Water in Your Oil Tank? How to Keep Your Operation Afloat

We’ve recently seen an increase in the number of enquiries on how to remove water from an oil tank, usually because a boiler or machine has started to play up. Read on to find out more about how water gets into an oil tank, how to test for it, how to remove it and how to avoid it in the first place. 

Could your oil tank have water inside?

Although an oil-based system is a safe and reliable way to heat your home or power your business, good maintenance is key in ensuring it lives a long and happy life.

Oil tanks aren’t impenetrable. So, while you may externally inspect your tank every few months, it’s just as important to take a look inside, because you could be missing the most fatal problem that lurks within – water contamination.

Water in Tank

What happens if water gets in your oil tank?

Water doesn’t mix with oil; it’s heavier, so it sinks to the bottom of a tank. Therefore, you won’t be able to see if there’s an issue by simply looking into the top of your oil tank. Often, the first sign of water contamination is when a fuel user has problems with their fuel system, and then it can be too late, as the damage has already been done.

You should take preventative measures to avoid these troublesome problems; by properly maintaining your tank and arranging regular fuel testing, you can avoid the hassle and costs associated with fuel contamination.

How does water get in an oil tank?

The two main sources of water in an oil tank are condensation and rainwater.

Rainwater can get inside an oil tank in a number of ways, including:

  • Ill-fitting cap / not being closed correctly
  • Corroded / faulty seals
  • Openings left open
  • Damaged vents
  • Rusting or perforations, cracks/splits in the body
  • Lack of maintenance
  • Purchasing old fuel or from an unreliable source  

Outdoor tanks are more exposed to severe wind and rain and are therefore more at risk from rainwater contamination.

Condensation is more common during the spring and summer months. It occurs when oil is left in storage through temperature fluctuations between the tank’s interior and its surroundings.

When external temperatures increase, the temperature within the tank increases at a much slower rate. Empty space within the tank fills with warmer air from outside the tank, drawing in moisture which then cools to form water droplets on the interior walls and eventually settled on the base of a tank.

Over time, this water can build up enough to enter the fuel line and damage burner components, affect combustion and freeze during cold months, blocking the oil tank outlet or fuel supply.

Letting your tank run on low fuel leaves more space for air, which increases the risk of humidity and condensation within.

Water in Oil

How to detect water in your oil tank

Regular OFTEC tank inspections

Having your tank checked regularly by an OFTEC registered engineer ensures your tank is in good condition and in the best possible state to deal with adverse weather conditions.

Water-finding paste

In the meantime, you can use some water-detecting paste to determine if any water is present. Using a dipstick or rod that is long enough to reach the bottom of the tank, smear the paste onto the end and lower it into the tank. It’s important to make sure it can touch the bottom as this is where water settles.

Leave the stick upright for the required time stated on the manufacturer’s instructions (usually around 30 seconds). If the paste changes colour, it will indicate how much water has been detected.

If water is detected, you’ll need to contact a registered OFTEC engineer company to come out as soon as possible before the problem worsens and causes damage.

How do you get water out of an oil tank?

Metal tank – drain it

Does your tank have a sludge valve or run-off tap attached to the bottom? This is an effective way to remove any water ingress as you can use it to drain off the water into a bucket or bund. This method will not remove every droplet of water, but it’s a good start before an engineer can come out.

You mustn’t let any oil seep onto the ground or dispose of it in a drain or waterway as this can cause environmental damage. Contact your local council who will be able to point you in the right direction of where to dispose it.

Plastic tank – pump it

The water will need to be “lifted” out using a pump, as most plastic tanks do not have a draining facility. Again, remember that you cannot dispose of the contaminated water down the drain. The best option is to get an OFTEC registered engineer to clean your fuel as this method is not effective to remove all of the water. Your feed pipe will also need flushing and your fuel filters changing.

When in doubt – polish it

Your fuel is lifted from your tank and run through a series of filters designed to remove water and other contaminants, by “fuel polishing” it. In the meantime, an OFTEC registered engineer performs a full tank clean, which removes any traces of water and contaminants left behind in the tank. After the engineer has finished, and your fuel has polished back to usable quality, your fuel is pumped back into your tank. 

