Oil Boilers Explained: How They Work and What They Cost

If your home isn't connected to the mains gas network, an oil boiler is probably heating your water and radiators right now. That's true for a fair few rural properties across the Midlands, where gas pipes never reached. Oil boilers explained simply: they work in a very similar way to gas boilers, just with a different fuel source and a tank in the garden instead of a pipe under the street.

That difference matters when it comes to running costs, servicing and what happens when something goes wrong. Here's what you actually need to know.

How an oil boiler works

An oil boiler burns kerosene (sometimes called heating oil) stored in a tank outside your home, usually a plastic bunded tank with a capacity of 1,200 to 2,500 litres. A supply line feeds oil to a burner inside the boiler, which ignites it to heat water for your radiators and taps.

Beyond the fuel source, the mechanics are close to a gas system. You'll still find combi, system and regular versions, and the differences between those types work the same way whether you're on oil or gas. Our guide to combi vs system vs regular boilers covers that in more detail if you're not sure which setup suits your home.

What oil boilers cost to run and replace

Oil prices move more than gas prices, since there's no fixed tariff and you're buying in bulk deliveries. Many households top up a few times a year and shop around for the best delivered price, which gives more control than a fixed tariff but also more exposure to market swings. The Energy Saving Trust has useful guidance on typical oil consumption and how to budget for it.

Replacement costs are broadly similar to gas, though tank condition and access can push the price up. Our new boiler cost guide breaks down typical fixed prices by boiler type, and if the number feels steep, boiler finance can spread it over manageable monthly payments.

Servicing and safety rules

Oil boilers need annual servicing just like gas ones, ideally by an OFTEC-registered engineer (the oil equivalent of Gas Safe). Skipping services risks soot build-up, inefficient burning and, in the worst cases, carbon monoxide leaks. Our guide on how often a boiler should be serviced applies to oil systems too. The Gas Safe Register site is aimed at gas engineers, but it's a good starting point for understanding why regular checks matter across any fuel type.

Tanks need checking as well: look for rust, cracks, and a bund that hasn't cracked or filled with rainwater. A leaking tank is an environmental hazard and can be expensive to remediate.

Common oil boiler problems

Many of the faults are familiar from gas systems. If pressure keeps dropping, our pressure guide still applies. Banging noises usually mean sludge or air in the system, covered in our banging noise guide. Oil-specific issues include a blocked fuel line, a failed nozzle, or simply running out of oil, which is easy to miss if you're not checking the tank gauge.

When to replace an oil boiler

Most oil boilers last 10 to 15 years with good maintenance. If yours is older, breaking down often, or struggling to heat the house, it's worth reading signs you need a new boiler. Manufacturers like Worcester Bosch and Ideal both make reliable oil-fired models worth considering, and Which? publishes independent reliability data if you want a second opinion before choosing.

Sizing matters too. An oversized or undersized oil boiler wastes fuel and money, so it's worth checking what size boiler you need before you commit.

If your oil boiler is on its way out, or you just want a straight comparison against gas alternatives, get a fixed price from a Gas Safe and OFTEC registered engineer who covers your area. We work across the Midlands, so check our coverage to see if that includes you.