Fireplaces are one of the oldest methods of heating a home and can still serve a purpose in a modern residence. In warmer areas of the world, this simple method can still provide all the heat that is necessary. In colder areas or in larger homes, more than one unit may be required. Traditionally, the units were built of stone or masonry as an integral part of the home's construction. They typically burned wood or coal and vented the combustion products to the outside through a brick or stone chimney. These heating systems had a few disadvantages such as the work and mess of cutting and keeping wood available. They also were not very energy efficient as air from the inside of the home was used for combustion and the rooms where the units were located were typically a lot warmer than other areas of the home. Gas logs were invented to allow the homeowner to have the atmosphere without the mess. These systems basically consisted of fake logs made of ceramics and a gas burner system. These systems used a pilot light which was a small flame that burned constantly and was used to light the main burner when desired. These units eliminated the mess and inconvenience of the traditional systems but did nothing to improve the efficiency as the combustion air was still taken from the inside of the home. To improve the home's energy efficiency, inserts were developed. These units allowed the use of the existing fireplace for heat but minimized the escape of conditioned air up the chimney. At first they only burned wood but today units are offered for various types of fuel. With the increasing demand for energy efficient products, the modern gas system was born. These units use metal vent pipes instead of the masonry chimneys of the past. B vent units are typically used in situations where the unit can be vented vertically through the roof of the home. Direct vent models can be installed in situations where the combustion products must be vented horizontally through the side wall of the home. The latest models have variable speed blowers to maximize their efficiency as well as remote controls for convenience. The variable speed blowers allow the heat to be more evenly distributed throughout the room and/or home. For situations where venting the combustion products is not practical, unvented gas-fired models are available. These units use an oxygen depletion sensor to sense the quality of the air in the room. If the amount of the oxygen in the room's air drops, it is a sign that the unit is not burning properly and the gas valve is shut. For situations where a gas burning appliance is not suitable, electric fireplaces were developed. These units use the typical ceramic logs to simulate the fire but actually the heat is produced by electricity. The latest innovations include units that burn ethanol or biomass fuels.