02.Heat transfer - a law of nature
Heat always travels from warm to cold areas (for example, from the heated interior to the winter air outside your home) — until both areas have the same temperature. Heat can be transferred in three ways:
Convection: Heated air moves about — usually upwards. The smoke of an open fire, for example, is carried upwards by convection currents.
Radiation: Heat transfer from a warm body to a cold one through direct radiation. The air between the bodies isn’t warmed up in this process. For example, if you stand in front of an open fire, its radiation will warm you although the air around you is still cold.
Conduction: Direct heat transfer through solid materials. For example, a poker that’s partially thrust into an open fire will gradually become hot along its whole length due to conduction.
If you want your home to be warmer than the outside, you need to reduce and/or replace heat losses. Insulating your ceiling and walls can reduce loss; a heater can replace them, using radiation and/or convection.
A heater that uses radiation only won’t efficiently heat the air in a room — so use it as a personal heater. Convection alone also may not be enough to evenly heat a room, but may lead to stratification. Additional air movement (for example, using a fan) can improve this.
The heat map of a test room clearly shows the stratification effect created by a convection heater when there’s little air movement in the room: the yellow bar at the ceiling represents about 22°C, the light blue bit (where your cold feet would be) about 14°C.
Watt's a joule?
Energy is measured in joules (J). The rate at which energy is used over a period of time is measured in watts (W), with one watt being the use of one joule per second.
Joules and watts are used to rate different types of heater. Gas heaters are rated according to the energy that goes into them each hour in megajoules (MJ = one million joules). Electric heaters, on the other hand, are rated in kilowatts (kW = one thousand watts).
You can compare the capacity of heaters with such different ratings. For example, an electric heater with a rating of 1 kW running for one hour uses one kilowatt hour (kWh). This is equal to 3.6 MJ (1000 Wh x 3600 seconds per hour).