Glow-worm F1

Why a Glow-worm boiler shows F1

Each time there is a call for heat or hot water, the boiler opens its gas valve, sparks the ignition electrode and waits for the flame sensor to confirm a flame. If no flame is confirmed within a few attempts, it shuts the gas off and locks out with F1. The code tells you the ignition sequence failed. It doesn't tell you which part of the sequence failed, which is why the same code can have very different fixes.

The usual causes, roughly in the order they turn up: the gas supply isn't reaching the boiler (an isolation valve knocked off, a prepayment meter out of credit, or a supply problem at the meter); a frozen or blocked condensate pipe in cold weather; a worn, dirty or badly gapped ignition electrode; a flame sensing fault; a gas valve that isn't opening; or, less often, a blocked flue or a fault on the main circuit board.

Checks you can safely make before calling anyone

Check that your gas hob or other gas appliances work. If they don't, the problem is the supply, not the boiler: check the meter, and if you have a prepayment meter make sure it has credit. If you smell gas at any point, stop, open windows, don't use electrical switches and call the National Gas Emergency Service on 0800 111 999.

In freezing weather, look at the plastic condensate pipe where it leaves the house. If it's frozen, pour warm (not boiling) water along it or hold a warm cloth on it until it thaws. A blocked condensate pipe is one of the most common reasons for F1 in winter, and it's the one cause you can clear yourself.

Once you've made those checks, you can reset the boiler once using the reset button described in your user manual. If F1 comes straight back, or returns within a day or two, stop resetting it. Repeated resets won't fix an ignition fault and can make the underlying problem harder to diagnose.

• What to do about a frozen condensate pipe
• Why a boiler won’t reset and what it means

What an engineer does about F1

A Gas Safe registered engineer will check the gas pressure at the boiler, then watch an ignition attempt to see whether there's a spark, whether gas is arriving and whether the flame is being detected. That narrows F1 down to the electrode, the ignition lead, the flame sensing circuit, the gas valve or the board. Cleaning and re-gapping an electrode is a common fix; replacing a gas valve or PCB is more involved, which is why it's worth diagnosing properly before any part is ordered.

If F1 keeps coming back alongside other lockout codes, or the boiler also locks out with F4, F28 or F29, the engineer will treat them as one ignition problem rather than separate faults.

• Why a Glow-worm boiler keeps locking out
• Ignition faults explained: pilot, electrode, gas valve

Glow-worm F1: common questions