A full oil boiler service means the burner comes out of the appliance, the nozzle is replaced, both filters are changed, the combustion chamber and flueways are cleaned, and the boiler is recommissioned with a calibrated flue gas analyser. You should be given the readings on paper.
Why the burner comes out
The single clearest difference between a real oil boiler service and a superficial one is whether the burner was removed from the appliance.
An oil burner is a self-contained unit: a motor, a fan, a pump, a nozzle, two ignition electrodes, a photocell and a control box, all mounted on a plate that bolts to the front of the boiler with a blast tube projecting into the combustion chamber. Everything that wears is inside it, and almost none of it can be inspected, let alone serviced, while it is bolted in place.
So the service happens on a bench or a dust sheet, not while kneeling in front of the boiler. If nobody took the burner off, the nozzle was not changed, the electrodes were not gauged and the combustion chamber was not cleaned.
The sequence
- A reading before anything is touchedThe boiler is run from cold and a combustion reading taken before any cleaning. This is the number that tells you how the appliance has actually been running for the past year. Clean first and you have destroyed the evidence.
- IsolateElectrically at the spur, and the fuel at the tank valve. Then the burner comes off.
- New nozzleEvery time. Nozzles are not cleaned and refitted – the orifice is precision-machined and once it has eroded, its spray pattern has widened permanently. The correct size, spray angle and pattern for that specific appliance.
- Electrodes gaugedGap and position set to the manufacturer’s figures using a gauge. Electrodes pit and erode, and a gap set by eye is a gap that will cause a lockout in February.
- Pump pressure checkedA gauge on the pump’s pressure port, and the pressure set to the appliance data plate. This directly determines the firing rate – a pump running below specification underfires the boiler.
- Photocell cleanedThe sensor that proves the flame. A film of soot on it means the burner lights and then locks out because it cannot see its own flame.
- Combustion chamber and fluewaysBaffles out, chamber brushed and vacuumed, flueways cleared. The dirty half-hour that determines whether heat goes into your water or up the flue.
- Both filtersThe tank-side filter and the burner filter. Changing one and not the other is common and it is not a service.
- Flue and safety checksFlue integrity, terminal position, seals, fire valve operation and sensor position, and the condition of the oil line and any flexible connections.
- Recommission and analyseBurner refitted, boiler run to temperature, smoke test, and a full combustion analysis with a calibrated analyser.
The readings, and what they actually tell you
The analyser prints a ticket. Ask for it, and keep it – the value is in the trend across years, not in any single visit.
| Reading | What it measures | What a poor figure means |
|---|---|---|
| Smoke number | Soot in the flue gases, measured on a filter paper against a standard scale | Incomplete combustion. Soot insulates the heat exchanger and efficiency falls quickly |
| CO2 or O2 percentage | How much excess air the burner is drawing | Too much air heats the outdoors; too little makes soot and carbon monoxide |
| Flue gas temperature | How hot the exhaust leaves the appliance | A figure rising year on year means the heat exchanger is fouling |
| Net efficiency | The proportion of fuel energy actually delivered | This is the number that shows on your oil bill |
| CO | Carbon monoxide in the flue gases | A safety reading as well as a combustion one |
A single set of readings tells you the boiler is in tune today. Four years of readings tell you whether the heat exchanger is fouling, whether the burner is drifting, and roughly when the appliance is going to need something significant. Keep the tickets in the boiler’s folder.
What should also be checked while somebody is there
A service visit is the natural moment to look at the things that will cause the next call-out, and a good engineer will do this without being asked:
- The oil tank – bund condition, base, sight gauge, vent, fill point, and any signs of the tank deteriorating. See eight signs an oil tank is near the end of its life.
- The condensate pipe route, particularly any external section, before winter rather than during it.
- Water in the filter bowl, which indicates water in the tank and is a tank problem rather than a boiler one.
- The fire valve sensor, which frequently ends up dangling behind the boiler rather than clipped in position.
- System pressure and the expansion vessel, on a sealed system.
- Any CO alarm in the property – present, in date, and sited sensibly.
How to tell it did not happen
- It took ten minutes.
- You were given no readings.
- The engineer did not ask you to turn the boiler off in advance, or did not isolate it.
- No old nozzle, filters or soot appeared – a service produces waste.
- The engineer never went near the oil tank.
- There is no record beyond a sticker.
None of these prove bad faith on their own, but together they describe a visit that did not do what a service does. It is a reasonable thing to ask about, and any engineer doing the job properly will be pleased to explain the readings.
Booking a service? Ask for the combustion readings when you book, not afterwards – it tells you immediately what kind of visit you are getting.
Questions we get asked about this
How long should an oil boiler service take?
Long enough to remove the burner, replace the nozzle, clean the chamber and flueways, change both filters and recommission with an analyser. A visit measured in minutes has not done those things.
Does the nozzle really need changing every year?
Yes. The orifice is machined to thousandths of an inch and it erodes in normal use, widening the spray pattern. It cannot be restored by cleaning, and it is a small part relative to the efficiency it costs when worn.
What is a smoke test?
A measured volume of flue gas is drawn through a filter paper and the resulting stain compared against a standard scale. It is a direct, physical measure of how sooty the combustion is.
Should I get the readings in writing?
Yes, and keep them with the appliance. The year-on-year trend is genuinely useful and it is the only objective record of how the boiler is ageing.
Can I service an oil boiler myself?
Setting pump pressure and electrode gaps and verifying combustion requires calibrated equipment and competence, and getting it wrong produces carbon monoxide. Changing the tank-side filter is within reach of a confident owner; the rest is not.
When is the best time to book?
Late spring to early autumn. The boiler has just come off a hard winter so wear shows clearly, the diary is open, and anything that needs replacing can be dealt with in months rather than days.
You might also need
- Bunded or single-skin? How to tell which tank you are allowed
- Buying heating oil: order size, timing and tank management
- Eight signs an oil tank is near the end of its life
- Frozen condensate pipes: why it happens and how to stop it recurring
- Grant, Firebird, Warmflow, Worcester Danesmoor and Mistral, compared honestly
- Internal, external or boiler house: where an oil boiler should go
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.