Not something you see everyday

3" plumbing stack vented into chimney with the water heater

Was it a working chimney?

yes it was shared with the water heater

Oh good methane in a chimney.

I find plumbing vents penetrating chimneys quite often. It causes excessive spalling of the liner (if equipped) and bricks.

I simply recommend removal.

Most of these were done this way due to the slate roof. The Plumber didn’t want to penetrate the roof.

You are correct the garage did have a slate roof. This one had been replaced a while ago.

That’s usually the case when I find these plumbing vents penetrating chimneys in the attic.