said by ArgMeMatey:Have you gotten on the roof to check both of those gray pipes? Or do you already know what they're for?
Could one of them be the exhaust from that fan? I've seen 3" PVC used for fan exhaust "duct".
I don't know a lot of plumbers who would run two sanitary system vents out that close to each other unless there were some code issue involved.
One is for the heat/hot water. Other one I believe is the tube for water smell? I don't know the correct term.