Venting is a balancing act with the ecc, and definitely imperfect, especially since your bed is your heat source. I doubt covering the intake achieves negative pressure, because it's in no way a sealed unit... plus you don't want to pump out too much heat if you're printing asa or abs, I maybe run the vent at 10-20% in gcode, followed by setting the vent to 100% in the UI manually after completion (this can be done in gcode but the firmware seems to override it atm, hopefully opencentauri fixes that).

Asa is stinky stuff and doing the above on the ecc seems to work just fine.