Most oil installations have two filters – one at the tank and one on the burner. Both are changed at every service. A blocked filter starves the burner, and what comes out of the bowl tells you a great deal about the condition of your tank.
Two filters, and why both matter
A typical oil installation has a filter at the tank end, protecting the whole supply line, and a second smaller filter on the burner itself, immediately before the pump. They do different jobs and both need changing.
Changing one and not the other is common and it is not a service. The tank-side filter catches the bulk of what comes out of the tank; the burner filter is the last defence before a pump with very close tolerances and a nozzle with an orifice measured in thousandths of an inch.
What comes out of a filter tells you the condition of your tank. Clean element, clear fluid – the tank is in good order. Heavy sludge, dark sediment, or water sitting in the bottom of the bowl – the tank is shedding, and the filter is the symptom rather than the problem.
What a blocked filter does
- Starves the burner of fuel, so the pump cannot deliver the pressure the nozzle needs.
- Causes lockouts, typically presenting as a boiler that fires and then loses flame, or fails to prove a flame at all.
- Makes the fault intermittent, because a partly blocked filter passes enough fuel at low demand and not enough under load – which is why it gets worse as the weather gets colder.
- Stresses the pump, which is working against a restriction.
- Produces poor combustion before it produces a lockout, so you are burning more oil for less heat first.
The pattern to recognise: a boiler that has been slightly unreliable through the autumn and then fails properly in the first real cold snap. That is frequently a filter that has been progressively blocking since the summer.
What we find in them
| What comes out | What it means |
|---|---|
| Clean element, clear fuel | Tank in good order. Nothing further needed |
| Fine dark sediment | Normal accumulation over a year. Expected |
| Heavy black sludge | The tank is shedding. Common after a run-dry, and it may continue for weeks |
| Water in the bowl | Water in the tank – from condensation, a missing vent cap, or a failing steel tank corroding from the inside |
| Rust particles | A steel tank corroding internally. Worth an inspection |
| Waxy deposits | Cold-weather waxing, usually from an exposed section of supply line |
| Debris after recent work | Disturbed sediment. Should settle after one or two changes |
Water is the finding that matters most. It will not burn, it causes lockouts that look like ignition faults, and in a steel tank it is actively corroding the floor from the inside. See eight signs an oil tank is near the end of its life.
When to change them
- Annually, as part of the service. Both filters, every time. If your service record does not mention both, ask.
- After running the tank dry, without exception. The run-dry drew the tank floor into your supply line and the filters caught it – see running out of oil.
- After a tank replacement or any work on the tank, because disturbed sediment goes straight into the line.
- When lockouts start, particularly intermittent ones that worsen in cold weather.
- More often if the tank is shedding. A tank near the end of its life will block filters faster than annually, and that is information rather than an inconvenience.
What the job involves
- IsolateThe tank valve closed, and the boiler electrically isolated.
- Change the tank-side filterBowl off, old element out, bowl cleaned, seals checked and replaced, new element in.
- Inspect what came outAnd tell you what it means. This is the useful part.
- Change the burner filterWhich on most burners means removing the pump cover or the filter housing.
- Bleed and primeAir will have entered the line, and the burner needs bleeding before it will fire reliably.
- Fire and verifyBoiler run, combustion checked, and left working.
It is a short job on its own and it is included in every oil service we carry out. If you are having a service, you should not be paying separately for filters.
Boiler getting less reliable as the weather turns? A blocking filter is the commonest cause and it is a small job.
Questions we get asked about this
How often should oil filters be changed?
Annually, as part of the service, and both of them. More often if the tank is shedding sludge or after any run-dry.
Can I change them myself?
The tank-side filter is within reach of a confident owner with the right seals and somewhere for the spilled fuel to go. The burner filter needs the burner opened and the line bled afterwards, which is less straightforward.
Is water in the filter bowl serious?
It tells you there is water in the tank. It will not burn, it causes lockouts, and in a steel tank it is corroding the floor from the inside. It is worth acting on rather than draining each year.
Why does the filter block so quickly?
Because the tank is shedding – usually after a run-dry, or because the tank is nearing the end of its life. Repeated rapid blocking is a tank problem, not a filter problem.
Are filters included in a service?
In ours, yes – both of them. A service that does not change both filters is not a service, and it is worth checking what you are being quoted for.
Will a blocked filter damage the boiler?
It stresses the pump and produces poor combustion before it produces a lockout. Neither is catastrophic if dealt with, and both cost you fuel in the meantime.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
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.