vent size increase

I am either having a brain fart or reading this wrong.
According to G2428.2.11 the vent for a gas appliance can increase but only 2 sizes and it must be at the draft hood outlet.(G2427.10.3.5)
I have seen many times a 3 inch mechanical draft system for a boiler jump from 3 to 4 to 5 to 6 to 8 and then enter a vertical B vent to go through the roof. Each collar for the increase is at the end of a 5 foot horizontal run then turn 45 degrees to go up.
Have no picture but… attached is a picture of a natural draft DWH that comes in from the left, then joins a vent for 2 furnaces each not HE so all is natural draft. You can see the 3 in. eventually and quickly jumping to 8 in. to vent vertical.
What am I missing or what have I forgotten?

What I see in the picture is a vent connector entering the main B vent. The installer did not have the correct fitting, or the skills to field fabricate one, so he used a series of reducers instead. I don’t see anything wrong other than a waste of money. More info is needed to determine if the main B vent chimney is sized correctly.


Thanks Erik. You are right it should not take 4 pieces of equipment to increase. But I still have a questions about 2428.2.11 which seems to contradict the table you showed and limits the increase to 2 sizes. That is why I have the question.

I use a different gas code cause I am in Canada, but they are more or less the same, except for numbering, so I have no idea what 2428.2.11 is.
To repeat, if I am seeing your picture correctly a vent connector is entering the chimney or possibly a breeching through a saddle tee. The connector ends at the tee, so the connector isn’t being changed in size other than by using two fittings instead of just one.
If you are still worried about it refer it to the AHJ, some code inspectors might fail it, some would maybe just shrug it off.

thanks - will try that