How does an oil boiler work?
An oil boiler does the same job as a gas boiler, heating water for the radiators and taps, but the burner works differently. Most domestic oil fired boilers use a pressure jet burner:
- When the thermostat calls for heat, a small pump in the burner draws oil from the tank through a supply pipe and filter.
- The pump forces the oil through a fine nozzle at high pressure, turning it into a mist of tiny droplets.
- A fan blows air in to mix with the mist, and a pair of ignition electrodes creates a spark that lights it.
- A photocell watches for the flame. If it does not see one within a few seconds, the burner shuts down and locks out for safety.
- The flame heats water in the heat exchanger, and the pump sends it round the radiators or to the hot water cylinder.
Because the burner has a pump, nozzle, electrodes and photocell, oil boilers have more wearing parts than gas boilers. That is one reason annual servicing matters more. The nozzle in particular is often replaced at every service.
The fuel
Domestic oil boilers in the UK almost always run on kerosene, often called 28-second oil. Some older boilers, particularly in farm and commercial settings, run on gas oil, also known as red diesel, but kerosene is standard for homes. OFTEC also promotes HVO, a renewable liquid fuel made from waste fats and oils, as a lower carbon replacement that many existing boilers can use after conversion.
Types of oil boiler
Oil boilers come in the same three basic types as gas boilers, plus a choice of indoor or outdoor casing.
- Oil combi boiler. Heating and hot water from one unit. Many oil combis include a small internal store of hot water to boost flow. Worcester Bosch's Greenstar Heatslave II, for example, is an oil combi sold in 12/18, 18/25 and 25/32 kW sizes.
- Oil system boiler. Heats a separate hot water cylinder, with the pump and expansion vessel built in. Suits homes with more than one bathroom. See our system boiler guide for how the set-up works.
- Heat-only (regular) oil boiler. Works with a cylinder and loft tanks, common in older properties.
- Internal or external. Internal models go in a kitchen or utility room. External models sit outside in a weatherproof casing, which frees up space and keeps the burner noise outside.
Since 2007, new oil boilers fitted in England and Wales have had to be condensing models under the Building Regulations. Like gas condensing boilers, they have a condensate pipe that must be protected from freezing. Our guide to a frozen condensate pipe applies to oil boilers too.
Oil tank rules
The oil tank is a big part of owning an oil boiler. It needs to be sited safely, away from anything that could catch fire and from anywhere a leak could pollute water. OFTEC's guidance says a domestic tank should be:
| Keep the tank at least | From |
|---|---|
| 1.8 m | Non-fire-rated eaves of a building |
| 1.8 m | A non-fire-rated building or structure, such as a timber shed |
| 1.8 m | Openings such as doors and windows in a fire-rated building |
| 1.8 m | Oil fired appliance flue terminals |
| 760 mm | A non-fire-rated boundary, such as a wooden fence |
| 600 mm | Screening, such as trellis and foliage |
Where closer siting cannot be avoided, fire protection can be added, but that needs a technician's assessment. OFTEC recommends tanks made to its own standards, OFS T100 for plastic tanks and OFS T200 for steel.
Bunded tanks
A bunded tank has a second outer skin that can catch the oil if the inner tank leaks. OFTEC says the bund must hold at least 110% of the tank's contents. Whether you need one is decided by a risk assessment, carried out by an OFTEC technician, that looks at things such as nearby rivers, wells and other controlled waters. In practice most new domestic tanks are bunded.
Larger stores are covered by stricter law. In England, the Control of Pollution (Oil Storage) Regulations apply to oil stored at private homes only when the tank holds more than 3,500 litres. Most domestic tanks are smaller than that, but the Building Regulations still apply when a tank is installed or replaced.
Running costs
Heating oil is not covered by Ofgem's energy price cap, so the price changes with the market, and you buy it in bulk, typically several hundred litres at a time. Prices can move sharply over a winter, so many households compare quotes from several suppliers before each delivery or join a local oil buying club. A tank gauge or remote level monitor helps you order when prices suit you rather than when the tank is nearly empty in midwinter. A modern condensing oil boiler is typically around 90% efficient or better, similar to a gas condensing boiler, so how its running cost compares with gas or electricity depends mostly on the fuel price when you order.
How often should an oil boiler be serviced?
Every year. OFTEC recommends an annual service by a registered technician, and manufacturers require it to keep the guarantee valid. A typical service includes cleaning the heat exchanger, replacing the nozzle, checking the electrodes and photocell, setting the pump pressure, cleaning or replacing the oil filter, checking the tank and supply pipe, and a combustion test with an analyser.
Skipped services on an oil boiler tend to show up as soot, smoke from the flue, a smell of oil, and lockouts. Book the service in late summer or early autumn, before technicians get busy and before you need the heating every day.
How long do oil boilers last?
A well maintained oil boiler often lasts around 15 years, and many run for longer. Burner parts wear and are replaced as part of normal servicing, so the lifespan is usually set by the heat exchanger and casing rather than the burner. Signs that replacement is getting closer include repeated lockouts after servicing, corrosion or leaks from the heat exchanger, and difficulty getting parts for an older model. Our guide to repairing or replacing a boiler covers how to weigh it up.
Are oil boilers being phased out?
Not yet, and not in 2026. In 2021 the government consulted on phasing out new fossil fuel heating in homes off the gas grid from 2026. In September 2023, the then Prime Minister delayed that plan, and the target became 2035. Existing oil boilers do not have to be removed, and a broken oil boiler can still be replaced with a new one.
Government policy can change, so check GOV.UK for the latest position before a big decision. For now, the main push is through grants: the Boiler Upgrade Scheme offers up to £7,500 in England and Wales towards a heat pump. A heat pump can suit a well-insulated rural home, and the alternatives also include an electric boiler in smaller properties or LPG where oil is not wanted.
Staying safe with an oil boiler
Oil boilers produce carbon monoxide if they burn badly, just like gas boilers. Fit a carbon monoxide alarm in the room with the boiler; in England, the Building Regulations require one when a new or replacement fixed combustion appliance such as a boiler is installed. Never block the flue or the air supply, and keep the area around an external boiler clear of garden rubbish and plants.
Only an OFTEC registered technician, or another competent person registered for oil, should service or repair an oil boiler or tank. If you see oil leaking from the tank or pipework, stop using the boiler and contact a technician straight away. If your carbon monoxide alarm sounds, turn the boiler off, open the windows, leave the property and get medical advice if anyone feels unwell.