Chimney flue heights

Most houses in my neighborhood have chimneys with three flues. Some have the flue heights staggered, while some are all the same. Does the staggering help prevent smoke exiting one flue from going down another? If so, how different do the heights have to be to be effective? Does the order matter (SML or SLM)?
Also, how much space does there have to be from the highest flue to a chimney cap?
Thanks.
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user wrote:

Are you sure the two of the flue liners aren't just decoration? There is no reason to have three operating flues unless you have several heat sources.
The height of the flue above the chimney cap is primarily a function of looks (and maybe how much snow you get). Three or four inches is probably a minimum. The actual height of the flue is based on a minimum horizontal distance from the roof line and height related to that point. It is based on draw capability and blockage of air flow by a near surface. Local codes may vary.
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heat rises.
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Hi. Thanks for the response.
The three flues are operational: two fireplaces and furnace.
Sometimes when I use the upstairs fireplace, the downstairs family room smells of smoke. I've heard that this can happen when smoke exiting one flue goes down the one next to it. I also heard that by varying the heights of the flue caps, that is less likely. My flues are flush with each other and the chimney. I'm thinking about adding extensions to create different heights.

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