What is Heat Transfer?

Submitted by DavidAndersen on Mon, 03/16/2009 - 08:47.

What form of heat transfer causes the indoor air of a building to rise on a clear sunny day?

Lets say this is a residential building with a gable roof.
There is a 5 foot attic space with flat ceilings.
The air conditioning is set at 68 F. It is 85 F. outdoors.

Actually there is no definitive answer because there are more possible scenarios than there are answers. The purpose of my question is to make you think about the process of heat transfer. It's source, the means of transfer and its effect on the objects we are viewing with our thermal cameras.

My point in posting this question is to point out that we must perceive heat transfer from all three sources together. Just as the total energy we see from an object in our infrared camera which comes from three sources; emissive, transmissive and reflective energy.

Total energy = Emissivity + Reflectivity + Transmissivity = 1

The transfer of heat occurs from conduction, convection and radiation.

Q (Total Heat Transfer) = Radiation + Conduction + Convection = 1

What is heat?

Heat is energy, so the amount of heat is the amount of energy stored in an object. It is measured by temperature.

What is temperature?

Temperature is the rate of molecular movement within an object depending on its quantity of heat energy stored. It is the state an object is in, it is not a form of energy.

What is cold?

There is no such thing as cold. Cold is relative. Things are hot, hotter or not so hot. Just like the Kelvin scale of temperature, 0° Kelvin is no heat, everything above that is more or less heat. But we will use "colder" as a term for a an object that has less heat energy in comparison to another.

Heat energy always moves from hotter to colder (a higher temperature to a lower temperature). The amount of heat transfer is the "net difference" between two objects. We often use the term "delta T".

In Barry's excellent example:

Radiation; is heat energy that passes through space by electromagnetic waves. It does not require molecular contact to transfer. All objects radiate electromagnetic energy. Remember this! "All objects" not just the sun or objects that we perceive as a heat source.

Conduction; is heat energy that passes through a substance from one molecule to another. The more densely compact the molecular structure of the object, the higher the rate of heat transfer. It is a direct transfer of heat energy which can occur in solids, liquids and gases. It is the only means of heat transfer through a solid object.

Convection; is heat energy that passes by a moving fluid (a liquid or a vapor).

Heat transfer through convection is not efficient within the fluid. Hot water and cold water do not mix well, nor does hot air and cold air. Air stratification, stack affect and other conditions frequently cause uncomfortable conditions within a building because the air does not mix well.

Heat transfer through convection causes significant changes in an adjacent object's temperature because of the constant circulation of the fluid which maintains the high delta T between the fluid and the adjacent object required for rapid heat transfer. This can cause significant problems for the thermographer due to the high heat transfer rate associated with convection.

Why does air rise?

Air rises due to convection. In order for convection to take place, a change in energy must be effected on the fluid. Where does this energy come from? Remember, we must consider all forms of heat transfer in every application.

Could it be conduction? Most of the time it is. Any object hotter than the air will begin to heat the air molecules which it comes in contact with through conduction. An example is Barry's post where the sun's radiation strikes the floor through a window. Radiation of heat energy through the air strikes the floor and is absorbed, raising its temperature. It could also be a space heater or appliance.

You do not always have to "add" heat energy to the fluid to create convection. Remember to consider that a loss of energy may also result in convection. An uninsulated attic in the wintertime may remove heat from the air causing it to fall to the floor forcing the warmer air to rise.

How about radiation? Air is made up of molecules of different substances and radiation from a heat source or object of a higher temperature than the air will transfer heat energy to the air molecules. Molecules increase in temperature, which by definition is molecular motion, causing them to bang into each other transferring heat from one molecule to another (sounds like conduction?). This violent activity causes the molecules to separate from one another causing the air to become less dense and lighter than the air with less molecular movement. Air that has less density begins to rise due to displacement and floats above the cooler air because it is more buoyant.

As the air gives up its heat when it comes in contact with an object such as the ceiling for example, the molecular activity slows and becomes more dense and falls back to the floor where the process is repeated.

Though we frequently perceive heat transfer in the air as convection, a percentage of "transfer" occurs through conduction within the air, as well as from the air to another object. We should always perceive convection and conduction together when considering heat transfer.

So in looking back to our initial question, we see that all energy comes from our sun. Due to the emptiness of space, almost 100% of the sun's energy is radiated. Its first encounter is the Earth's atmosphere. Some of the energy is absorbed and raises the temperature of the air. If the air temperature is raised high enough, it will conduct as well as infiltrate by convection through the building envelope and raise the temperature inside the house.

The remainder of the sun's energy strikes the buildings roof and exposed walls. The energy is absorbed by these materials and heat is conducted towards the indoor air which is cooler. On the way, the energy encounters an airspace. This may be between the sheathing in the wall insulation or the roof deck and the ceiling insulation. Heat conduction produces convection (if the conditions are right) and transfers heat to the insulation. It is also radiated from the sheathing directly through the air to the insulation. The insulator conducts heat to the interior wall/ceiling. There is also convection occurring (at a much slower rate (R-value)) within the insulation. Once reaching the interior Sheetrock wall/ceiling the energy is again radiated conducted and convected throughout the room resulting in a rise in temperature within the space.

So if you did not evaluate the entire path from the sun to the indoor air of the house and all the potential means of heat transfer along this path you are only perceiving a portion of the process.

I encourage all infrared thermographers to evaluate all aspects of infrared science (regardless of how insignificant it may appear) because the accumulative effect creates a greater error factor which may eventually become significant.

Though the qualitative analysis of infrared scans can produce sufficient information to locate a thermal anomaly which can be further investigated and determine its cause and effect on the building, understanding quantitative information will greatly improve your ability to identify conditions that are less obvious.

Go to the initial  conversation at: Your Leaking House.com

Submitted by DavidAndersen on Mon, 03/16/2009 - 08:47.