How Often Should an Oil Boiler Be Serviced

An oil boiler should be serviced at least once a year, or more often if its manufacturer says so, by an OFTEC registered technician. That is OFTEC's own guidance for oil heating, and an oil service has more to do than a gas one. Alongside the combustion checks, the nozzle and the fuel filter need attention, and the tank and the fuel in it are checked on the same visit.

It matters most in the parts of Kent away from the gas grid. Nearly one home in four in the Ashford borough is off the grid (24%), and so is about one in five in Tunbridge Wells (23%) and Sevenoaks (20%), against 16% across the South East, going by the government's 2024 figures. Those homes run on oil, LPG or electricity. We are OFTEC certified for oil and Gas Safe registered for gas, LPG included, based in Bearsted, and below is what an oil service involves and what to watch between visits.

Why at least once a year

An oil burner sprays fuel through a fine nozzle and burns it as a mist, and how cleanly it burns depends on that nozzle, the air setting and the fuel reaching it. All three drift over a year of running. A burner that is slightly off still heats the house, but it burns more oil to do it, leaves soot in the boiler that insulates the heat exchanger, and moves closer to a breakdown in the coldest week of the year.

OFTEC's guidance is that appliances should be serviced at least once a year or as the manufacturer recommends, and manufacturer warranties generally ask for an annual service too. If your boiler's instructions ask for more, follow them. The interval for a gas boiler service is the same once a year, but the work inside it is different.

What an oil boiler service covers

OFTEC sets out the shape of it. The appliance is inspected and cleaned, consumable parts such as the fuel nozzle and the flexible fuel lines are replaced as required, and the fuel filter element is replaced. The combustion is tested and the results written down. Open flues and chimneys are swept as often as the engineer advises.

The visit ends with paperwork. OFTEC's guide says an OFTEC registered technician will complete a CD/11, the service and commissioning report, recording the work done, the combustion results and any parts replaced. Keep it with the boiler's other papers, because it is the evidence of maintenance a manufacturer will want to see if there is ever a warranty claim.

The tank is part of the service

In our experience, having the tank looked at on the same visit as the boiler works better than treating them as two separate jobs, because OFTEC's guidance is that the tank should be visually checked at the annual service and the fuel tested for water, and water in the fuel is a fault that starts at the tank and ends at the burner. OFTEC warns that water can seize the burner's fuel pump, which is a costly repair.

OFTEC also recommends a visual check of your own between services, and gives a tank an expected working life of around 20 years. Walk round it every so often and look for anything that has changed, staining on the ground, damage to the tank or its base, or a bund holding water. If the tank is getting on, it is worth knowing its age before it becomes urgent.

On oil, the boiler and the tank are one system. A fault that starts at the tank finishes at the burner.

Letting the tank run low

The same OFTEC warning covers running low. Water, particles and other contamination settle at the bottom of a tank over time, and a tank run close to empty is more likely to draw them towards the boiler. Ordering before the level gets low is the cheap fix. If the tank does run dry, or it has been moved, have it checked before the boiler is relied on again.

OFTEC also points out that where the water is not taken out each year, water, particles and other contaminants can turn into sludge or even biological growth in the tank. That is the reason the fuel is tested at the service rather than only the boiler, and the reason a tank that has never been checked is worth looking at even when the boiler seems fine.

Signs it needs looking at sooner

• A smell of oil or fumes indoors
• Soot or black marks around the boiler or the flue outside
• Smoke from the flue when the burner starts
• A burner that struggles to light or keeps locking out
• A carbon monoxide alarm sounding

Any of those is a reason to call rather than wait for the service date. A burner that keeps stopping is behaving like any boiler that locks out, a safety device doing its job, and the cause needs finding rather than the reset button pressing. OFTEC's carbon monoxide guide says a detector should be fitted in the same room as any new or replacement heating appliance, and lists the symptoms to know, including headaches, dizziness, tiredness and nausea. An alarm that sounds means getting out and getting it checked, not opening a window and carrying on.

Oil and gas are different registrations

Oil boiler work comes under OFTEC, while mains gas and LPG come under Gas Safe, and an engineer can hold one, the other or both. Out in the villages it is worth asking which before booking, the same way checking a Gas Safe engineer's categories is worth doing for gas. What changes when a home is off the grid goes beyond the boiler too, from flue positions to the tank's place in the garden.

We hold both, so a house on oil gets the same visit as one on the mains, and our LPG and oil boiler work covers servicing, repairs and replacement. Around Ashford, Tunbridge Wells and Sevenoaks, the three districts with the largest off-grid shares in that list, a home on oil gets the same service as one on the mains.

When you book, it helps to have last year's service report to hand, the make and model of the boiler, and clear access to both the boiler and the tank, including any gate or cover in the way. If the boiler has been showing a fault code, or the burner has locked out, note it down, and say if you have noticed a smell of oil anywhere. It all shortens the visit and means the right nozzle and filter come with us.

So, have an oil boiler serviced at least once a year by an OFTEC registered technician, have the tank and the fuel checked on the same visit, keep the CD/11, and do not let the tank run low. CTL Plumbing & Heating services oil boilers across Kent from Bearsted, with free home visits and free quotes across Kent. To book a service before the weather turns, get in touch.