I have a direct vent also, have sealed all joints (even the elbow joints and screw holes) and these DV's do require more maintenance/cleaning.
I purchased a draft gage from Keystoker for about $80, works great. My boiler is at -.02 over fire, -.04 idling. Had a very slight odor but found the installer actually installed the flue incorrectly
and I was probably getting a little exhause out of the upper elbow because of that. I will fix that this weekend and re-fire it.