unit or bund that has at least 110% of the capacity of the inner tank. If the tank has a fill pipe that is not contained within the secondary unit, a drip tray must be installed. They also require the use of an automatic overfill prevention if it is not "reasonably practical" to monitor the oil levels within the tank. The Building Regulations Approved Document J covers the legal requirements for the installation of the tanks within the premises of a building. The regulations state that any new tank larger than 2,500 litres must be stored within a bunded tank or secondary containment that is a minimum of 110% of the tank's capacity. If a tank is single skinned and smaller than 2,500 litres, it must be given an individual site pollution risk assessment. This highlights any pollution or hazard risks such as the possibility of the oil escaping and reaching a river or stream, or the risk of a collision if the storage tank is located near a road. They further state that all tanks must be installed on a surface strong enough to support a full storage tank. The surface must be flat, even and fire-resistant, and should extend at least 300mm beyond the boundaries of the tank. A paving stone surface must be at least 42mm thick, and a concrete surface must be at least 100mm thick. The document also states that the tank should be situated at least 1800mm away from any potential hazards, such as doors, windows, appliance flue terminals, non-fire rated buildings such as garden fences, and at least 760mm from non-fire rated smaller structures such as wooden fences. A safe, secure tank that complies with all regulations will look similar to the diagram above. It details the different parts of the tank that need to be checked in order to ensure the tank is legal, including where the ancillary equipment should be located and the presence of an automatic overfill prevention. K-factor The degree day system is based on the amount of fuel a customer has consumed between two or more deliveries and the high and low outdoor temperatures during the same period. A degree day is defined as one degree of temperature below in the average temperature of one day. In other words, to arrive at the number of degree days in one day, the official high and low temperatures for that