Water in Your Oil Tank: Causes, Signs and Fixes
Water is one of the most common causes of heating oil problems. Here’s how it gets into your tank, how to check for it, and what to do about it.
The short answer
Water gets into oil tanks through condensation, worn lids and seals, and cracks. Because it’s heavier than heating oil, it sinks to the bottom, where it causes sludge, corrosion and blocked filters. Check for it with water-finding paste, and have it pumped out by a technician before it reaches your boiler.
How does water get into an oil tank?
Condensation
Air in the empty space above the oil contains moisture. As the temperature swings between day and night, that moisture condenses on the inside of the tank and runs down into the oil. A tank that is often left low, especially through the summer, collects more condensation.
Damaged lids, seals and fill caps
A cracked or badly fitting lid, a perished seal or a missing fill cap lets rain straight in. On bunded tanks, rain can also collect in the bund itself, which then needs pumping out.
Vent and fill point problems
A damaged or blocked vent, or a fill point that isn’t properly sealed, can let in rain during heavy weather.
Cracks and splits
Older plastic tanks can split, and steel tanks can rust through. As well as letting water in, a crack can let oil out, so treat any crack as urgent. See our guide to a leaking oil tank.
Signs of water in your heating oil
- The boiler locks out or needs resetting more often than usual
- The burner sputters, smokes or runs unevenly
- Oil in the filter bowl looks cloudy or has droplets or a layer of water at the bottom
- The filter keeps blocking with black or brown sludge
- The oil line or valve freezes in cold weather
- Rust on the inside of a steel tank, or around the outlet
How to check for water in your oil tank
The standard test uses water-finding paste, which is available from heating and plumbing suppliers:
- Spread a thin layer of paste on the last few centimetres of a clean dipstick or wooden rod
- Lower it slowly through the fill point until it touches the bottom of the tank
- Hold it there for the time stated on the paste, then lift it out
- The paste changes colour wherever it touched water, showing how deep the water layer is
Don’t lean over or climb on the tank, and never use a naked flame near it. If you’re not comfortable doing the test, a technician can check it as part of a service.
Why water in the tank matters
| Problem | What happens |
|---|---|
| Microbial growth | Bacteria and fungi grow where oil meets water, forming a thick sludge sometimes called “diesel bug” |
| Blocked filters | Sludge and water clog the filter, starving the boiler of fuel |
| Boiler damage | Water reaching the burner causes misfires, lockouts and corrosion of the pump and nozzle |
| Tank corrosion | Steel tanks rust from the inside where water sits at the bottom |
| Frozen fuel lines | Water in the line can freeze in winter and cut off the oil supply |
How to get rid of water in an oil tank
Professional water removal
A technician pumps the water and sludge out from the bottom of the tank and disposes of it as hazardous waste. This is usually quick and leaves your clean oil in place.
Tank cleaning
If there’s a lot of sludge, the oil is pumped into holding tanks, the inside of the tank is cleaned, and the filtered fuel is returned. The filter is replaced at the same time.
Fixing the cause
A new lid, seal or fill cap often stops the problem coming back. If the tank itself is cracked, corroded or near the end of its life, replacement is the safer option. Find out more about our oil tank servicing and cleaning and oil tank replacement.
How to stop water getting into your tank
- Keep the tank reasonably full, especially in summer, to reduce condensation
- Check that the lid and fill cap are closed and the seals are in good condition
- Keep the vent clear and undamaged
- Check bunded tanks for rainwater in the bund
- Have the tank inspected every year with your boiler service
- Replace the filter at the recommended interval
Frequently asked questions
How do I know if there is water in my oil tank?
The most reliable check is water-finding paste on a dipstick lowered to the bottom of the tank. The paste changes colour where it touches water. Boiler lockouts, a sputtering burner, cloudy oil and a blocked filter are common warning signs.
Can I remove water from my oil tank myself?
We don’t recommend it. Water and sludge are contaminated with oil, so they count as hazardous waste and must be disposed of properly. Disturbing the bottom of the tank can also stir sludge into the fuel line. A technician can pump out the water and sludge safely.
Will water in the tank damage my boiler?
It can. Water blocks filters, causes the burner to misfire or lock out, and corrodes the pump and nozzle over time. In cold weather it can freeze in the fuel line and stop the oil supply altogether.
Does a little condensation matter?
Small amounts of condensation are normal, but they build up over years. An annual tank check catches it before it reaches the outlet. Keeping the tank fuller in summer reduces condensation.
Think there’s water in your tank?
Our OFTEC registered technicians cover Lincolnshire, Nottinghamshire, Yorkshire and Rutland. Call 01673 572222 or send a few details for free, no-obligation advice.
More advice: all oil tank guides