Heat, temperature, perception and thermal comfort

What links heat and temperature, mass and thermal energy, what we feel and thermal comfort?

These terms get mixed up all the time, so it is worth defining each of them and seeing how they connect.

Heat is a form of energy that transfers between two bodies at different temperatures, and it always travels from the warmer body to the cooler one.

Heat is the transfer of energy from one object to another. It is the energy of the movement of the particles in an object.

Temperature is the measure of how hot a body is. It measures how agitated the atomic particles within the material are. The faster those atoms or molecules move, the higher the temperature.

Mass is the quantity of atoms contained in the material.

The more molecules an object contains, the more thermal energy it holds.

The higher the temperature of an object, the more thermal energy it holds.

And the reverse in both cases.

Take a glass of water at 70 °C and a bath filled with water at 25 °C. Which quantity of water holds the most thermal energy?

The temperature is far higher in the glass, but the glass contains far fewer molecules. The thermal energy is therefore lower in the glass than in the bath, even though the bath is at a lower temperature.

Now suppose we want to warm a swimming pool using a bucket of water at 35 °C, and another pool using a glass of very hot water at 70 °C.

Replace the water molecules with banknotes:

a bucket filled with 50 euro notes and a glass filled with 100 euro notes.

The total amount in the bucket is far greater than the total in the glass.

In the glass, the quality of the hot water, like the value of each note, is higher.

In the bucket, the quantity of hot water, like the number of notes, is greater.

It is the speed of the molecules that determines the quality of what is hot. That quality depends only on their speed and has nothing to do with their total mass.

Even though the water molecules in the bucket move more slowly, there are far more of them. What they lack in speed they make up in number. The quantity of what is hot depends on both mass and speed.

The great difference between temperature and heat is that temperature depends on speed alone, whereas heat depends on both speed and mass.

Quality of what is hot = temperature = speed

Quantity of what is hot = heat = speed + mass

The water in the glass is at a very high temperature but holds very little heat. The water in the bucket is at a much lower temperature but holds a great deal more heat.

In conclusion, a bucket of water at 35 °C warms a swimming pool more effectively than a glass of water at 70 °C.Quantity at a lower temperature beats quality at a higher one.

Thermal energy depends on the mass of a body and on how agitated the particles within it are.

Thermal energy = mass + temperature

A body at 25 °C with a heat exchange surface forty times greater than that of a hot body at 70 °C holds considerably more thermal energy.

That greater mass combined with temperatures that stay low, close to that of the skin, is what we call thermal comfort.

What we feel is the link between the idea of heat and the idea of temperature.

Here is a short experiment to show it.

Take three containers of equal volume, each at a different temperature.

Put your left hand in the one at 5 °C and your right hand in the one at 35 °C, for thirty seconds.

Then take both hands out and put them into the middle container, whose water is at 20 °C.

What will you feel?

Your left hand will register a sensation of warmth, while your right hand registers a sensation of cold. And yet the container is at 20 °C for both hands.

Which shows that what we feel is a genuine sensation that is nonetheless wrong in thermal terms, and that heat and temperature are two different things.

Thermal comfort is a close relationship between the mass of a body and its temperature.

A radiant ceiling offers the largest possible surface for heat exchange at the lowest possible temperature.

That match between area and working temperature, together with the absence of any obstacle to the spread of that energy, is what makes a radiant ceiling the most effective form of heating of those available.

It puts to work the same principles that govern the effect of sunlight, which is where our idea of comfort comes from in the first place.