Gas Furnace Exhausting 4” type B into 3”

Is it acceptable to route a 4” type B vent pipe into a 3” type B just prior to the roofline and seal the connection with foil tape?

This just doesn’t seem right to me but wanted to get an opinion. I think the installer took some other shortcuts and am wondering if this is even code?

Thanks!

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source https://hvac-talk.com/vbb/threads/2238446-Gas-Furnace-Exhausting-4”-type-B-into-3”?goto=newpost