The main or primary heating system in any building is generally accepted as being the system that heats the largest proportion of that building, usually by means of a central heating system. This is a system where either water or air is heated in a primary heating source. The furnace or boiler is fuelled by oil or gas or occasionally by electricity. Many buildings, particularly private dwellings, often require a secondary heating system in one or more rooms to supplement the primary source of heat.
Fixed Secondary Heaters
Secondary heaters may be either portable or fixed. Fixed heaters are usually in the form of a fireplace, a stove or a hearth and must be included in the description of the property if it is to be sold. Secondary heaters are often wood- or coal-burning and require a chimney or flue to remove any fumes or dangerous gases produced by the heater. Many fireplaces and hearths that once burnt wood or coal have been converted to burning gas, using artificial logs and displaying naked flames to simulate the visual warmth of a natural fire.
A secondary heating device that is completely self-contained and at the same time portable is generally known as a space heater. The space heater can be one of several forms. Gas heaters typically have an LP gas cylinder in a cabinet at their rear, and fire elements at the front that must be lit. These heaters pose a possible risk from carbon monoxide poisoning and should be used only in rooms that have plenty of ventilation. The more common space heaters are electric, either using direct radiation from a heating element or in the form of an oil-filled radiator that is heated by electricity. Paraffin, or kerosene, heaters are still used in some countries but are considered dangerous, often causing death by fire or carbon monoxide poisoning.
Underfloor heating is becoming more and more popular. Although in many houses it is the primary heating source, it is often used in a single room as an alternative secondary heating source. The concept of underfloor heating goes back thousands of years, having been used by ancient Chinese and later by the Romans. Heating in those days was often achieved by an external fire with flues that distributed the hot gases. Today, it usually is either by underfloor electric wiring, in a method similar to an electric blanket's wiring, or in some cases by sending hot water through underfloor pipes.