And as the chemistry of combustion takes place the fuel + air react with the individual elements in each to produce gases which have different pressures. All hopefully ends up in the chimney producing what we call draft. The nitrogen in the air is usually undesirable because it plays no part in the combustion chemistry equation and just conducts heat out the flue.coaledsweat wrote:Yes warm air rises because its lighter but actually a chimney should work without the warm air rising thing. The reason being is that it works not because of air temperature, but air pressure. At the bottom of a 30' tall chimney, let's say the air pressure is 14.7 PPSI. Now if you climb the chimney, you may find that the air pressure only produces about 14.4 PPSI Because there is 30' less air squeezing it down. The draft is created by a long cylinder seeking to equalize pressure through out its length. So any long length of pipe stood up should create a draft of sorts even without a fire. It's Bernoulli's law!
It's the engineer in me.