The signs that matter are crazing and deformation on plastic, rust weeping and internal corrosion on steel, an unsound base, water in the filter, failing fittings, and a bund that is not doing its job. Most are visible from two metres away.
Go and look at it
Most people last looked closely at their oil tank on the day it was installed. It is worth ten minutes with a torch, once a year, ideally in the spring.
The eight signs
- Crazing on a plastic tankA fine web of surface cracks, worst on the sun-facing side. Early crazing is a warning; deep crazing with any deformation is a tank near the end. Look at the side that faces south.
- Bulging, deformation, or a tank that no longer sits flatPolyethylene creeps under sustained load. A tank that has changed shape is a tank under stress, and stress plus UV is how splits form. Often caused by an inadequate base.
- Rust weeping through a steel tankParticularly below a seam or near the base. Steel tanks corrode from the inside out, so visible rust coming through means the metal there is already thin. Not a paint job.
- Water in the filter bowlThe clearest evidence of water in the tank. On steel it is actively corroding the floor. On any tank it will eventually reach the burner and cause lockouts.
- A base that has moved, cracked or was never adequateSlabs on sand, settled corners, a tank overhanging its base, or water pooling underneath. The base is the most common underlying cause of tank failure and the easiest thing to ignore.
- Perished or damaged fittingsA sight gauge that has gone brittle or cloudy is one knock away from emptying the tank onto the ground. A missing vent cap lets rain in. A seized outlet valve means you cannot isolate the tank in an emergency.
- A bund that is full of water, cracked, or has its plug outA bund holding a season’s rainwater has lost most of its containment capacity, and on a steel tank that water is corroding the tank. A cracked outer skin means no containment at all.
- Any staining, smell or vegetation die-back around the baseThis is the one not to wait on. Oil in the ground is a pollution matter, and it gets more expensive by the week.
Which of these mean acting now
| Sign | Urgency |
|---|---|
| Staining, smell, or dead vegetation near the tank | Now. Possible leak – stop using it and call |
| Visible weeping or dripping from anywhere | Now |
| Perished sight gauge | Within days – it is a whole tank of oil waiting to empty |
| Rust weeping through a steel tank | Weeks. Plan a replacement, do not wait for winter |
| Deep crazing with deformation | This season. Replace before next winter |
| Water in the filter bowl | Investigate now, plan accordingly |
| Base settled or inadequate | This year. It is causing the next failure |
| Bund full of water | Empty it now, then work out why |
| Light surface crazing on a ten-year-old tank | Monitor annually |
Do not dig it over and do not hose it down – both spread it. Stop using the tank if you safely can, keep it away from drains and ditches, photograph it, and call. Contaminated ground is a different and far more serious job than a tank replacement, and time matters.
Age, and what it actually tells you
Broadly, domestic oil tanks last somewhere between fifteen and twenty-five years, and manufacturers’ warranties are commonly around a decade. But age is a weak predictor on its own.
What actually determines lifespan:
- The base, more than anything else.
- Sun exposure, for plastic.
- Water ingress, for steel.
- Whether the vent cap has been on for the whole life.
- Whether it has ever been run right down, which concentrates sludge and water against the tank floor.
- Whether anybody has looked at it.
So a fifteen-year-old tank is not automatically due, and a twelve-year-old tank in a bad position may be. Condition is what to assess. What age does usefully is tell you when to start looking properly – which is around the fifteen-year mark.
Planning rather than reacting
The difference between a planned replacement and a reactive one is substantial, and it is worth understanding before you are in the second situation.
| Planned | Reactive | |
|---|---|---|
| Timing | Spring or summer, tank deliberately run low | Whenever it failed, often mid-winter with a full tank |
| Oil in the tank | Minimal, cheap to recover | Full, and possibly contaminated |
| Ground conditions | Dry, easy for base work | Saturated |
| Heating | Continuous – the changeover is planned | Off until it is resolved |
| Cost | Predictable | Higher, plus possible contamination |
| Choice | Time to compare and decide | Whatever can be done quickly |
We inspect the tank as part of every oil boiler service, precisely so that this conversation happens in September rather than in February.
Have a look at yours this weekend. The sun-facing side of a plastic tank, or the seams and base of a steel one, tell you most of what you need to know.
Questions we get asked about this
How long does an oil tank last?
Roughly fifteen to twenty-five years, but position matters far more than age. A tank on a poor base in full sun will not reach the lower end; one well sited and kept dry can exceed the upper end.
My tank looks fine. Should I still get it checked?
The two most serious failure modes – internal corrosion in steel and stress in plastic – both look fine from a distance until fairly late. A proper look at the base, seams, bund and fittings takes minutes and it is worth having done.
What is the most common cause of tank failure?
The base. An inadequate or settled base stresses a plastic tank continuously and lets water sit under a steel one. It is the single most consequential thing to get right and the one most often skimped.
Is a perished sight gauge really urgent?
Yes. A sight tube that splits will empty the tank contents onto the ground, and it usually does it when nobody is there. It is a small part and it should be replaced at the first sign of brittleness or clouding.
Can a leaking tank be repaired?
No. A tank is a pressure-free vessel but it is still a containment vessel, and a repair to a corroded or split tank is not a repair, it is a delay. The fittings can be replaced; the tank cannot be patched.
What does a tank replacement involve?
Oil recovery, drain and clean the old tank, remove and dispose of it, assess or rebuild the base, site the new tank to current requirements including containment, renew the supply line and fire valve, and recommission. See oil tank replacement.
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
- 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
- Oil boiler lockouts: what the reset button does, and when to stop pressing it
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.