Two Appliances in One Flue

Rumford Technical Discussion
Two Appliances in One Flue?

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The code seems to allow it:

R1003.14 Flue area (appliance). Chimney flues shall not be smaller in area than that of the area of the connector from the appliance [see Tables R1003.14(1) and R1003.14(2)]. The sizing of a chimney flue towhichmultiple appliance venting systems are connected shall be in accordance with Section M1805.3.
M1805.3 Size of chimney flues. The effective area of a natural draft chimney flue for one appliance shall be not less than the area of the connector to the appliance. The area of chimney flues connected to more than one appliance shall be not less than the area of the largest connector plus 50 percent of the areas of additional chimney connectors.
But generally it would be preferred to run a separate flue for each appliance to insure adequate draft for each appliance especially if the two appliances would be in different rooms or in different pressure zones.