What creates thermal draft in a chimney?

The phenomenon of thermal draft in a chimney is primarily created by the difference in temperature between the air inside the chimney and the air outside. When the air within the chimney is hotter than the surrounding outside air, it generates an upward movement due to its lower density. This heated air rises, creating a pressure difference that pulls cooler air into the combustion chamber from the room, thus maintaining a continuous flow of air necessary for fuel combustion.

This principle applies to the operation of various heating systems that utilize chimneys for venting exhaust gases. A proper thermal draft ensures that harmful gases are expelled efficiently and that fresh air is drawn in for combustion.

The other options do not accurately describe the mechanics of thermal draft. While cooler outside air can influence conditions, it is the relative temperature inside the chimney, specifically that it is hotter than the outside air, that is fundamental in establishing a thermal draft.