Oil Tank Problems — Leaks, Low Level Alarms & What to Check
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →An oil tank leak is an environmental emergency — oil can contaminate soil and groundwater. Contact your oil supplier and an OFTEC engineer immediately. You may also need to report it to the Environment Agency.
Oil tank problems often creep up quietly until something goes wrong — a gauge that stops working, a musty smell around the boiler, or worse, a visible puddle beneath the tank. Whether you're dealing with an ageing steel tank, contaminated fuel, or a faulty level indicator, these issues are more common than you'd think, particularly in rural properties and those off the mains gas grid. The good news is that many problems are straightforward to diagnose and prevent with a bit of know-how. This guide walks through the main culprits — stuck gauges, diesel bug, leaks, and compliance issues — and explains what you can safely handle yourself and when to call in an OFTEC registered engineer. If you spot a leak or suspect serious tank damage, professional help isn't optional; it's essential.
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Most likely cause & what to check
Oil tank gauge not reading correctly: plastic bung-and-float gauges can stick or fail over time. Gently tap the gauge housing — if the float is stuck it may free itself. For accurate readings, use a clean dipstick to measure the tank directly. Digital smart monitors (such as Watchman Sonic) are more reliable and send alerts to your phone.
Sludge or contamination in the oil: diesel bug (a microbial growth that forms in the presence of water) is a common problem in oil tanks, particularly in plastic tanks with poorly fitting lids that allow condensation. Symptoms include dark, gelatinous sludge in the fuel filter and the boiler blocking up repeatedly. An OFTEC engineer can treat the tank with biocide and fit a water-removing filter.
Tank leaking: a visible leak from the tank body, fittings, or supply pipe is serious. Isolate the oil supply valve immediately (turn the valve handle at the tank outlet). Contain the spill with absorbent material and call an OFTEC engineer and your oil supplier. Do not try to repair a leaking oil tank yourself.
Tank size and positioning requirements: domestic oil tanks must meet Building Regulations Part J (England and Wales) — typically a bunded (double-walled) tank is required if within 10 metres of a watercourse, within 50 metres of a public water supply, or in a flood-risk area. An OFTEC engineer or your oil supplier can advise on compliance.
Replacing an old single-skin tank: single-skin steel tanks more than 10–15 years old are a leak risk. A bunded plastic tank replacement typically costs £800–£2,000 installed. OFTEC registered engineers can carry out tank replacements to the required standard.
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Frequently asked questions
How do I know if my oil tank is leaking?
Look for dark staining or puddles around the tank base, supply pipes, and fittings — particularly after the heating has run for a few days. A strong oil smell near the tank is also a red flag. If you spot any of these signs, stop using the boiler, close the isolation valve at the tank outlet immediately, and contact your oil supplier and an OFTEC engineer straight away.
What is diesel bug and how do I prevent it?
Diesel bug is microbial growth (bacteria and fungi) that thrives in oil tanks when water is present, often caused by condensation seeping through poorly fitting lids on plastic tanks. Prevent it by ensuring your tank lid seals properly, keeping the tank in a dry, sheltered location, and having it inspected annually. If contamination does occur, an OFTEC engineer can treat the tank with biocide and fit a water-removing filter.
Why does my oil tank gauge keep sticking?
Plastic bung-and-float gauges can jam over time as the float mechanism wears or sediment builds up inside the housing. A gentle tap on the gauge casing sometimes frees a stuck float temporarily, but the most reliable fix is to install a digital smart monitor or simply use a clean dipstick to check the level manually. If your gauge fails regularly, replacement is inexpensive and worth doing.
Do I need a bunded tank, and what's the difference?
A bunded (double-walled) tank has an outer protective shell that catches leaks before they reach the soil — it's required by Building Regulations if your tank is within 10 metres of a watercourse, 50 metres of a public water supply, or in a flood-risk area. Single-skin tanks are cheaper upfront but offer no spillage protection; if yours is over 10–15 years old, upgrading to a bunded replacement is a sensible investment.