Mr Jim Olson (Facebook GMG Group) has done a lot of testing with one of those units and here is one of his findings.
Jim Olson
Admin · January 7
Down Draft testing is complete. Today ended up being perfect as the temp fluctuation all day was very minimal. Both tests the ambient outside air changed less then 2 degrees during the test. We did have a slight breeze, as well and the test ranged from 8-10 degrees. The grill used was a completely stock DB, heat shield measurement was 4.25 drip pan vent was closed but not covered in aluminum foil. I weighed out 5lbs of GMG Texas Blend pellets (I zeroed out the container the pellets were in to get as close as possible). I am running firmware 6.5, which is the icy setting in the climate slider for 6.7. The grill was running at 250 degrees during this test, I also pushed all the pellets to the auger to make sure it used up all 5lbs and then let it run out and burnout on its own as you will see in the graphs. Also on a side note, I did not dial the grill in, I set it at 4.25 knowing that whatever happened in one test, should replicate itself in the other test.
I tried to cover as many variables as possible, but I likely did not hit them all
I will quickly run through the Downdraft claims below and give