Why is this ok?

[mhix (Matthew Hix, CMI MD -32331, VA - 3380000698)]

This diagram, taken from an ashi reporter article, is from simpson dura-vent and an example of an acceptable configuration for common venting of a hwh and furnace.

It has always been my understanding that the hwh (if lower btu) must enter the common vent above the furnace. Is this not the case?

http://www.ashireporter.org/HomeInspection/Articles/The-Venting-in-Common-of-Multiple-Gas-Appliances/2067

[cevans (Chuck Evans, CMI TREC 7657)]

It does in the diagram I’m looking at.

[mhix (Matthew Hix, CMI MD -32331, VA - 3380000698)]

Ah, so if the draft hood of the hwh is higher than the furnace collar, it can join the common vent anywhere? That doesn’t seem right.

[mhix (Matthew Hix, CMI MD -32331, VA - 3380000698)]

Think I found the answer.

The
smaller appliance should enter the common flue above the larger BTU rated appliance and as high as possible whenever practical. However, if the smaller appliance must be below, the increase in vent size for the common vent must occur below the larger appliance inlet.

http://www.ambest.com/directories/bestconnect/CaseyOMalley.pdf

[lkage (Larry Kage, CMI)]

Think I found the answer.

http://www.ambest.com/directories/bestconnect/CaseyOMalley.pdf

Correct: