This system is the grand of the more recent systems. It is simply laid on top of the concrete slab by using specially notched spacers on which the heating cables are simply hooked. Cables are usually spaced from each other. A screed of around 25mm to 50mm thick usually a mixture of river sand and standard cement to a ratio is poured over the wires and leveled using conventional tools. All types of flooring can be used: from ceramic tiles to wood. This system is also used lately with the now fashionable colored screeds using chemical oxides. Oil furnaces have a pilot light that warms a heat exchange unit, which in turn warms the air before it is circulated back through the house. These furnaces have a flue where exhaust vent to the outside. An electric furnace uses heating strips, or elements, to warm the air.