[esp-r] air flow problem in double-pane window

[Leepaul]

Dear all,
 
I simulate a natural air flow in a double pane window, which has a upper exit and lower entry.
I run simulation in Summer typical week, Spring typical week, and Autum typical week, each air flow rate seems ok, then keep anything unchanged, and only run the simulation in the Winter typical week, then the air flow rate is always zero, it's obviously not correct.  Does anyone have the similar experience? and what results in this? 
 
Thank you.
Paul

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