An oil fired boiler burns heating oil, usually kerosene, stored in a tank outside your home: a pump in the burner draws oil from the tank, sprays it through a nozzle as a fine mist, mixes it with air from a fan and lights it with an electric spark. The flame heats water in the boiler’s heat exchanger, and that hot water is pumped round your radiators and used to heat your hot water, so apart from the fuel supply and burner it works much like a gas boiler.

From tank to radiator: how it works step by step

1. The tank. Heating oil is delivered by tanker and stored in a tank, usually in the garden. A gauge on the tank, or a remote monitor, shows how much is left.
2. The supply line. Oil runs from the tank to the boiler through a pipe. Manufacturers such as Grant require a remote sensing fire valve where the line enters the building, which shuts off the oil if it detects a fire at the boiler, and a filter near the boiler to stop dirt and water reaching the burner. Where the tank sits lower than the burner, a de-aerator removes air from the oil.
3. The burner pump and nozzle. When the controls ask for heat, the burner motor turns a small pump that raises the oil to high pressure and forces it through a precision nozzle. This atomises it into a fine mist that burns cleanly.
4. The fan. The same motor drives a fan that blows air into the combustion chamber, so the oil mist and air mix in the right proportions.
5. Ignition. Electrodes create a high-voltage spark that lights the mixture. The burner should light within seconds.
6. Flame detection. A light sensor (often called a photocell) watches for the flame. If it does not see one, or the flame goes out, the control box shuts the burner down and locks out. On many oil boilers a lockout reset button on the burner then lights up.
7. The heat exchanger. Hot gases from the flame pass through the heat exchanger, heating the water inside it. In a condensing boiler, a second stage cools the gases further, until the water vapour in them condenses and gives up more heat.
8. Round the system. The pump sends the hot water round your radiators, and either through a coil in a hot water cylinder or, in an oil combi, through a hot water heat exchanger to your taps.
9. The flue and condensate. The cooled gases leave through the flue. The condensate, which is slightly acidic, drains away through a trap and a condensate pipe.

The whole sequence is controlled by your timer and thermostats, exactly as it is on a gas system. If you have a lockout and want to rule out the controls first, the checks in our guide to a boiler not firing up apply to oil boilers too, apart from the gas supply.

Types of oil boiler

Type	How it works	Best suited to
Regular (heat only)	Heats the radiators and a separate hot water cylinder, usually with a cold water tank in the loft	Larger homes and existing open vented systems
System	Like a regular boiler, but the pump and expansion vessel are built in, so no loft tank is needed	Homes with high hot water demand and an unvented cylinder
Combi	Heats radiators and heats hot water on demand, with no cylinder	Smaller homes where space matters
Internal or utility	Floor-standing or wall-hung inside the house, often in a kitchen or utility room	Homes with indoor space for it
External	The whole boiler sits outside in a weatherproof casing	Homes short of indoor space, and to keep burner noise outside
Boiler house	A model designed for garages, cellars and plant rooms	Outbuildings and dedicated boiler rooms

Most modern oil boilers are floor-standing, although there are wall-hung models. External boilers are popular because the burner is noisier than a gas boiler and the oil line stays outside the house.

How an oil boiler differs from a gas boiler

Feature	Oil boiler	Gas boiler
Fuel supply	Stored on site in a tank, delivered by tanker	Piped continuously from the gas network
Burner	Pressure jet burner with pump, nozzle and photocell	Fan-assisted gas burner with a gas valve
Running out of fuel	Possible if the tank is not refilled in time	Not an issue, apart from prepayment meters
Noise	The burner is usually more audible	Usually quieter
Competent person rules	Building Regulations, with oil installers usually registered with a scheme such as OFTEC	Must be done by a Gas Safe registered engineer
Lockout reset	Usually a reset button on the burner	A reset button on the control panel

Oil boilers are not covered by the Gas Safe Register, because they do not burn gas. Installing one is building work that must meet the Building Regulations, and it is either self-certified by an installer registered with a competent person scheme for oil, or notified to building control. Our oil boiler installation page explains how a replacement is planned.

