
5. Look at the condensate pipe if it is freezing outside. Our guide on thawing a frozen condensate pipe explains how, using warm water only.
6. Reset once. Follow the reset steps in the boiler manual.

If you smell gas at any point, do not reset anything. Open doors and windows, do not use switches, turn off the gas at the emergency control valve if it is safe, leave, and call 0800 111 999. If the meter is in a cellar or basement and you smell gas there, Cadent’s advice is not to go in.

If F28 comes back

If the boiler locks out with F28 again after one reset, leave it off and book a boiler repair. Repeated resets will not fix a faulty part.

A Gas Safe registered engineer will usually:

• measure the gas pressure at the meter and at the boiler while it tries to light
• inspect the ignition and flame sensing electrodes, which can wear or get dirty
• check the gas valve is opening
• check the flue and air supply are clear
• read the boiler’s fault history

Low gas pressure at the boiler is worth a mention for London homes. In older houses, gas pipework has often been extended over the years for new kitchens, extra floors or a moved boiler. If the pipe is too small or too long for the boiler, pressure can drop when it fires. That needs a pipework fix rather than a boiler part. Our page on gas pipework covers that work.

Is it worth repairing?

F28 is often a simple fix such as an electrode or a supply issue. On an older boiler with other problems, the engineer may suggest weighing up a repair against a replacement. They can explain what they found so you can decide. The price is confirmed after a look at the job.

Who can work on your boiler

Gas work in the UK must be done by a Gas Safe registered engineer. Ask to see their Gas Safe ID card. The back of the card lists the types of gas work the engineer is qualified for, and “domestic boiler” is one of them. You can check the card number on the Gas Safe Register.

If you rent, tell your landlord or agent. HSE says landlords are responsible for repairs from general wear and tear on gas appliances they provide. Private renting is common across our area: 31.2% of Islington households rent privately, according to the Census 2021. See our Islington area page for more on local homes.