Black smoke means incomplete combustion and needs attention. White smoke at start-up is usually normal condensation. Soot marks around an appliance mean combustion products are entering the room and need immediate action. Oil smells mean a leak somewhere.
Smoke from the flue
| What you see | What it means | What to do |
|---|---|---|
| Brief white or grey puff at start-up, clearing in seconds | Normal. Condensation and a momentarily rich mixture during ignition | Nothing |
| Visible white plume in cold weather, persisting | Normal for a condensing boiler. Water vapour condensing in cold air | Nothing – though consider where the plume goes |
| Black or dark grey smoke, continuous | Incomplete combustion. Too little air, worn nozzle, wrong pump pressure, or a fouled chamber | Turn it off and call. Every minute is soot on the heat exchanger |
| A large black cloud on ignition, then clearing | Unburnt fuel in the chamber igniting – usually after repeated resets | Stop resetting. It needs the chamber cleaned |
| Blue haze | Unusual on domestic oil – worth investigating | Have it looked at |
| Smoke from anywhere other than the flue terminal | A flue leak. Combustion products escaping where they should not | Turn it off immediately |
Incomplete combustion means carbon monoxide is being produced, and it means soot is depositing on the heat exchanger at a rate that will cost you real efficiency within weeks. A boiler producing black smoke should be turned off and looked at, not left until the next service.
Soot marks
Soot staining on or around the boiler, on the wall near the flue, or on the ceiling above an appliance means combustion products are going somewhere other than up the flue.
- Soot around the boiler casing or door – a failed door seal, a cracked combustion chamber, or positive pressure in the chamber from a flue restriction.
- Soot on the wall around an external flue terminal – usually poor combustion, sometimes a terminal in a position where the plume recirculates.
- Soot inside the house near the appliance – combustion products entering the room. This is the serious one.
- Soot deposits inside the boiler casing found at service – tells the engineer the appliance has been running rich.
Soot in the room is the finding that should stop everything. It means the flue path is not containing combustion products, and where soot goes carbon monoxide goes.
Smells, and where they come from
Smell of oil, boiler not running
A leak. The usual candidates are a weeping flexible connection at the burner, a pump seal, a filter bowl seal, a compression joint on the supply line, or the tank fittings. It is worth tracing rather than living with – a slow weep is oil on the floor and, if the line is buried, oil in the ground.
Smell of oil when the boiler starts
Usually unburnt fuel briefly during ignition. A small amount momentarily is not unusual on some appliances. A strong smell each start, or one that lingers, suggests a poor ignition sequence – electrodes, nozzle or pump pressure.
Smell of oil after repeated lockouts
Unburnt fuel in the combustion chamber from failed ignition attempts. Stop resetting, ventilate, and get it cleaned before firing. See oil boiler lockouts.
Acrid, burning or ‘hot’ smell
Could be dust burning off after a period of disuse, which passes. Could also be an electrical fault, overheating, or something in contact with a hot surface. If it persists, turn it off.
Sweet or chemical smell in the house
Worth investigating rather than assuming it is the boiler. Kerosene has a distinctive smell; something sweeter may be a different source entirely.
Carbon monoxide, and why oil counts
Carbon monoxide is produced by incomplete combustion of any carbon fuel. Oil appliances produce it exactly as gas ones do, and the idea that CO is a gas-only concern is both common and wrong.
It has no smell, no colour and no taste. The warning signs are indirect:
- Headaches, nausea, dizziness, drowsiness or flu-like symptoms that improve when you leave the house and return when you come back.
- Several people or pets affected at once, which is the pattern that most often identifies it.
- Symptoms that track with the heating being on.
- Soot marks, yellow or unusual flame colour, or excessive condensation on windows when the appliance runs.
Audible, sited to the manufacturer’s instructions, and in date – they have a finite life, typically seven to ten years, after which the sensor is unreliable and the unit must be replaced rather than just having its battery changed. Check the replace-by date on yours. If an alarm sounds: get everyone out, ventilate, turn appliances off, and seek medical advice mentioning suspected CO exposure.
What actually causes bad combustion
- A worn nozzle, widening the spray pattern so fuel is not properly atomised.
- Incorrect pump pressure, changing the firing rate away from what the appliance was designed for.
- Electrode gap or position wrong, producing unreliable ignition and a poor flame.
- Insufficient combustion air, from a blocked vent or an over-sealed room.
- A restricted or partially blocked flue, including birds’ nests, which are more common than people expect.
- A fouled heat exchanger, reducing heat transfer so the flue gas temperature rises and efficiency falls.
- Contaminated fuel, particularly water, which will not burn.
- A badly set burner, which is what an annual service with a combustion analyser exists to prevent.
Almost all of these are found and corrected in a proper annual service – see what actually happens during an oil boiler service.
Seeing black smoke or soot marks? Turn it off and call. That is not something to leave until the next service.
Questions we get asked about this
Is white smoke from my oil boiler normal?
A brief puff at start-up, yes. A persistent white plume in cold weather is water vapour from a condensing appliance and is also normal. Black or dark smoke is not.
My boiler smells of oil when it starts.
A faint momentary smell at ignition is not unusual on some appliances. A strong smell, or one that lingers, points at ignition or atomisation – electrodes, nozzle or pump pressure – and is worth having set up properly.
There are soot marks on the wall by the flue.
Poor combustion, or a terminal position where the plume recirculates against the wall. Either way it needs the burner checking, because soot outside means soot inside the heat exchanger too.
Do oil boilers produce carbon monoxide?
Yes. Incomplete combustion of any carbon fuel produces it. Every property with an oil-fired appliance should have a working, in-date CO alarm.
My boiler is noisy and smoky. Is it dangerous?
Turn it off and have it looked at. A boiler running that badly is producing carbon monoxide and depositing soot on the heat exchanger, and both get worse quickly.
How do I know if my CO alarm is still working?
Test it with the test button, and check the replace-by date printed on the casing. The sensor has a finite life regardless of the battery, and an expired alarm gives false reassurance – which is worse than having none.
You might also need
- Bunded or single-skin? How to tell which tank you are allowed
- Buying heating oil: order size, timing and tank management
- Eight signs an oil tank is near the end of its life
- Frozen condensate pipes: why it happens and how to stop it recurring
- Grant, Firebird, Warmflow, Worcester Danesmoor and Mistral, compared honestly
- Internal, external or boiler house: where an oil boiler should go
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.