Sludge is a consequence of heating with oil. It forms when waste products in the oil such as carbon and sulfur separate and sink to the bottom of the tank. Also, condensation forms in the delivery pipe, which is exposed to the outside air. The condensation drips into the tank, rusts the interior, and the rust particles also sink to the bottom. Over time, this combination of waste products and rust accumulates and turns into a thick, soupy, and sticky layer of sludge.
A small amount of sludge in the tank is normal and can usually be managed by changing the filter regularly, filling the tank slowly to avoid stirring up sludge and keeping the tank as full as possible to reduce the surface area that's exposed to condensation. There are also some additives you can put in the tank that will help break down the sludge and keep the oil flowing smoothly through the system. However, sludge can cause serious and expensive problems, especially if you have an older tank. Here's why replacing it might be the cheapest option in the long run.
Sludge can hide corrosion.
Sometimes the tank corrodes underneath the sludge layer. This creates a weak area that can break under pressure – either when the tank is being filled or when you use a suction device to remove the sludge. While this is a worst-case scenario, it is also the most dangerous and expensive to remedy. Many heating oil companies can inspect the tank and check for small leaks, but tanks aren't designed to last forever. If it's old and sludge has been a problem for years – or decades – it's a safe bet that there is a dangerous rust spot underneath the sludge. Replacing the tank is simpler and cheaper than cleaning up a major oil spill in your basement.
The problem won't get better.
As time goes by and more waste products and condensation are added to the mix, the sludge layer gets deeper and thicker. Eventually, it will clog the lines, valves and burner. While filters can prevent sludge from getting into these small parts, when the layer is thick enough, it will clog even the filter. Sludge doesn't reverse itself -- the older the tank gets, the more sludge builds up and the more parts you'll need to replace. Each time the clogs shut the system down and leave you without heat, you'll pay for a service call. If this is happening several times per year, it's probably time to retire your old tank and start over with a new system.
Even if your tank served you faithfully for many years without a major problem, sludge buildup is practically inevitable. Steel tanks that have been in place for more than 20 years are especially at risk, particularly if they are in a damp or humid basement. If you are changing filters more frequently, noticing more soot or losing heat regularly, ask your heating oil company about replacing your old tank. Modern oil tanks have features such as condensation-reducing valves to reduce sludge and double-walled fiberglass construction to prevent spills, both of which can make a newer tank cheaper in the long run.
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