Two gas appliances vented into a common flue with the smaller connector entering below the larger is:

Common venting of two atmospheric appliances is ordinary practice, and the rule about which connector enters higher follows directly from how draft works. Hot flue gases rise, so the appliance producing the larger volume of hot gas establishes the draft that carries both. Its connector enters the common vent above the smaller appliance's, so the smaller unit's gases join a stream already moving upward rather than being pushed down toward it. Reversing that order lets the larger appliance's output pass across the smaller connector and potentially drive gases back down it. The arrangement described here is the correct one, which is why the answer is that it is conventional — subject to the real requirement, which is that the common vent, the connectors and the vent height be sized to the combined input using the published vent tables.

Entering the common vent with the smaller connector below the larger is the conventional and correct arrangement, because the larger appliance establishes the draft that carries both. Its acceptability depends on the vent and connectors being correctly sized to the combined input.

Two atmospheric appliances may share a common vent when sized using the published vent tables for combined input. The smaller appliance's connector enters below the larger, so the larger unit establishes draft, and each connector requires adequate rise before joining.

Big One Enters High. The larger appliance pulls the draft, so the small connector joins underneath and rides the stream upward.

Watch for the orphaned water heater as the related scenario: replace the furnace with a sidewall-vented unit and the shared flue becomes badly oversized for what remains.

• •Treating all common venting as prohibited
• •Checking entry order without checking combined sizing
• •Confusing this with the orphaned-appliance problem after a furnace upgrade