enter image description here

As in the picture, we have a room with vents at the bottom to let cold air in and at the top to let hot air out. Normally the air flow is limited by the temperature gradient (correct me if I'm wrong). If we place a hot object in the vent on top, won't that create a faster draft? That way we could get very high "wind" speed indoors to replace fans.

Is this possible, and if yes, what's the limit for air flow per second?

  • $\begingroup$ You need to do some calculations. Upward forces are a result of expansion of air and pressure from it being lower density. Some downward flow can also result from expansion but it will eventually cease at steady state since air needs to get there from somewhere (and it's probably not coming from above thanks to that density unless the chimney is ridiculously wide). It'd be a matter of how much the hot object cools by the time things get there. $\endgroup$
    – Abel
    Sep 29, 2023 at 14:03
  • 1
    $\begingroup$ isn't that exactly how a fireplace chimney works? $\endgroup$
    – jsotola
    Sep 29, 2023 at 17:35

1 Answer 1


I take a stab at this very unfocused but interesting question.

  • Yes a hot object like a small flame near or even inside the vent would accelerate the airspeed and potentially help move the heat from the room if the air pressure shock it creates would ride behind the temperature gradient flow, pushing it out like a rocket nuzzle. Also the heat it radiates or converts is blown away by the flow faster than it gets a chance to be absorbed by the duct and walls or even backfire.
  • such a system would work under a limited set of controlled parameters such as initial velocity, temperature, and laminar flow. It may need priming to get to those conditions and people and furniture in the room or opening a door may break the cycle!
  • The design of the vent and stack has to accommodate this (rocket nuzzle) action.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.