Over time, a number of factors can contribute to a build up of water in your oil fuel tank, for example:

  • If your tank has been damaged or is old
  • The filler cap has been left open
  • During severe weather, such as strong winds and heavy rain
  • If there are fluctuations in the external temperature

Boilers, machinery and vehicles can be affected significantly by water contamination in your oil tank, so it’s important to know how to spot it and what steps you can take to avoid it.

How does water get in your oil tank?


More common during the warmer months and in partially filled fuel tanks, water contamination can occur when cold water vapour meets a warmer surface.

When the external temperature starts to increase, for example during the spring and summer, the tank’s internal temperature is cooler than the outside climate. This causes moist air to cool inside of the tank and water vapour in the air to produce water droplets on the inner walls. Since water is heavier and denser than most liquid fuels, this produces a layer of water which settles on the bottom of the tank.

Leaving your tank half full leaves room for air which increases the risk of humidity and condensation. We recommend ensuring your tank is topped up year-round to help reduce the risk of condensation in your tank and to also reduce the risk of running your tank dry.


Old tanks and a lack of regular maintenance vastly increases the risk of rainwater ingress in your tank. There are many ways that water can get inside, including:

  • Damaged vents
  • Faulty or corroded seals
  • Cracks in the tank
  • Poorly fitting filler caps

Oil tanks have a lifespan of around 10-15 years, providing they have regular tank maintenance. At Speedy Fuels, we offer a wide range of OFTEC-standard plastic and steel fuel tanks and employ a team of engineers who carry out a wide range of industrial tank services. Get in touch today to find out more.

The problems of water in your oil tank

  • Microbial contamination – water creates the perfect breeding ground for bacteria to form. Bacteria release acid that corrodes tanks, fuel lines, filters and causes sludge build-up on the bottom of tanks. Eventually, this will cause problems with equipment such as difficulty starting or even worse, failure.
  • Corroded oil tank – with metal tanks, corrosion forms as a result of water contamination, from the inside out – so without regular checks, you won’t be aware of any issues that are occurring within. Water produces iron oxide (rust) when it comes into contact with iron and steel surfaces, causing abrasive wear to machinery and equipment parts. Corrosion causes sediment and bacteria to build-up, causing injector problems, damaged burner components and eventually, a full tank replacement.
  • Frozen fuel lines – if your fuel is contaminated with water, it can freeze at just 0oC, creating ice crystals. These can create wear in fuel systems, blocking fuel filters and increasing corrosion by expanding inside larger cracks.

How can I spot water in my oil tank?

During the winter, we receive an increasing number of calls due to high rainfall and freezing temperatures, when unfortunately, equipment or machinery have started to show issues.

Regular fuel inspections

External inspections cannot adequately detect water contamination, so it’s important to invest in regular inspections by a professional company . Our engineers can come to your site and test your fuel to determine the level of contaminants and the course of remedial action to take.

If only a small level of contamination is found, our engineers can clean your tank and polish your fuel back to its former glory. Unfortunately, if it’s severely contaminated, you will need a fresh supply of fuel. Our engineers will always aim to clean your tank but you may also need a replacement fuel tank, to avoid putting clean fuel back into a dirty, corroded tank.

Water detection paste

A water detection paste can help determine if there’s water present in your tank during your routine checks. The paste will change colour when it comes into contact with water and is available in both red and green.

To check for water in your oil tank, apply some of the water detection paste on a stick and gently place it inside the tank until it hits the bottom. Leave it for approximately 30 seconds and then carefully take it out of the oil tank. You will know if water is present if the paste has changed colour. The depth of the water will also be indicated on the stick. If the stick has dots on it then water is held in suspension.

To ensure that all the water is removed, you should change the oil filter and flush the boiler feed pipe. Depending on your expertise, this is perhaps an action that ought to be carried out by facility engineer.

Want to find out more about removing water from your oil tank? Then give us a call today on 0330 123 3773 and we will help you in any way we can.