How to avoid water contamination

Prevention is always better than a cure; particularly for your pockets when it comes to fuel storage.

  • Have 6 monthly tank inspections by an OFTEC registered engineer – particularly before winter to ensure your tank is working in good order ahead of the cold weather
  • Check your tank and pipework for signs of physical damage – look for signs of corrosion, aging, paint chips, rusting and cracks to avoid the risk of leaks. To avoid damage, ensure your tank is sheltered from the elements; snow, high winds, ice and falling branches, which all have the potential to damage an oil tank
  • Secure lids on your tank – it seems obvious, but they can become loose which will allow rainwater to easily get inside, especially as seals wear over time
  • Check your oil tank for bulging – this can mean weakness in exterior walls which can lead to cracks and leaks
  • Check your tank bund for oil, rubbish, water & plant matter – it must be in good condition and hold 110% of its interior in the event of an oil spill. Bunded tanks often have water trapped in the gap between an internal and external tank. Water, rust and oil can cause sludge to collect and erode the tank from the inside out
  • Give your tank some TLC – remove any signs of rainwater ingress and leaves from the bund, and cut back any overhanging shrubbery or trees as they can drip water onto your tank
  • Replace your tank – if it’s showing signs of wear and tear, it’s time to get an upgrade. Tanks need to be replaced typically every 15 years
  • Have regular tank cleans by a professional – this will eradicate any water and sludge from your tank and give the full picture of its health 
  • Keep your tank topped up – leaving it at low levels increases the chance of condensation to occur inside a tank
  • Buy your fuel from a trustworthy, reliable source – buying fuel from a disreputable or unreliable source can lead to water and other contaminants being pumped directly into your tank

Water in oil tank testing

What happens if I leave the water in my oil tank?

Water may seem harmless, but it can cause mayhem on your equipment or boiler if left untreated. Water within your fuel tank will create an ideal environment for bacteria to grow within your system, decreasing your efficiency.

Bacteria will cause issues with your tank, fuel lines and most importantly your fuel. It can be detrimental to your business operations if water within your tank isn’t removed as it may break a vital piece, which will come at a cost.

Rust & corrosion

If you leave water in a tank, over time it will cause corrosion to form which can lead to further problems such as leaks and a decline in your system’s efficiency.

Your equipment and fuel pump will also become damaged. The rust, water and oil on the bottom of your tank will form a bacteria-ridden sludge substance that will contaminate the fuel supply that feeds your system.

Bacteria-ridden sludge

Water in an oil tank presents the perfect breeding conditions for bacteria to thrive. Bacterial growth forms a sludge that settles on the bottom of your tank, producing an acidic by-product that corrodes the interior of your tank, its fuel lines, burners and filters. This leads to fuel degradation, leaks, severely decreased efficiency and ultimately, system failure.


Oil has a low freezing point, so cold temperatures rarely pose a threat unless they’re extreme. When contaminated with water, however, its resistance to the cold is much lower. Ice crystals form in the fuel at just 32 degrees Fahrenheit, helping cool it from within. The ice can also block supply pipes and prevent the flow of fuel, leading to a loss of power or even bursting pipes in severe cases.


Water’s viscosity is lower than diesel which provides a reduced lubricating cushion between the opposing surfaces of moving parts. This increases the chance of abrasive wear.

Suspect that there’s water in your oil tank? Get in touch today

Remember, the removal of water or any contaminant from an oil tank is a specialist job and correct disposal of the waste material is controlled by environmental legislation. Therefore, it’s always best to get an expert to carry out the works for you.

Leaks can lead to increased fuel costs and environmental damage. Fix the problem before it worsens and you have to dig into your pockets to replace your tank and your fuel.

If you suspect water contamination in your oil tank, get in touch today on 0330 123 1144 before the problem becomes too severe to rectify. Our OFTEC registered engineers can come to your site and clean your tank and its contents, and provide help and advice to stop future issues in their tracks.

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