What causes an updraft in a chimney even with no furnace or fireplace on?

Question

i was told by an hvac mrf rep that draft is caused by the slight difference in air pressure between the top and bottom of chimney,,,but a knowledgable family member said thats not true......question also assumes there is no wind outside creating a venture [sp?] effect...

Answer 1

Air flowing across the end of a tube causes a drop in pressure at that end and hence a flow of air along the tube. This is called the venturi (or scent bottle) effect. Without the flow of air across the opening of the chimney there can be no pressure difference between the top and bottom.

Answer 2

Bristolman has a good answer but ... you say yourself that it assumes there is no wind. And without wind, no venturi effect (unless, of course if the said chimney is on a moving truck! :-)

The only thing that can make air to rise in a chimney, apart from the venturi effect, is heat. True - you say that there is no furnace. But if the room is heated and the temperature higher than the atmosphere outside, then it would be enough to see air rising in the chimney.

On the other hand, if the temperature in the room is lower than outside (air-conditioning) then no air will move in the chimney.

It can be compared to the convection in the atmosphere due to the heating of the earth surface, and the the subsidence of the air sinking in a high pressure, creating an inversion, i.e. warm air above cold one.

Answer 3

Warm air rises and the air outside is cooler and even cooler as you rise toward the ceiling but a chimney provides for a release of heat from the house.