Oil boiler not firing or locked out: what to check

Safety first

Oil boilers are robust, but the burner is a live electrical and fuel system. As a homeowner you can safely:

• check the oil level and the tank;
• check power, the timer and the thermostat;
• press the reset button once, and once more if needed after a short wait;
• check whether a condensate pipe outside is frozen.

You should not:

• take the boiler casing off or open the burner;
• bleed the fuel pump or loosen fuel connections;
• keep pressing the reset button;
• try to light the boiler with the burner door open.

Stop and call an engineer straight away if you smell oil fumes or smoke inside the house, see smoke from the boiler or flue, hear a bang on ignition, or your carbon monoxide alarm sounds. In those cases turn the boiler off at the switch, open windows and do not reset it. See oil boiler smoke or smell.

What a lockout actually means

When an oil boiler fires, the burner sprays atomised oil into the combustion chamber and ignites it with a spark. A small light sensor called a photocell checks that a flame has formed. If it does not see a flame within a few seconds, the control box shuts off the oil and goes to lockout, usually with a red light on the burner or boiler.

So a lockout is not the fault itself. It is the boiler protecting itself because something stopped it lighting properly.

The checks you can do yourself

Work through these in order.

1. Is there oil in the tank?

It sounds obvious, but it is the most common reason. Gauges can stick, and sight tubes can be misleading. Check the tank level directly if you can do so safely.

If you have run out, order oil. When it is delivered, wait at least an hour for any sediment and water stirred up in the tank to settle before trying the boiler. Then press reset once. If the boiler still will not fire, there is probably air in the fuel line and it needs bleeding by a technician.

2. Is the oil getting to the boiler?

Check that any valve on the outlet of the tank is open. If you can see the fuel filter near the tank (a clear or metal bowl on the fuel line), look for water or sludge in the bowl. Do not open it; just note what you see and tell the engineer.

The fire valve is a safety device on the fuel line that cuts the oil supply if it detects a fire at the boiler. They occasionally trip on their own. If yours is a visible lever type and has dropped, do not reset it yourself; tell the engineer, as it may need replacing or the cause checking.

3. Power, timer and thermostat

• Is the boiler switch or fused spur on?
• Has a trip switch gone in the consumer unit?
• Is the timer set to on, and is the clock showing the right time (a power cut can reset it)?
• Is the room thermostat turned up above the current room temperature?

If the boiler shows no lights at all, it is a power problem, not a lockout.

4. Frozen condensate pipe (condensing boilers)

Condensing oil boilers have a plastic condensate pipe that drains to outside. In a hard frost it can freeze and block, and the boiler will lock out. You may hear gurgling before it stops.

If the pipe runs outside and you can reach it safely at ground level, you can pour warm (not boiling) water along it or hold a hot water bottle against it to thaw it. Then press reset once. Do not climb ladders to do this.

5. Press the reset button, once

Press the reset button once and wait. The boiler may take a minute or two to run through its start-up cycle.

If it locks out again, wait a few minutes and try one more time. If it fails again, stop. Each failed attempt sprays oil into the chamber that does not burn. Too much of it can cause a flare or bang when the boiler finally lights, and can flood the chamber.

Common causes an engineer will look for

If your checks do not get the boiler going, the cause is usually one of these:

• Air in the fuel line after running out of oil, or from a small leak on the suction side.
• Blocked fuel filter from sludge or water in the tank.
• Worn or blocked nozzle. Nozzles are replaced on every service; one that has gone two or three winters without changing is a common cause.
• Dirty photocell. If the photocell cannot see the flame through soot, the boiler locks out even though it lit.
• Electrode gap or condition. Worn or wrongly set electrodes give a weak spark.
• Water in the oil from condensation in the tank, which can also freeze in winter.
• Burner pump or motor faults, or a failed control box.
• Frozen or waxed oil in exceptionally cold spells, usually on exposed fuel lines.

Most of these are found and dealt with on a normal service, which is why boilers serviced every year lock out far less. See what a boiler service includes.

How to bleed an oil boiler (and why to leave it to an engineer)

Bleeding means letting air out of the fuel line at the burner pump. It involves running the burner with the pump bleed screw open and catching oil in a container, then closing it at the right moment. It is a common enough job for a technician, but done wrong it can leave oil on hot surfaces or cause the boiler to fire with air in the line. We advise homeowners not to try it.

How to stop it happening again

• Service every year, before winter, so the nozzle, filter, photocell and electrodes are fresh. See how often to service a boiler.
• Do not let the tank run very low. The bottom of the tank is where sludge and water collect.
• Insulate exposed condensate pipes and fuel lines on external boilers.
• Ask about the tank at your service: water in the oil and a tank in poor condition are worth dealing with early.

Get your oil boiler looked at

If your boiler will not hold a reset, book a repair visit: £85 for the first hour, then a fixed quote before any further work. If it is due a service anyway, book the oil service at £89 with a CD/11 record. Book online or email [email protected]. OFTEC To confirm: oftec.