If we want to make and keep our homes warm, both by bringing heat into them and then making sure it doesn’t easily leave, it can help to have some knowledge of how heat moves from place to place. This happens in three ways, which some of us might remember from science lessons at school – conduction, convection and radiation – and all of them are relevant to day-to-day heating issues in the home.
Conduction
When moving by conduction, heat is transferred from one part of a substance to another by vibration of its particles. If you place one end a metal cooking utensil in a flame, for example, the particles at that end will gain energy and begin to vibrate, passing the energy on to adjacent particles which also begin to vibrate, spreading heat throughout the utensil. The more energy there is in an object’s particles, the hotter it feels, so when you place your hand on pipes and radiators or on a car bonnet that has been heating up under the sun, you are feeling something whose particles contain a great deal of energy.
Energy is always transferred from hotter objects to colder objects (or from the hotter parts of a substance to the colder parts), not the other way round. So when you put ice cubes into a glass of water to cool it down, it’s not actually that the ice is transferring coolness to the water, but that the water, being warmer than the ice, is transferring heat energy to the ice, itself losing energy and becoming cooler in the process.
Solid objects – particularly metal objects – are good conductors, while fluids (liquids and gases) are poor ones. The contact of your hand upon a hot radiator or your foot upon a heated floor are examples of direct bodily experience of conduction. Heat may be lost from the home in this way though the walls, roof, floor and windows.
Convection
When moving by convection, heat is also transferred by means of particles, but this time by their free movement rather than vibration. As such, convection takes place in liquids and gases, where there is fluid movement of particles, rather than in solids.
As in conduction, energy is always transferred from the hotter parts of a substance to the colder parts. When heat is applied to a fluid, the density of particles in the area nearest the heat source decreases, while their volume increases, causing these particles to move into denser, lower-volume areas. The particles in those areas are displaced, eventually coming nearer the source of heat and gaining energy themselves – at which point they push back into their original areas, engendering a cycle of displacement between hot and cold areas known as a convection current, by which heat is continuously transferred and the temperature of the fluid becomes more uniform.
In our homes, we can feel the effects of convection with the water heating in our radiators and convection currents circulating warm air through the house. These same convection currents carry warm air towards colder areas, where it is lost by conduction through walls etc.
Radiation
Unlike in conduction and convection, when moving by radiation heat isn’t transferred by means of particles but by means of electromagnetic waves. Because it’s not reliant on particles, heat transfer by radiation can occur through a vacuum (where there are no particles), and this is how heat from the sun is carried to earth through space. This energy is absorbed by the earth and objects thereon, then can move from place to place by one of the other two methods.
Because of their name, we might suppose that heat from radiators is transferred by radiation, but in fact radiators mostly transfer heat through convection. Heat may be lost from the home by radiation through the windows, walls and roof.
