Plastic tanks degrade from ultraviolet light and mechanical stress, failing by crazing and splitting. Steel tanks corrode from the inside out, driven by water settling beneath the oil. Site conditions should decide which is appropriate.
How plastic tanks fail
Polyethylene tanks are light, corrosion-proof, easy to handle, and by far the most common domestic choice. They fail in two related ways.
Ultraviolet degradation
Sunlight breaks down the polymer at the surface over years. The visible sign is crazing – a fine web of surface cracks, always worst on the side that faces the sun. It starts as a dulling of the surface, progresses to visible fine lines, and eventually to cracks that propagate through the wall.
A tank in full southern sun will show this considerably sooner than an identical tank in shade. It is the single strongest argument for thinking about orientation at installation.
Mechanical stress
A plastic tank supported unevenly is being twisted by its own weight continuously. Polyethylene creeps under sustained load, so the tank slowly deforms to match the imperfect base, and the stress concentrates at particular points.
Combine stress concentration with UV embrittlement and you get the classic plastic tank failure: a split, usually near the base, usually on the sun-facing side, usually on a tank that has been standing on inadequate slabs for fifteen years.
- Watch for: crazing, chalky surface, colour fading, bulging or deformation, any weeping at seams or around fittings, and a tank that no longer sits flat.
How steel tanks fail
Steel tanks are robust, fire-resistant, much less affected by sunlight, and generally have a longer potential life. They fail by corrosion, and the important point is the direction.
Internal corrosion
Water gets into an oil tank through condensation – warm humid air enters through the vent as the tank breathes, and condenses on cool steel overnight – and occasionally through a wet delivery or a missing vent cap. Water is denser than kerosene, so it sinks to the bottom and forms a layer under the fuel.
That water sits against the base of the tank permanently, and it corrodes the steel from the inside. The tank looks perfectly sound from the outside while its floor is thinning. By the time rust weeps through to the exterior at a low seam, the metal at that point is already very thin.
External corrosion
Where paint has failed, where water sits on a flat top, and critically where the tank meets its supports – a steel tank sitting in standing water on its base corrodes there first and it is the hardest place to inspect.
- Watch for: rust streaks below seams, blistering paint, any weeping, standing water on the top or underneath, and water in the filter bowl at every service – which is the clearest early evidence of water inside the tank.
Whatever your tank is made of, water appearing in the filter bowl tells you there is water in the tank. On a steel tank, that water is actively corroding the floor. It is worth acting on rather than simply draining the filter each year.
Which suits which site
| Factor | Favours plastic | Favours steel |
|---|---|---|
| Full sun exposure | No – UV is the enemy | Yes – largely unaffected |
| Shaded, damp position | Yes | Less good – external corrosion |
| Fire separation tight | No | Yes – non-combustible |
| Access difficult, tank must be carried in | Yes – far lighter | No – heavy |
| Coastal or exposed location | Yes – no salt corrosion | Less good |
| Very long intended life | Reasonable | Better, if kept dry |
| Budget | Generally cheaper | Generally dearer |
| Bunded version needed | Readily available integrally bunded | Available, heavier and costlier |
For most domestic Norfolk installations, an integrally bunded plastic tank on a proper base in a position that is not in full unrelieved sun is the sensible default. Steel makes a strong case where fire separation is tight, where the position is exposed to impact, or where the tank will sit in a boiler house.
Making either last
- The base. Level, non-combustible, extending beyond the tank, draining away from it. This matters more than the material.
- Keep the vent cap on. A missing vent cap lets rain straight in, and water is what kills steel tanks and contaminates fuel in both.
- Shade a plastic tank if the position is unavoidably sunny – but with something non-combustible and at a proper separation distance, not a timber screen against the tank.
- Keep a steel tank painted and keep water off the top and out from under the supports.
- Keep a working reserve in the tank, so the sludge and water layer at the bottom is not drawn into the line.
- Have it inspected, particularly past fifteen years – see oil tank inspection.
- Check the filter bowl at every service, and treat water there as information rather than a nuisance.
Tank over fifteen years old? A condition assessment tells you what you are looking at, and when – rather than finding out on a wet morning.
Questions we get asked about this
Which lasts longer, plastic or steel?
It depends entirely on the position. A plastic tank in shade on a good base can outlast a steel tank standing in water. A steel tank kept dry and painted can outlast a plastic tank baking in the sun. Condition, not material, is what to assess.
Is crazing on my plastic tank serious?
It is the first stage of UV degradation and it is a warning rather than an emergency. Fine surface crazing on a tank of ten years is worth monitoring; deep crazing with any deformation on a tank of twenty is worth planning a replacement around.
My steel tank has rust streaks. How bad is that?
Rust weeping through from inside means the metal at that point is already thin, because steel tanks corrode from the inside out. It is not a paint job, and it warrants an inspection.
How does water get into an oil tank?
Mostly condensation – the tank breathes through its vent as temperature changes, and humid air condenses on cool steel or plastic overnight. Also from a missing vent cap, a failed fill cap, or occasionally a wet delivery.
Can water be removed from a tank?
Yes, by draining from the lowest point or pumping it out, depending on the tank. It is worth doing if there is a significant quantity, and worth establishing why it is getting in.
Should I paint my plastic tank to protect it from UV?
No. Paint will not adhere reliably to polyethylene and it can trap moisture. Shading with a non-combustible screen at a proper separation distance is the better approach if the position is unavoidably sunny.
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?
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