Boiler Not Firing Up: Gas, Ignition and Oil Supply Checks

Why is my boiler not firing up?

A boiler fails to fire up for one of three reasons: it is not being asked to light, it cannot get fuel or air to the burner, or it lights but cannot confirm the flame. The first is a controls problem. The second and third are what this guide covers.

When the thermostat or a hot tap calls for heat, a modern gas boiler runs through a fixed sequence. The fan starts and the boiler checks the air flow. The gas valve opens and the ignition electrode sparks. A flame sensing electrode then has a few seconds to detect the flame. If it does, the boiler runs. If it does not, the gas valve shuts and the boiler tries again. After a set number of failed attempts, it stops and locks out.

If you can hear the spark ticking but nothing lights, our guide to a boiler clicking but not lighting covers that symptom in detail. If the boiler shows no signs of trying to start at all, and the display is lit, check the thermostat, timer and summer mode first, because the boiler may simply not be receiving a call for heat.

What is an ignition lockout?

An ignition lockout is a safety shutdown that happens when the boiler has tried to light and failed to detect a flame. The gas valve closes, a code appears and the boiler stays off until someone presses reset. It will not clear on its own, even if the cause goes away, because the boiler wants a person to check before it releases gas again.

Every manufacturer has its own code for this. The table lists the common ones, with our full pages on each.

Code names and meanings vary between model ranges and software versions, so always check the user manual or the label inside the front flap for your exact boiler.

Gas supply checks you can do

Most ignition lockouts that homeowners can fix come down to the gas supply. Work through these before you reset:

1. Try another gas appliance. If the gas hob or fire will not light either, the problem is the supply to the house, not the boiler.
2. Check a prepayment meter. Make sure there is credit and that emergency credit has not run out. After a top-up, some meters need a button pressing before the gas flows again; your supplier's meter instructions explain this. British Gas advises that when a meter runs out of credit and switches off, the boiler may need a reset using the manufacturer's instructions.
3. Check the meter valve. The emergency control valve handle next to the gas meter should be in line with the pipe. If it is across the pipe, the gas is off.
4. Check the boiler's own gas valve. Many boilers have a gas isolation valve on the pipe underneath. It should also be in line with the pipe.
5. Think about recent work. A new meter, a new hob or work on the gas pipes can leave air in the supply. The boiler may fail to light the first few times afterwards. Leave purging the pipework to the Gas Safe registered engineer who did the work.

Can a frozen condensate pipe stop a boiler firing?

Yes. A frozen condensate pipe is one of the most common reasons a condensing boiler will not fire in cold weather. When the pipe freezes, the acidic water that the boiler produces cannot drain away. It backs up inside, and the boiler either detects the blockage directly or struggles to keep a stable flame. On many models this shows as an ignition or flame code rather than a specific condensate warning, which is why an Ideal L2 or a Vaillant F28 on a freezing morning often turns out to be ice in the pipe.

Signs include gurgling from the boiler, a pipe outside that is frozen solid and a lockout during a cold snap. Thaw the outside section with warm water from a kettle that has cooled for a while, or a hot water bottle, then reset the boiler once. Our guide to a frozen condensate pipe gives the full steps and explains how to stop it happening again.

Other reasons a gas boiler will not ignite

If gas is reaching the boiler and the condensate is clear, the cause is inside the boiler. These all need a Gas Safe registered engineer:

• Worn or dirty ignition electrode. The spark weakens or jumps to the wrong place. This is a common failure on boilers that have missed services.
• Faulty flame sensing electrode. The boiler lights but cannot detect the flame, so it shuts off within seconds.
• Gas valve fault. The valve does not open, or opens but does not deliver the right pressure.
• Fan or air pressure fault. The boiler will not try to light unless it proves enough air is moving through the flue.
• Blocked flue or air intake. Birds' nests, debris or a damaged flue can stop the boiler lighting safely.
• Low gas pressure at the boiler. The engineer measures this at the gas valve. It can be caused by an undersized gas pipe or a problem at the meter.
• PCB fault. The control board runs the ignition sequence, and a fault there can stop it starting.

Low system water pressure can also stop a boiler firing, but most boilers show a separate low pressure code for that rather than an ignition code. If the gauge reads under 1 bar with the system cold, top it up before resetting.

Worcester boiler not firing up

A Worcester Bosch Greenstar that will not fire up usually shows EA 227 on the display, meaning the boiler tried to light and did not detect a flame. The checks are the same as for any gas boiler: gas to the hob, prepayment credit, the meter valve and, in winter, the condensate pipe.

Once you have checked those, reset the boiler once. On most Greenstar models the reset button is on the front control panel, but the exact method varies by model; our guide to a Worcester boiler reset covers each range. If EA 227 returns after 2 or 3 resets, the engineer will usually look first at the ignition and flame sensing electrodes, then the gas valve and the condensate siphon.

Oil boiler not firing up

An oil boiler that will not fire up is most often short of fuel or has a blockage between the tank and the burner. Oil boilers are not covered by Gas Safe, and anything beyond the checks below is a job for an OFTEC-registered technician. Our main oil boiler guide explains how the burner and fuel supply work.

If the tank ran dry, air and sludge from the bottom of the tank may have entered the fuel line. The burner will then need bleeding and the filter changing, which is technician work. On an oil burner, press the lockout reset once and follow any reset limit in your boiler's user instructions. Do not keep pressing it: each failed attempt sprays oil into a chamber that has not lit, and that unburnt oil then has to be dealt with by a technician.

When to call an engineer

Call a Gas Safe registered engineer if the boiler locks out again after 2 or 3 resets, if other gas appliances work but the boiler will not light, or if you see soot, scorch marks or a yellow flame. For oil, call an OFTEC-registered technician. For LPG, use a Gas Safe engineer with LPG on their ID card.

Tell the engineer the code, what you have checked and whether the problem started after a cold night, a meter change or a fuel delivery. Typical UK prices for the usual parts are below; they vary with brand, access and region.