Oil boiler not working? What to check first
An empty tank is the most common cause of no heat in winter. For everything else, find your symptom below.
Published 10 October 2026 · General guidance, not a substitute for a registered technician
Stop and get help
Smell of fumes, or someone feels dizzy, sick or drowsy
- Get everyone outside now, opening doors and windows on the way only if it takes seconds. Don't stay to look for the cause.
- Call 112 or 999 if anyone feels unwell. Get medical help even if symptoms ease outside.
- Do not use the boiler again until a registered technician has checked it.
- A carbon monoxide alarm near the boiler and in bedrooms is the only reliable way to know. Test yours monthly.
Stop and get help
Smell of oil, or oil leaking at the tank or boiler
- If it is safe, close the valve on the fuel line at the tank to stop the flow.
- Keep flames, cigarettes and sparks away. Do not wash the oil into a drain.
- Soak up small amounts with sand, cat litter or absorbent pads, and keep children and pets away.
- Call an OFTEC-registered technician and tell your home insurer.
- If oil reaches a drain, stream, well or a neighbour's property, tell your local authority.
Take care
The tank ran dry, or the boiler stopped after you ran low
- Find an oil supplier delivering near you and ask for their earliest slot.
- Take at least 300 litres if you can. A small top-up can pull sludge from the bottom of the tank into the line.
- Wait about an hour after the delivery before firing the boiler, so the sediment it stirred up can settle.
- A boiler that ran dry usually has air in the fuel line and needs it bled before it will light. The supplier or an OFTEC technician can do it.
- Then press reset once. If it locks out again, read the lockout section before a second try.
Take care
It won't start after an oil delivery
- The fuel-line valve at the tank is sometimes closed during a delivery or tank work. Check it's open.
- Give it about an hour after the delivery, then press reset.
- If the tank was nearly empty beforehand, expect air or sludge in the line: a technician can bleed it and change the filter.
- If it still won't fire, call an OFTEC technician rather than resetting again.
Take care
A red lockout light is on
- Hold the reset button for a few seconds, then release. If the boiler fires and keeps running, book a service if it ever locks out again.
- Never reset it more than twice. That is OFTEC's safety advice. Each failed attempt can leave unburnt oil in the combustion chamber, and repeated resets can cause a dangerous ignition. If it locks out again after a second try, switch it off and call an OFTEC technician.
- A technician's first two questions: is there oil in the tank, and is the fuel valve open? Have the answers ready.
Safe to check yourself
No lights on the boiler at all
- Check the fused spur (the switch on the wall beside the boiler) and the fuse board. Reset a tripped switch once.
- If it trips again straight away, leave it off: that's an electrical fault for an electrician or heating technician.
Safe to check yourself
Power is on but there is no heat or hot water
- Set the programmer to constant for heating and hot water and the thermostat above room temperature, then give it 10 to 15 minutes: some boilers take that long to fire.
- If it still makes no attempt to start, check the tank and the lockout light, then call a technician.
Safe to check yourself
Hot water works but some radiators stay cold
- With the programmer on heating (not hot water only), bleed the cold radiators until water comes out and check their valves are open.
- A whole floor or zone staying cold points to a failed zone valve or pump: a job for a plumber or heating technician.
Safe to check yourself
Banging, kettling or a loud hum
- A kettle-like rumble usually means trapped air or sludge in the system. Bleeding the radiators can help.
- A new loud hum or vibration is often the pump or burner fan wearing out. A service fixes it, and a serviced boiler burns less oil.
Finding a registered technician
Work on the burner, pump, nozzle or fuel line is for an OFTEC-registered technician. Search the OFTEC register by area. An annual service catches most lockout causes before winter.
Don't want to run dry again?
The planner works out when you'll need oil from your tank reading and local weather, and puts the date in your calendar.
Common questions
How many times can I reset an oil boiler?
No more than twice. OFTEC advises never attempting more than two resets. If the boiler still locks out, leave it off and call an OFTEC-registered technician, because repeated resets can leave unburnt oil in the combustion chamber.
Why won't my oil boiler start after I ran out of oil?
Running dry lets air, and sometimes sludge, into the fuel line. After the delivery, wait about an hour, check the fuel valve is open, then press reset. If it doesn't fire, the line needs bleeding.
How long after an oil delivery should I wait to turn on the boiler?
Many technicians suggest about an hour. A delivery stirs up sediment at the bottom of the tank, and waiting lets it settle before the boiler draws oil again.
Who should I call to fix an oil boiler in Ireland?
An OFTEC-registered technician. OFTEC is the registration body for oil-heating technicians in Ireland and the UK, and its website lists registered technicians by area.