To put it simply, draft is the force that moves the gases up the stack to get rid of them. Clearly, draft is one of the important factors in a combustion system. Draft can be either negative (atmospheric burners) or positive (forced-draft burners). There are many factors that will affect the draft on a combustion system, most of which cannot be altered, but changes in draft can be a sign of problems in the flue system. Draft in atmospheric burners is basically caused by the pressure differential due to the height of the stack. In operation the heat of combustion will also contribute, but there must be a basis draft when starting. Many modern burners will not start if the draft falls below a certain level.