EGEE 102
Energy Conservation and Environmental Protection

Mechanisms of Heat Loss or Transfer

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Heat escapes (or transfers) from inside to outside (high temperature to low temperature) by three mechanisms (either individually or in combination) from a home:

  • Conduction
  • Convection
  • Radiation
Examples of heat transfer by conduction, convection, and radiation.
Examples of Heat Transfer by Conduction, Convection, and Radiation

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  • Conduction: heat moving through walls of a home from high temperature inside to low temperature outside.
  • Convection: heat circulating within the rooms of a house.
  • Radiation: Heat from the sun entering a home.

Conduction

Conduction is a process by which heat is transferred from the hot area of a solid object to the cool area of a solid object by the collisions of particles.

In other words, in solids the atoms or molecules do not have the freedom to move, as liquids or gases do, so the energy is stored in the vibration of atoms. An atom or molecule with more energy transfers energy to an adjacent atom or molecule by physical contact or collision.

In the image below, heat (energy) is conducted from the end of the rod in the candle flame further down to the cooler end of the rod as the vibrations of one molecule are passed to the next; however, there is no movement of energetic atoms or molecules.

Click play to start animation.

Conduction Candle Animation

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Example of Conduction

A hand holds a metal rod above a lit candle. The molecules quickly heat up at the point where the flame touches the rod. The heat then spreads across the entire metal rod and the heat is then able to be felt by the hand.

With regard to residential heating, the heat is transferred by conduction through solids like walls, floors, and the roof.

Example of Conduction in Regard to Residential Heating

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Example of Conduction in Regard to Residential Heating

Picture the cross section of a wall in a house. Inside the house it is 65°F and outside it is 30°F. Two arrows point from inside the house to the outside to show how heat is transferred from the inside of the house to the outside through the wall via conduction.

Heat loss across a solid wall by conduction

Convection

Convection is a process by which heat is transferred from one part of a fluid (liquid or gas) to another by the bulk movement of the fluid itself. Hot regions of a fluid or gas are less dense than cooler regions, so they tend to rise. As the warmer fluids rise, they are replaced by cooler fluid or gases from above.

In the example below, heat (energy) coming from candle flame rises and is replaced by the cool air surrounding it.

Example of Heat Transfer by Convection

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Example of Convection

A hand is held above a lit candle. As the candle heats the air, the heat rises to the hand. Eventually, it gets too hot and the hand pulls away from the candle.

In residential heating, convection is the mechanism by which heat is lost by warm air leaking to the outside when the doors are opened, or cold air leaking into the house through the cracks or openings in walls, windows, or doors. When cold air comes in contact with the heater in a room, it absorbs the heat and rises. Cold air, being heavy, sinks to the floor and gets heated, and thus slowly heats the whole room air.

Instructions: Press the play button below and observe what happens to the cold air (blue arrows) as it enters the house and encounters the warm air (red arrows) coming from the heating vent:

Convection in a Room Animation

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Example of Convection in Regard to Residential Heating

Picture a room with an open door letting in cool air on the left and a radiator creating heat on the right. As the radiator heats the air around it, the air rises and is replaced by cool air. Once the warm air hits the ceiling, it travels left towards the open door, cooling as it moves. The cool air from the open door travels to the right across the floor towards the radiator to be heated. The overall effect is a circular convection current of air within the room.

Radiation

Radiation is transfer of heat through electromagnetic waves through space. Unlike convection or conduction, where energy from gases, liquids, and solids is transferred by the molecules with or without their physical movement, radiation does not need any medium (molecules or atoms). Energy can be transferred by radiation even in a vacuum.

In the image below, sunlight travels to the earth through space, where there are no gases, solids, or liquids.

Radiation Example Animation

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Example of Radiation

Picture the Sun and the Earth with arrows traveling from the Sun to the Earth through space. The arrows represent the energy that travels to the Earth via radiation, which does not require any medium (atoms or molecules) to do so.

Test Yourself

First, identify the type of home heat loss pictured in images A-J as either: conduction, convection or radiation. Then click and drag each image down to the correct category at the bottom of the screen.

Test Yourself Activity

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Test Yourself: Types of Heat Loss

Identify the type of heat loss (conduction, convection, or radiation) for each of the following examples:

  1. Heat escaping through the roof of a house
  2. A hot stove burner
  3. Boiling water
  4. A torch halogen lamp producing light and heat
  5. A door hanging wide open, letting in cold air
  6. A fire creating heat
  7. Heat escaping through a wall
  8. A mirror reflecting sunlight
  9. Heat escaping through a window
  10. Heat escaping through a chimney

Answers:

  1. Conduction
  2. Radiation
  3. Convection
  4. Radiation
  5. Convection
  6. Radiation
  7. Conduction
  8. Radiation
  9. Conduction
  10. Radiation

Reducing Energy Consumption

There are two ways in which we can reduce energy consumption.

  1. The most cost-effective way is to improve the home’s “envelope”—the walls, windows, doors, roof, and floors that enclose the home—by improving the insulation (conduction losses) and sealing the air leaks with caulking (convection losses).
  2. The second way to reduce the energy consumption is by improving the efficiency of the furnace that provides the heat.
Line drawing: house with arrows pointing out from the walls and roof showing conduction & arrows flowing in a circular motion inside the house showing convection
Conduction and Convection

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Line drawing of a house with arrows pointing out from the walls and roof showing conduction & arrows flowing in a circular motion inside the house showing convection.