Is this vent stack reduction OK?
Not reduction they have increased the size, reason is because in Cold weather the moist vent air could close a 1 1/2 or 2 inch stack .
So it is required to be 3 inch so the frost can not close the vent . Many older ones I see are still 1 1/4 or 1 1/2 and have been this way for 50+ years . No big deal
Thanks so much Roy!
Yep, just as Roy stated.