The three types of heat transfer are conduction, convection and radiation.

Conduction heat transfer is heat energy traveling from one molecule to another. A heat exchanger in an HVAC system or home furnace uses conduction to transfer heat.

Your hand touching a cold wall is an example of heat transfer by conduction from your hand to the wall. However, heat does not conduct at the same rate in all materials. For example, all HVAC copper conducts at a different rate than iron or aluminum, etc.

Heat transfer by convection is when some substance that is readily movable such as air, water, steam, or refrigerant moves heat from one location to another. Compare the words “convection” (the action of conveying) and “convey” (to take or carry from one place to another).

An HVAC system uses convection in the form of air, water, steam and refrigerants in ducts and piping to convey heat energy to various parts of the system. When air is heated, it rises; this is heat transfer by “natural” convection.

“Forced” convection is when a fan or pump is used to convey heat in fluids such as air and water. For example, many large buildings have a central heating plant where water is heated and pumped throughout the building to the final heated space. Fans then move heated air into the conditioned space.

Heat transferred by radiation travels through space without heating the space. Radiation or radiant heat does not transfer the actual temperature value. The first solid object that the heat rays encounter absorbs the radiant heat.

A portable electric space heater that glows red-hot is an example of heat transfer by radiation. As the electric heater coil glows red-hot it radiates heat into the room. The space heater does not heat the air (the space)—instead it heats the solid objects that come into contact with the heat rays.

Any heater that glows has the same effect. However, radiant heat diminishes by the square of the distance traveled; therefore, space heaters must be placed accordingly. Another good example of radiant heat is the sun; the sun heats the earth, but not the air around the earth.

The sun is also a good example of diminishing heat. The earth does not experience the total heat of the sun because the sun is some 93 million miles from the earth.

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