There are three types of heat transfer; conduction, convection, and radiation.
Conduction
Conduction is the transfer of heat between objects that are in direct contact.
Example: A pot on a stove burner. As the pot absorbs heat, heat will transfer through the pot handle to your hand.
Convection
Convection is when a fluid, such as a liquid or air moves from one location to another and carries its thermal energy along with it.
Example: As the space heater heats the air surrounding it near the floor, the air will heat in temperature and rise to the top of the room. This forces down the colder air, so it becomes heated.
Radiation
Radiation is heat transfer that does not rely on contact between a heat source and the heated object. Heat is transferred through empty space by thermal radiation.
Example: Heat from the sun
What this means on a real service call
Most heating and cooling problems involve all three types of transfer at once, and sorting them out helps you find the real fault. Conduction moves heat through solid material: through a heat exchanger wall, through copper tubing, through duct metal, or through a wall. Convection moves heat with a moving fluid: warm air rising, a blower pushing air across a coil, or water moving through a hydronic loop. Radiation moves heat without contact or air movement: a hot heat exchanger radiating to nearby surfaces, or sunlight warming a roof and then the rooms below it.
A furnace is a good example. Combustion heats the heat exchanger by conduction through its metal, air moving across the outside of the exchanger picks up that heat by convection, and the blower delivers that warmed air to the rooms. A cooling coil works in reverse: warm room air passes over a cold coil, heat moves into the refrigerant by conduction through the coil wall and by convection as air moves across the fins, and the refrigerant carries that heat outside.
Numbers and measurements a technician actually sees
You will rarely measure conduction or radiation directly on a service call. You will measure their effects: temperatures on both sides of a component, and temperature differences across it. A useful check is the temperature drop across a heat exchanger, across a coil, or along a duct run. If air leaves a furnace at, for example, 120 degrees Fahrenheit and arrives at a distant register at only 95 degrees, a large part of that heat has been lost on the way, often by conduction through uninsulated ductwork and by convection to a cold attic or crawl space.
Also watch surface temperatures with care. A supply duct that feels hot in summer or cold in winter is a sign of heat transfer you do not want. Insulation works by slowing conduction and convection, not by stopping heat completely. That is why duct insulation, sealing, and keeping ducts inside conditioned space when possible make such a difference to comfort and to run time.
A common mistake
A common mistake is to blame the equipment when the problem is the path heat must travel. A room that is always cold may not need a larger furnace. It may have a long, uninsulated duct run, a disconnected duct, or a return that pulls cold air from an unconditioned space. Likewise, a room that overheats in summer may be gaining radiant heat through a sunny window or an uninsulated roof, not suffering from a weak air conditioner. Fixing the path is often cheaper and more effective than upsizing equipment.
A practical checking step
For a room that will not stay comfortable, work through the path heat takes. First, check airflow and the filter, because convection depends on moving air. Second, feel or measure temperatures at the equipment and at the farthest register, to see how much heat is lost in the duct run. Third, look for conduction and radiation problems you can see: uninsulated ducts or pipes, a hot attic, a sunny window, or a missing panel that lets air bypass a coil. Fourth, check for air leaks at accessible duct joints, because leaked air is heat that never reaches the room.
Safety caution: do not reach into a running blower, touch a hot heat exchanger, or remove a guard to feel a surface. Let hot surfaces cool, turn power off before opening a blower compartment, and use a thermometer or an infrared thermometer where it is safe to do so. If a heat exchanger is cracked or damaged, do not keep running the furnace; shut it down and get it checked by a qualified technician, because combustion products can enter the air stream.
Related posts
For the heat itself, see About heat and Types of Heat. For how a furnace makes that heat, see About Combustion.
One more distinction helps on the job: natural convection happens on its own, as warm air rises and cooler air falls, while forced convection is driven by a blower or a pump. If a blower fails, you lose forced convection, and the system may overheat, freeze, or short-cycle even though the heat source itself is working fine.