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Radiators and convectors are heat exchangers designed to transfer thermal energy from one medium to another for the purpose of space heating. The heating radiator was invented by Franz San Galli, a Prussian-born Russian businessman living in St. Petersburg, between 1855–1857.
In practice, the term "radiator" refers to any of a number of devices in which a fluid circulates through exposed pipes (often with fins or other means of increasing surface area), notwithstanding that such devices tend to transfer heat mainly by convection and might logically be called convectors.
The term convection heater or convector refers to a class of devices in which the source of heat is not directly exposed. As domestic safety and the supply from water heaters keeps temperatures relatively low, radiation is inefficient in comparison to convection.
A hot-water radiator consists of a sealed hollow metal container filled with hot water by gravity feed, a pressure pump, or convection. As it gives out heat, the hot water cools and sinks to the bottom of the radiator and is forced out of a pipe at the other end. Anti-hammer devices are often installed to prevent or minimize knocking in hot water radiator pipes.
Traditional cast iron radiators are no longer common in new construction, replaced mostly with forced hot water baseboard style radiators. They consist of copper pipes which have aluminum fins to increase their surface area. These conduction boiler systems use conduction to transfer heat from the water into the metal radiators or convectors.
The radiators are designed to heat the air in the room using convection to transfer heat from the radiators to the surrounding air. They do this by drawing cool air in at the bottom, warming the air as it passes over the radiator fins, and discharging the heated air at the top. This sets up convective loops of air movement within a room. If the register is blocked either from above or below, this air movement is prevented, and the heater will not work. Baseboard heating systems are sometimes fitted with moveable covers to allow the resident to fine-tune heating by room, much like air registers in a central air system.
Steam has the advantage of flowing through the pipes under its own pressure without the need for pumping. For this reason, it was adopted earlier, before electric motors and pumps became available. Steam is also far easier to distribute than hot water throughout large, tall buildings like skyscrapers. However, the higher temperatures at which steam systems operate make them inherently less efficient, as unwanted heat loss is inevitably greater.
Steam pipes and radiators are prone to producing banging sounds often incorrectly called water hammer. The bang is created when some of the steam condenses into water in a horizontal section of the steam piping. Subsequently, steam picks up the water, forms a "slug" and hurls it at high velocity into a pipe fitting, creating a loud hammering noise and greatly stressing the pipe. This condition is usually caused by a poor condensate drainage strategy and is often caused by buildings settling and the resultant pooling of condensate in pipes and radiators that no longer tilt slightly back towards the boiler.
A fan-assisted radiator contains a heat exchanger fed by hot water from the heating system. A thermostatic switch energises an electric fan which blows air over the heat exchanger to circulate it in a room. Its advantages are small relative size and even distribution of heat. Disadvantages are fan noise and the need for both a source of heat and a separate electrical supply.
Also known as "radiant heat", underfloor heating uses a network of pipes, tubing or heating cables is buried in or attached beneath a floor to allow heat to rise into the room. Best results are had with conductive flooring materials such as tile. The large surface area of such room-sized radiators allows them to be kept just a few degrees above desired room temperature, minimizing convection. Underfloor heating is more expensive in new construction than less efficient systems. It also is generally difficult to retrofit into existing buildings.
The Roman hypocaust employed a similar principle of operation.
Skirting board radiators are a form of heating which involves placing radiators inside a skirting board. Hot water is piped though the system, usually taken directly from the central heating system. 
Similar in configuration to forced hot water baseboard—low profile units running along the base of a wall with a central heating element surrounded by radiating fins—electric baseboard heaters are inexpensive to produce and install. They offer instant heat and great reliability, but may be more or less cost-effective relative to other forms of heat depending on electricity prices.
Electrically powered portable radiators come in two basic forms:
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