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Heat loss is the net loss of heat through transfer from warmer to cooler areas. The environment usually works toward thermodynamic equilibrium, where no heat transfer occurs because everything is at an equal state. Along the way, heat loss occurs. This can be important for everything from understanding how refrigeration works to insulating a home to keep its internal temperature as stable as possible.
It is possible to transfer heat in several ways. One is through conduction, where a hot item comes into contact with a cooler one. The excited atoms in the hot object slam into those in the cool one, transferring energy and creating a net loss of heat. Convection with gases and fluids allows warm energy to circulate and leads to heat loss. In thermal radiation, charged particles emanate from an object and transfer to the closest object, like a dog lying in front of a warm stove.
In something like home design, heat loss is a cause for concern. The large surface area of the home creates a great deal of room for conduction to the outside air, while convection inside the home will encourage warm air to rise and then escape through the roof via conduction. Some steps designers can take to address these concerns include heavily insulating the home to create a barrier, as well as reflecting heat from the outdoors to prevent conduction through the walls and into the inside in hot climates.
Equations can model heat flow and loss for the purpose of design or research. Different materials have their own properties and these can determine the rate of heat loss and transfer. Researchers may be able to turn differing rates of conduction to their advantage in some settings. This can be seen with mechanical controls on some thermostats where two strips of metal cycle the heat on and off as they expand and contract in response to temperature changes.
Heat loss can also be a concern in medicine, where core temperatures can drop very quickly in bad weather and may be difficult to bring back up. Patients in a hypothermic state may need blankets, hot water bottles, and other supportive care to slowly warm back up again. As anyone who has come into contact with a person just out of a cold body of water knows, heat exchange can also occur between people. The person who was sitting on the shore will feel colder after contact with the swimmer because he is experiencing heat transfer.
I agree with @Sporkasia that old windows let heat escape and increase your heating bills during cold weather. However, just as much heat can be loss through an attic that is not insulated, or one that has some insulation but not enough. You should use the recommended thickness of insulation in your attic, and then add insulation underneath the house if possible.
Insulating the floors cuts down on a lot of heat loss. Unfortunately, too many people aren't even aware they are losing heat this way.
Living in an old house, I know a good deal about heat loss. In old homes that were built before builders knew anything about energy efficiency, the windows are usually one of the biggest concerns when talking about heat loss.
We have a sun room and two of the four walls in the room are all windows. This is great for letting in the sun, but it was also great for letting in cold air and letting heat escape during the winter. Before we replaced the windows you could put your hand on the inside of the window panes and they would be cold, even though the heat was on high in the house.
Once we replaced the windows, our heating cost went way down and we were much more comfortable in the house because we didn't have the cold drafts that we would get from the cold air entering from the outside.
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