How do I know if there is water in my oil tank?

Where the water comes from

UKIFDA, the fuel distributors' association, says water is very unlikely to arrive with your delivery: heating oil has to be bright and clear to meet its British Standard, and most delivery pumps stop automatically if they detect water. The usual causes are:

• Condensation. A temperature difference between the inside of the tank and the air around it makes moisture form, and a tank in direct sun gets more of it. UKIFDA says keeping the tank topped up helps prevent condensation.
• Rain. It gets in through openings left open, faulty or damaged seals, or a tank that has split, cracked or rusted. Overhanging trees that drip onto the tank add to the risk.

Signs there may be water in the tank

• The boiler cuts out or locks out for no clear reason, especially soon after a delivery, which can stir up water at the bottom.
• Trouble in a cold snap. Kerosene stays liquid in normal UK winters, but UKIFDA notes that water in a tank can freeze and block the flow of oil to the boiler.
• The lid, seals, gauge or sensor are loose or damaged, or the tank shows rust, cracks or splits.
• Your technician finds water or sludge when checking the tank or filter.

How to test for water

Water-finding paste is made for this. Smear a thin layer on the end of a clean, dry dipstick or cane, lower it straight down through the fill point to the bottom of the tank, hold it there for as long as the pack says, then lift it out. The paste changes colour where it touched water, and the height of the change shows how deep the water is.

Only do this if you can reach the fill point safely from the ground, and refit the cap firmly afterwards. If you would rather not, ask for a water check at your annual boiler service: UKIFDA says an OFTEC-registered engineer inspecting the tank will check for water or condensation.

Getting it out

Removing water from a tank is a job for an OFTEC-registered technician or your oil supplier, not a DIY task. The water sits under the oil, it comes out mixed with oil, and it has to be disposed of properly. UKIFDA says that once water is detected it can be pumped out, and that you should get specialist advice on safe removal. A good technician will also find and fix how it got in, or it will come back.

Never pour oily water down a drain. If oil or oily water reaches a drain or watercourse, call the pollution hotline: 0800 80 70 60 in England, Scotland and Northern Ireland, or 0300 065 3000 in Wales. If the tank itself is damaged or old, ask your technician whether it should be replaced.

Keeping water out

• Keep lids, caps, gauges and sensors closed and tight.
• Keep the tank topped up through the colder months to cut condensation.
• Look the tank over regularly for cracks, splits, bulging or rust.
• Have it checked for water at every annual service.

Keeping the level up is the easiest fix of all. When you are due a top-up, compare prices for your postcode.