Heat balance is achieved when the amount of heat entering a system is equal to the amount of heat leaving it. The overall temperature of a system in heat balance remains stable over time. In some cases, the term energy balance may be used instead. Systems in the fields of biology, geology, manufacturing, and spacecraft design often must be kept at stable temperatures in order to function properly.
Homeostasis in animals includes the concept of keeping the animal's internal temperature relatively stable. In many organisms, including mammals and birds, the amount of heat that exists within the organism must remain constant because even slight fluctuations in heat can cause the organism to die. Animals lose heat through their skin and generate heat through their metabolism to make up for rising and falling external temperatures. When the amount of heat radiating off of the animal and the amount of heat generated or absorbed by the animal are relatively equal, that animal is in a state of heat balance.
In geology, the term heat balance refers to the amount of heat absorbed and released by an entire planet. A planet is heated by the sun and, to a small extent, by the release of magma from the planet's core. The planet radiates some of that heat back out to space. Heat balance is an important concept geologically because if the planet is not in a state of relative equilibrium with regard to heat, the average temperatures can rise or fall, creating either global warming or global cooling. Even a slight change in the heat balance of a planet can effect the organisms that inhabit the planet.
Heat balance is an important concept in engineering because many machines will only run within a certain range of temperatures. Devices called heat sinks are used to help radiate surplus heat from electrical components so that the machines can maintain a stable temperature. Likewise, spacecraft must maintain relatively stable temperatures but are exposed to more heat as they move toward a heat source, such as the sun or a planet. Designing these craft with adaptable heat balancing systems allows them to continue to function as they travel through space.
When engineers design factories, the heat balance of the factory's operations must be taken into account. In many cases, heat is emitted from a factory in the form of smoke or steam. This can waste energy that the factory could recycle, helping lower the cost of operation. A factory with a good heat balance finds a use for most of the additional heat that it creates.