Vaillant F28 Fault Code: Causes and What to Do

What the F28 fault code means

If your Vaillant ecoTEC boiler is showing F28, it has attempted to start up and ignite the burner but failed to light. After a set number of unsuccessful attempts, the boiler locks out for safety and displays the code rather than keep trying — so you'll usually have no heating and no hot water until it's resolved. F28 is one of the most common fault codes Vaillant owners see, and it relates to the ignition process during start-up.

The boiler isn't broken in a dramatic sense — it's doing exactly what it should by refusing to run when it can't light cleanly. The job is to work out why it couldn't ignite. Some causes are simple and homeowner-safe to check; others involve the gas valve, ignition electrodes or the control board, which are strictly for a registered engineer.

Common causes of a Vaillant F28

Because F28 simply means "failed to ignite", several different faults can trigger it. The usual culprits are:

• No gas reaching the boiler — a gas supply that's been turned off, an empty or low LPG tank, or a tripped emergency control valve.
• A frozen condensate pipe — very common in cold snaps. The blockage stops the boiler running and can show as F28 or, more often, F29.
• Low system pressure — if the gauge is well below 1 bar, some boilers won't fire.
• A faulty gas valve — the valve that lets gas into the burner isn't opening correctly.
• Worn or dirty ignition electrodes — the spark that lights the gas is weak, mispositioned or fouled.
• A flame-sensing or PCB fault — the boiler's control board isn't detecting a flame even when one is present.

The first three you can check yourself. The last three are internal gas-side faults that require the casing to come off — those are for a Gas Safe registered engineer only.

Safe checks you can do yourself

Before calling anyone out, there are a handful of genuinely safe checks. None of these involve opening the boiler or touching anything gas-related inside it.

1. Check your gas supply is on

Make sure other gas appliances in the home are working — try the hob or another gas appliance. If nothing gas-powered works, the issue is your supply, not the boiler. Check the gas isn't switched off at the meter and that you're not out of credit on a prepayment meter. On LPG, check the tank or cylinder isn't empty.

2. Check the boiler pressure

Look at the pressure gauge on the front of the boiler. When cold it should read around 1 to 1.5 bar. If it's sitting below 1 bar, low pressure could be stopping it from firing. You can usually re-pressurise using the filling loop — the two small valves or a key underneath the boiler — bringing it back up to about 1.5 bar. If you're not sure where the filling loop is or it keeps dropping, leave it to an engineer. Our guide on boiler pressure walks through this in more detail.

3. Check for a frozen condensate pipe

In freezing weather, the white plastic pipe running outside (often to a drain) can freeze and block. If you can safely reach it, pouring warm — not boiling — water along the outside pipe and over any visible blockage can thaw it. Once cleared, reset the boiler. This is more often an F29, but it's worth ruling out.

4. Reset the boiler — once

Most ecoTEC models have a reset button on the front panel (look for the reset symbol or hold the button shown in your manual). Press it once to clear the lockout and let the boiler attempt to fire again. If it lights and stays on, the F28 may have been a one-off. If it locks out again, do not keep resetting it. Repeatedly resetting a boiler that won't ignite can allow unburnt gas into the combustion chamber — stop and book an engineer.

When to call a Gas Safe registered engineer

If you've checked the gas supply, confirmed the pressure is fine, ruled out a frozen condensate pipe, and the F28 returns after a single reset, the cause is almost certainly internal — the gas valve, the ignition electrodes, the flame-sensing circuit or the PCB. All of these sit behind the boiler casing and form part of the sealed combustion and gas system.

Only a Gas Safe registered engineer may legally work on these. Never remove the boiler casing or attempt to clean, adjust or replace any gas-side component yourself. You can confirm an engineer is qualified for your specific boiler by checking their card and details on the Gas Safe Register. A registered engineer will test the gas supply pressure at the boiler, inspect and gap the ignition electrodes, check the gas valve operation and read the boiler's fault history to pinpoint the cause safely.

F28 vs F29: what's the difference?

The two codes are closely related and often confused:

• F28 — the boiler failed to ignite during start-up. It never managed to establish a flame.
• F29 — the flame was established but then went out (a flame loss during operation), which a frozen or blocked condensate pipe, or an interrupted gas supply, frequently causes.

In practice the homeowner-safe checks are the same for both: gas on, pressure healthy, condensate pipe clear, then a single reset. If either code persists, it's an engineer's job.

What does fixing an F28 typically cost?

Costs vary by region, the engineer and the part involved. The figures below are indicative ranges for 2026 to help you budget — always get a written quote first.

Job	What's involved	Indicative cost
Diagnostic call-out	Engineer attends, tests and identifies the fault	£70–£120
Ignition electrode replacement	Replace worn or fouled electrodes	£100–£180
Gas valve replacement	Parts and labour for a new gas valve	£250–£400
PCB (control board) replacement	Parts and labour for a new board	£300–£500

This is where boiler cover can take the sting out of an unexpected repair: instead of a one-off bill, a covered repair is handled for the cost of your monthly plan (subject to its limits, excess and any exclusion period). If you're weighing it up, our guides on what boiler cover is and cheaper boiler cover options explain how plans are priced and what they typically include. You can also compare boiler cover from our selected panel.