We get asked all the time about connecting an exhaust to the roaster. The manufacturer, as well as from our own testing, do not recommend attaching an exhaust directly to the back of the roaster as it creates a lot of back pressure which can have negative effects on the roast quality as well as the roaster including excessive smoke and moisture build up.
If your roasting setup requires some sort of an exhaust, we would recommend finding an in-line duct fan of some sort, and placing behind the roaster several inches from the exhaust port, so that it can draw ambient air as well as the smoke and moist air from the roaster.
Note DO NOT connect in line duct to the roaster, if the In-line duct fan is connected to the roaster, that will also pull all the heat from the roaster having a negative effect.
Below are several examples of proper ventilation.
An example of the fan can be found here. https://www.vevor.com/inline-duct-fan-c_11974/vevor-4-inch-inline-duct-fan-hvac-exhaust-blower-kit-ac-variable-speed-control-p_010488857098?adp=gmc&utm_source=google&utm_medium=cpc&utm_id=20083588474&ad_group=&ad_id=&utm_term=&gad_source=1&gclid=Cj0KCQiAwOe8BhCCARIsAGKeD557arsCC8A43b3vXxeGM3mEnw8xufe9cXrV4uv8rn26oS8u8azJ-fYaAtEWEALw_wcB