Vent joints no longer required to be sealed

[Mike Reno]

I'm told that taping the joints on exhaust vent pipes is no longer recommended, and that the joints won't leak because the furnace motor will push the exhaust through. Just trying to confirm.

[Jim Katen]

Taping the joints was never recommended or required. In fact, with listed B-vents it's prohibited - always has been. If we're talking about a Category I furnace (hot exhaust) there should be negative pressure in the vent anyway - never positive pressure. That's part of the definition of a Category I furnace. 

The furnace motor does not "push" the exhaust through. The exhaust rises by buoyancy, creating negative pressure behind it. The draft inducer (the thing you're calling the furnace motor) only serves to draw a regulated amount of air across the burners - it does not "push" air through the vent. 

Category IV furnaces are different. They require sealed pipes - but not with tape. Their pipes must be cemented plastic.