The fuel: kerosene, gas oil and HVO

Most modern domestic oil boilers burn kerosene, sometimes called 28 second oil or Class C2 kerosene. Some older boilers were set up for gas oil, a heavier fuel, and the two are not interchangeable without the burner being set up for the fuel. Grant’s condensing boilers, for example, run on Class C2 kerosene only.

Some manufacturers now make boilers that can be converted to run on HVO (hydrotreated vegetable oil), a lower carbon liquid fuel. Whether an existing boiler can be converted depends on the model, so ask before you order a delivery of anything other than kerosene.

The oil tank and the rules around it

The tank is often the part people forget. Building Regulations guidance for England (Approved Document J) sets minimum distances to reduce fire risk, including keeping a tank 1.8m from non-fire-rated parts of the house such as eaves, doors and windows, and from an oil boiler’s flue terminal, and 760mm from a non-fire-rated boundary such as a wooden fence.

A bunded tank (a tank inside a second skin that can hold the contents if the inner tank leaks) is needed in higher risk locations, such as for larger tanks or where the tank is close to a watercourse, a well or a borehole.

If you find oil leaking from a tank or pipe, stop using the boiler, contain the leak if you can do so safely, contact your oil supplier and insurer, and report any spill that could reach a drain or watercourse to the Environment Agency incident hotline on 0800 80 70 60.

Why oil boilers lock out

The most common reasons an oil boiler stops and shows a lockout are:

• Running out of oil. Check the tank gauge first. If the tank ran dry, air and sediment may have been drawn into the line, and after a delivery the line may need bleeding by a technician.
• A blocked filter or nozzle. Dirt or sludge from the tank restricts the oil flow.
• Water in the tank. Condensation or a damaged lid lets water in, which can reach the burner.
• A dirty photocell or worn electrodes. The burner lights but the flame is not detected, or it will not light reliably.
• A frozen condensate pipe. Just as with gas boilers, an external condensate pipe can freeze and shut the boiler down. Our frozen condensate pipe guide shows how to thaw one safely.

Servicing an oil boiler

Oil boilers need regular servicing, and manufacturers such as Grant advise that once a year is generally adequate. A service typically includes replacing the nozzle and the oil filter element, checking and cleaning the electrodes, photocell and combustion chamber, adjusting the burner using a flue gas analyser, checking the fire valve, and looking over the tank and supply line. Manufacturer warranties usually require regular servicing too.

How we price it

Installations are quoted as a fixed price after a free survey, so the price you agree is the price you pay. The survey looks at the boiler position, the flue, the oil supply line and the condition of the tank, because a replacement boiler is the right time to deal with a tank or fire valve that no longer meets the regulations.

Frequently asked questions

Are oil boilers being banned?

There is no ban on replacing an existing oil boiler today. In September 2023 the government pushed back its proposed phase-out of new fossil fuel boilers in off-gas-grid homes to 2035. Policy can change, so if you are replacing an oil boiler it is worth asking whether a heat pump would suit your home as well. If you are changing the whole system, our central heating installation page covers new pipework and radiators.

How often does an oil boiler need servicing?

Once a year is the usual recommendation, and it is normally a condition of the manufacturer’s warranty. Oil burners have parts, such as the nozzle and filter element, that are normally replaced at the service.

Does an oil boiler need electricity?

Yes. The burner motor, pump, fan, ignition and controls all run on electricity, so an oil boiler will not work in a power cut. It will restart when the power returns, although it may need a reset.

Is an oil boiler cheaper to run than gas?

The Energy Saving Trust says a modern condensing gas boiler is usually the cheapest to run of gas, oil, LPG and electric options. Oil prices also move with the market and you buy in bulk, so costs vary from year to year. For homes with no gas supply, the realistic comparison is oil against LPG, electric heating or a heat pump.

Why has my oil boiler stopped working?

Check the tank gauge first, as running out of oil is the most common cause. Then check the power, the timer and thermostat, and the condensate pipe in freezing weather. If the lockout light on the burner is on, press reset once, and if it locks out again, call a technician.

Replacing an oil boiler in Hertfordshire, Bedfordshire and Cambridgeshire

If your oil boiler is getting old (our guide to how long a boiler lasts explains the signs), or you are weighing up whether to replace it or move to a different system, our oil boiler installation page explains how we plan a replacement and what the survey covers. You can also call 01462 506374, answered 24 hours a day, 7 days a week.