What is sludge in an oil tank and is it a problem?

What sludge is

Heating oil is delivered to a British Standard, but a storage tank slowly collects other things. UKIFDA, the fuel distributors' association, lists the usual culprits: water from condensation, fine dirt and particles, and even biological growth. Over time these settle into a dark layer at the bottom of the tank. Aberdeenshire Council notes that sludge tends to build up where oil has been stored for long periods.

Most of the time it sits quietly at the bottom. The trouble starts when it moves.

When it becomes a problem

• The level runs low. With little oil above it, sludge is more easily drawn towards the outlet. UKIFDA advises keeping at least 12 inches (300 mm) of oil in the tank so the sediment is not disturbed, as it could clog the pipes.
• A delivery stirs it up. Fresh oil pouring into a near-empty tank churns the bottom layer, and UKIFDA notes that water in a tank can be stirred up during a delivery. The boiler may then lock out as debris reaches the filter.
• Water builds up. Aberdeenshire Council warns that sludge or water can block pipework, damage or corrode the heating system and make the oil less effective.

Signs of sludge

• The boiler cuts out or locks out, especially just after a delivery or when the tank is low.
• The burner starts and then stops, because a part-blocked filter lets through too little oil.
• Your technician finds a dark deposit in the filter or water at the bottom of the tank.

Bad fuel is rarely the cause. UKIFDA says delivered oil is made to a British Standard and problems blamed on fuel almost always come from contamination in the household's own tank.

Filters and tank cleaning

Oil systems have a filter between the tank and the burner to catch debris before it reaches the burner. UKIFDA says an annual inspection by an OFTEC-registered technician means any water can be found and removed, and filters and pipelines checked and changed if needed. If the boiler keeps cutting out, ask the technician to look at the filter first.

Where there is a thick layer, a suitable contractor can remove the water and sludge, clean the tank and work out why it built up. If the tank is old or damaged, replacing it may make more sense: OFTEC suggests seeking advice once a tank is over 20 years old. Changing filters, bleeding fuel lines and cleaning tanks are jobs for a professional, not DIY.

Keeping sludge in its place

• Do not run the tank low: UKIFDA recommends keeping at least 500 litres in where possible, and never less than 12 inches.
• Keep water out with tight lids, seals and sensors.
• Have the tank and filter checked at every annual service.

Topping up before the level gets low is the simplest defence. When you are ready, compare prices for your postcode.