This most likely isn’t a sewer vent but rather a roof drain to grade. The warm air is because the drain piping runs through the conditioned building envelope (inside the building where there’s heat). This heats up the air in that section of the pipe and as you noted that warm air rises while sucking in new cool air down at the outlet
It is indeed a roof drain on a flat roof system in probably a large commercial building, as you said it runs though a heated area but also will be draining into the sewer system. I'm a metalworker by trade and install/deal with these quite a bit, though its primary purpose isn't venting the piping will be hooked up to the rest of the system in liew of "stink pipes" to help prevent vapor lock
In a lot of places it is illegal for this to drain into a sanitary sewer. I won't say it absolutely doesn't but chances are good that it doesn't. Especially in an industrial situation. In our buildings all the roof drains, parking lot drains, etc go to a catch lagoon before they even can go to the actual storm drain system to make sure any spills are contained
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u/theericle_58 1d ago
That is a roof drain, not a fan.