Electric heating

Encyclopedia : E : EL : ELE : Electric heating

An electric heater is an electrical appliance that converts electrical energy into heat. The heating element inside every electric heater is simply an electrical resistor, and works on the principle of Joule heating: an electric current flowing through a resistor converts electrical energy into heat energy.

Contents

1 Design variations

1.1 Radiative heaters or \"space heaters\"
1.2 Convection heaters
1.3 Fan heaters or \"forced convection heaters\"
1.4 Domestic electrical underfloor heating

2 Mathematical analysis

Design variations

Although they all use the same physical principle to generate heat, electric heaters differ in the way they deliver that heat to the environment. There are three basic types.

Radiative heaters or \"space heaters\"

Radiative heaters contain a heating element that reaches a high temperature. The element is usually packaged with a reflector inside a glass envelope resembling a light bulb. The element emits infrared radiation that travels through air or space until it hits an absorbing surface, where it is partially converted to heat and partially reflected.

Convection heaters

In a convection heater, the heating element heats the air next to it by conduction. Hot air is less dense than cool air, so it rises due to buoyancy, allowing more cool air to flow in to take its place. This sets up a constant current of hot air that leaves the appliance through vent holes and heats up the surrounding space.

In the United Kingdom, these appliances are sometimes called electric fires, because they were originally used to replace open fires.

Fan heaters or \"forced convection heaters\"

A fan heater is a variety of convection heater that includes an electric fan to speed up the airflow. This reduces the thermal resistance between the heating element and the surroundings, allowing heat to be transferred more quickly.

Domestic electrical underfloor heating

These systems are called radiant heating systems, regardless of whether they include a heat exchanger (also called a radiator) or are electrically powered.

When a home radiant heat system is turned on, current flows through a conductive heating material. For high-voltage radiant heat systems, line voltage (110 V or 230 V) current flows through the heating cable. For low-voltage systems, the line voltage is converted to low voltage (8 to 30 V) in the control unit (which contains a step-down transformer) and this low voltage is then applied to the heating element.

The heated material then heats the flooring until it reaches the right temperature set by the floor thermostat. The flooring then heats the adjacent air, which circulates, heating other objects in the room (tables, chairs, people) by convection. As it rises, the heated air will heat the room and all its contents up to the ceiling. This form of heating gives the most consistent room temperature from floor to ceiling compared to any other heating system.

Mathematical analysis

According to Joule's Law, the heat power produced by a resistor is:

[P=IV]

where

P is the power in watts

I is the current in amperes, and

V is the potential difference in volts,

and according to Ohm's Law I and V are related as follows:

[V=IR]

where

R is the resistance of the heating element, in ohms.

We can combine these two formulae to obtain the heat output from the heating element in terms of either current or voltage: [P=I^2R=\frac R]

For heaters powered by AC mains, I and V are the root mean square (RMS) values of current and voltage.

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