furnace/water heater venting together

[Matt Barenbrugge]

I am working on a house where the furnace and hotwater heater are on the same vent. The vent from furnace and water heater are three inch. then they "Y" to a 6-7" double wall which makes sense to go through the floor and roof. Now here is where I am stumped. Above the floor behind a bathroom vanity the double wall reduces to four" then back to 6-7" before going through the attic. Is this OK?

[Jim Luttrall]

No, not from what I envision.

[Lon Henderson]

The short answer is no.

[Markus Keller]

NO
Also the original connection may or may not be proper depending on whether the furnace is power vented or not.