BOB,
I usually turn my vent box fan on @ ~10 minutes into the roast(the onset of 
smoke).  The vent box is 2ft tall with a 250cfm fan.  I have not notice any 
difference in roasting times with fan turned on @ 0 or 10 min. I also roast 
with an HWP. 2ft was too low.  The plastic vent screen deformed from the HWP 
heat output.
Noel V. Hong
email: nhong32590
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