So, what leads to the presence of water in a fuel tank, and why is it crucial to eliminate it promptly? The potential sources of water infiltration into a fuel tank primarily boil down to two main factors:

  1. Rainwater: One of the most prevalent causes of water contamination in fuel tanks is the entry of rainwater. This can transpire through various avenues, such as:
  • Incorrect closure of the tank cap.
  • Poor fitting of the cap, leaving gaps for water to seep through.
  • Corrosion around the seals compromising their effectiveness.
  • Faulty seals allowing water to ingress.
  • Damaged vents providing an entry point for rainwater.
  • Rust-induced cracks or perforations in the tank structure.
  • Inadequate maintenance practices. Additionally, there is a risk of rainwater contamination when purchasing oil from a non registered supplier.

Fuel tanks stored outdoors are particularly susceptible to the elements, including wind and rain. This exposure elevates the likelihood of rainwater infiltrating and compromising the integrity of the stored fuel.

  1. Condensation in the Fuel Tank: Another common cause of water accumulation in fuel tanks is condensation, a phenomenon that tends to be more pronounced during the spring and summer months. The fluctuation in temperatures during these seasons can lead to condensation formation. When oil is subjected to temperature variations while in storage, especially between the interior of the tank and its surroundings, condensation becomes more prevalent.

The process unfolds as external temperatures rise faster than the internal temperature of the tank. This temperature lag creates an opportunity for warmer air from outside to fill any vacant spaces within the tank. As this warmer air enters, it draws in moisture from the humid surroundings, which then condenses into water droplets on the inner walls of the tank. Initially, this moisture accumulates on the walls, eventually making its way down to the tank’s base. Over time, the water level can rise to hazardous levels, potentially entering the fuel line and causing a myriad of issues.

Preventative measures for condensation inside the fuel tank involve proper storage practices and avoiding allowing the tank to run too low on fuel. Maintaining an adequate fuel level increases the space within the tank for warmer air, consequently reducing the risk of condensation. You can also invest in a Tank Sponge, an environmentally-friendly tool designed to help prevent the build-up of water and sludge within your fuel tank, and maintain the quality and efficiency of your fuel.

The device consists of a hinged cage designed to hold replaceable TankSponge Eco inserts, each of which will soak up contaminated water from the bottom of your tank, whilst not soaking up any of the fuel, be that Kerosene, Gas Oil, or DERV.

The TankSponge Eco has also been designed with the environment in mind, with the specially moulded cage being made from 100% recycled plastic and being packaged in 100% recycled cardboard.

As part of your annual service, your boiler/aga engineer should be able to check the level of water in your tank for you to prevent further expensive repairs and let you know of the options available to you.

If you need any further assistance or advice, please find out more by visiting our Oil Tank Replacement or Boiler Servicing page or by calling one of the team on 01278 431451 or email

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