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Does circulation or mechanical movement of air within envelope…

Austin G | Posted in General Questions on

… affect stack effect at all?  Thinking more about air movement and ventilation than I intended in this final step.  I’m just curious if mechanical circulation of air within a building’s envelope can affect stack effect in a measurable way?

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  1. Expert Member
    Peter Engle | | #1

    To the degree that it mixes the air and reduces stratification, yes. The stack effect is caused by warm air being lighter than cold air. This buoyancy causes the warmer rise and cooler air to fall. if you mix the air vertically using mechanical equipment, the stack effect within the envelope is eliminated. But you've also got a stack effect with inside-outside air. In winter the indoor air is significantly warmer than outdoors. That warm air wants to flow out of the top of the building, drawing cooler in at the bottom. Mechanical mixing will reduce this effect, but only a small amount as the indoor-outdoor temperature difference is much larger than the vertical temperature difference inside the house. This is one more reason that good air sealing of the envelope is so important.

  2. Jon R | | #2

    +1 on what Peter said. The effect is small (eg, .1 pascal), so make sure that mixing doesn't do more harm than good.

  3. Tom May | | #3

    Though warm air does tend to rise, heat transfer can happen in any direction depending on the temperature difference. So say you have a very well insulated ceiling that can trap the warm air, then you mix it up, it may then conduct out your floor. Q= -kA(T2 - T1)

  4. Austin G | | #4

    Makes sense. Thank you both. I intended to circulate the air anyway, as I need to distribute the conditioned air to the areas furthest from mini split heads, but was just curious about this while working through it.

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