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Oil boiler servicing and maintenance guide


Jo Alsop

Heating Hero

The Heating Hub

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What is done during an oil boiler service? What are the maintenance requirements for my oil system?

Oil is a fuel that can be easy contaminated and this severely affecting the system’s efficiency and reliability. Water ingress to the tank can cause the oil to congeal and clog the nozzles. Frozen water or air will block the oil line and lock out the boiler. An annual service and regular maintenance are very important for an oil systems to ensure its continued operation. We guide you on the servicing and maintenance requirements needed to ensure your oil-fired heating system stays operational and minimises call out fees.

Annual Service

Unlike most gas boilers, an oil boiler services involves a lot of actual cleaning and regular renewal of perishable parts. Costs are around £120 - £180 depending on whether parts are replaced during the service.

Regular maintenance

During the service the Oftec engineer will undertake a visual inspection of the tank, oil line and other parts of the oil fired system. You should undertake any remedial or maintenance works detailed on the inspection sheet.

Common faults and their causes

OFTEC recommends that oil fired boilers, tanks and equipment are serviced at least once a year. Oil is a fuel that is easily contaminated and this can have a knock on effect on the efficiency and reliability of the system. At the very least, the boiler can lock out for a number of minor reasons (most of which can be prevented with an annual service) and will require an engineer to attend to reset it. Even clean burning boilers will dirty over time and an annual service is a must to keep it clean and prevent call outs.

The table below lists out common faults that require an engineer to attend, most of which are dealt with at the annual service. Oil boilers run so much more efficiently when they are clean and maintained and the annual service will deal with most of them preventing problems from arising.

Common problems Affects Faults Remedial works Preventative measures
Water ingress to the tank. Water is transferred with the oil to the boiler. This can carry debris from the tank. Oil congeals around the nozzle. Modern boilers will lock out when the photocell senses water in the oil. This will require an engineer call out to reset it.
Most older boilers will not sense water and will not therefore lock out. If left untreated, water in the oil will give rise to ‘incomplete combustion’, leading to soot deposits on the heat exchanger and a poor transfer of heat. In extreme cases the deposits will clog the baffles and make it impossible to service.
Soot deposits and incomplete combustion reduce efficiencies enormously, which means you will use more oil to heat your home.
Clean the nozzle and oil line, remove the water from the oil tank. NB the water will need to be taken to a local authority waste site. The OFTEC engineer will carry out a dip test in the oil tank during the annual service and arrange removal if necessary.
Frozen water in the oil line When the boiler is not running, usually overnight, water in the oil can freeze in the oil line during cold temperatures and block it. Boiler stops working as it cannot pull any oil through. Suction all of the oil and frozen water out of the oil line. Reignite the boiler and leave running until cold weather passes. As above
Clogged nozzles All oil boilers will accumulate congealed oil on the nozzles over time, which will cause them to perish Boiler lock out on newer models and incomplete combustion on older models driving down efficiencies and increasing oil consumption. Clean and replace nozzles Annual service with a nozzle change every 1-2 years
Soot deposits on the photocells The cell cannot detect whether the boiler is lit and will switch the boiler off Modern boilers only - this will lead to the boiler coming on and off, which is every inefficient. Clean the photocell Done during the annual service
Soot deposits on the electrode The electrode wears over time and gets covered in soot If it is worn or in poor condition it may not light the boiler again Clean or replace the electrode Check during the annual service – cleaned or replaced
Dirty condense trap Condensation and debris can collect in the trap Stops the boiler working Clean out Clean during the annual service


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Boiler checks and cleaning

The boiler works properly and efficiency when it burns the oil cleanly. When the boiler can achieve what engineers refer to as ‘complete combustion’, the boiler will extract get the most energy from the fuel. If the burner, fan, baffles or turbultors are dirty and sooted then ‘incomplete combustion’ will occur, which means not all of the energy from the fuel is transferred to your heating system. This leads to waste fuel products – soot - building up inside the boiler and reducing the efficiency further.

  • Check combustion chamber rope seal
  • Remove the burner and fan and clean
  • Clean and descale baffles
  • Clean and descale the primary heat exchanger
  • Remove, clean and replace turbulators
  • Check combustion levels after cleaning

Tank and system checks

  • Inspect and clean condensate
  • Test the fire valve
  • Clean the condense trap
  • Clean or replace the filter
  • Carry out a visual inspection of the tank and oil supply pipe to check for damage, deterioration and debris
  • Pressure test the oil supply pipe where it runs underground
  • Check system pressure

For more on your regulatory and environmental responsibilities as an oil tank owner go to our Guide to oil tank regulations. If you need to replace your tank try our Guide to fitted tank costs.

Jargon Buster

Component parts to an oil boiler system  
Burner and fan Located inside the boiler. Fires the boilers and draws in air to maintain the flame
Primary heat exchanger Located inside the boiler Transfer the heat to water
Turbulators Located inside the boiler. Improve the transfer of heat to water
Baffles Located inside the boiler. Used to control the air flow and maintain a healthy flame
Fire Valve Usually located externally, this will cut the fuel supply if temperatures reach over 85 degrees
Oil filter Fitted on the oil line, it removes impurities from the oil supply. NB not a solution to a sludgy/dirty tank that needs cleaning or replacing
Condense trap Fitted externally to the boiler and on the condense pipe. The condense trap collects condensation produced by the boiler and over time some sediment will build up
Tiger loop Maintains a constant supply of oil in the line to prevent air from entering the boiler.

Preventative maintenance checks

You can do your own checks throughout year to ensure the system is well maintained:

Tank checks to avoid water ingress to the tank:

  • Check there are no signs of corrosion on a metal tank or bulging on a plastic tank
  • Check for leaks and physical damage
  • Cut back any over-hanging trees that will continue to drip water onto the tank
  • Check pipe joints
  • Check for corroded seals around lids and hatches
  • Check lids and caps are secure
  • Check for water in the summer months that may build up due to condensation

Checking for water ingress:

  • Use a garden cane or any long stick
  • Coat in ‘water finding paste’ (available online)
  • Dip into the tank to the bottom
  • Wait the time period given on the instructions and pull out
  • Check the colour against the chart to see if water is present

Checking/preventing oil leaks

  • Visual check tank for leaks and physical damage
  • Check all of visible oil line for leaks
  • Fit a Watchman or similar to monitor for any sudden drops in oil which may be the result of a leak


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