The issue is the rear vents allow wind to blow directly into to the grill, while it will mostly blow over the top of a chimney. On my Napoleon propane grille the wind will blow in through the back of the "roll back" lid and out under the front vents, which has caused problems to the point that I usually don't bother trying to grill on it when it's windy. My Stampede hasn't had a serious issue yet, but I would think a smokestack would perform better if you can't have the back of the grill out of the wind.
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