Type B vent M/F end vertical positioning
I have a chimney that is used for two natural gas appliances: a 40 gal water heater and a hydronic radiator boiler (approx. 80K BTUs, give or take). The water heater uses 3" type B vent, the boiler uses 6". The two pipes are separated on their way from the appliances to the chimney (approx. 14 ft for the 6" boiler pipe and approx. 9 ft for the 3" water heater pipe, each making 3 90 deg bends). At the bottom of the chimney, I have a 6" type B tee which join the two incoming pipes into the chimney liner, also type B.
I was told that in case of type B vent pipes, the male should always be on top pointing down and the female the opposite, on the bottom and pointing up. That way the exhaust glides smoothly over the joint inside where the female inside wall of the bottom pipe laps over the male of the top pipe and the fumes coming from the bottom going up.
The problem I am having is this: one side of the hub tee is male and the other female, as it should be but the center is female and I need it to be male because it is pointing down. The 6" pipe comes from the bottom of the tee and the 3" pipe comes into the center through two adapters (3->4" and 4->6"). The adapters' M/F siding is also reverse from what I need it to be, i.e. the bigger side is male and the smaller side is female. This results is the entire 3" water heater piping being positioned reversely, which is (as I understand and would like to ask you to verify) unacceptable because the fumes can get trapped and escaped in joints where the female is on top (kind of like as if you put shingles on the roof pointing up and not down).
Since in my area there isn't a store willing to sell type B venting to a non-professional, I've been ordering my material from a store out of town. The sales rep I've been working with, who is knowledgeable but not a certified professional, cannot find an adapter which could solve my problem. I guess it would involve and adapter whose both sides are male and he says that is not acceptable because that scenario would still involve one reverse joint (the adapting one), which is still better than having 8 of them downstream but is still a problem.
So here I am, asking for ideas how to approach and solve this problem. It might even be that the M/F positioning is not even that big of an issue, I don't know but would like to verify since it is carbon monoxide that's at stake and a mistake could be lethal. I am attaching the picture with annotations for a better understanding.
Thanks in advance.