I don't know what heats the walls of a firebox, but i know that it is either air or water!

Nearly always, it is hot gases that flow in the air box of a fireplace that heats the walls. Water would be dangerous, as if it boiled, pressures would rupture the walls.

To determine what heats the walls of a firebox, we need to understand the principles behind a fireplace's operation. A firebox is typically designed to generate heat through the combustion of fuel, such as wood or gas. The heat produced during combustion is then transferred to the surrounding environment through various mechanisms.

In the case of a typical fireplace, it is the hot gases resulting from the combustion process that primarily heat the walls of the firebox. When fuel is burned, it releases energy in the form of heat. This heat causes the surrounding air molecules to become hot and expand, creating a flow of hot gases that rise upwards due to convection. As these hot gases rise, they come into contact with the walls of the firebox, transferring their heat energy to them.

The proximity of the hot gases to the walls allows for efficient transfer of heat through conduction. The walls of the firebox absorb the heat from the gases, causing them to become hot. This heat is then radiated from the walls into the room, contributing to the overall heating effect.

Water is not commonly used to directly heat the walls of a firebox for several reasons. Firstly, water is a poor conductor of heat compared to air. This means that it would be less effective in transferring heat from the combustion process to the walls. Secondly, if water were present in the firebox, it could potentially reach its boiling point and create high pressures, which could lead to the walls rupturing or other dangerous consequences.

In summary, it is the hot gases resulting from the combustion process that heat the walls of a firebox in a typical fireplace. This heat transfer mechanism allows for efficient heating of the surrounding environment. Water is not commonly used due to its poor conductivity and potential safety hazards.