Where does water in a fuel tank come from?

Here at Compass Fuel Oils, we are the UK’s leading fuel oil specialists, providing next day delivery across the UK. This includes heating oil as well as Red Diesel too. And we know that for many businesses and households, keeping fuel levels stocked up over winter is a crucial concern. This doesn’t just mean ordering enough fuel, it also means taking care of the fuel in storage. That’s why we also provide a range of effective, fully bunded, storage tank options and bowsers. These offer a high level of protection for your fuel, including protection from water contamination. But where does water in a fuel tank come from? And how can this be prevented?

So, where does water in a fuel tank come from?

Water inside a fuel tank can cause a lot of damage to any system, and needs to be removed as soon as possible. But how does it get in there in the first place? Well there are usually two main sources:

  • Rainwater
  • Condensation


One of the most common reasons for water in the fuel tank is rainwater entering. This can occur in a number of ways, including:

  • The cap not being closed correctly
  • The cap not fitting properly
  • Corrosion around the seals
  • Faulty seals
  • Damaged vents
  • Rust causing cracks or perforations
  • Lack of maintenance

There is also the risk of receiving rainwater with oil that is purchased from an unreliable source.

Generally, fuel tanks that are stored outdoors are often more exposed to severe weather conditions, including wind and rain, and this can increase the risk of rainwater contaminated your fuel.

Condensation in the fuel tank

Another common reason for water in the fuel tank is contamination. This is often more common during the spring and summer months when the oil experiences temperature fluctuations. When oil is left in storage through temperature fluctuations between the tank’s interior and its surroundings, condensation can be created and water can be found in the fuel tank.

This is because while the external temperatures are increasing, the temperature inside the tank is increasing at a much slower rate. This allows any empty space within the tank to fill with warmer air from outside the tank. This draws in moisture from the humid air, and then cools forming water droplets on the inside of the tank. At first this will be the interior walls, but eventually the water will make its way to the base of a tank. As this process continues, the water level can build up enough to reach a dangerous level, and enter the fuel line. Here it can cause a large number of problems.

Condensation inside the fuel tank can be prevented by storage methods, and by not allowing the tank to run too low on fuel. This increases the space inside the tank for warmer air, increasing the humidity and the condensation amount.

For more information about how we can help meet your fuel needs all year round, why not get in touch today?