Rules on building a manifold, vent connector

I don’t know anything about tapping into vent connectors, it just seems silly to have a bunch of large tees stacked one after the other. But if that’s how it has to be, I am fine with that.

Years ago, somebody tapped into this 9 inch.

Can I do the same general thing? What are the guidelines. I’ve got four small boilers, 4 inch diameter coming off each one.

The traditional water heaters are going away, I will indirect water heaters coming off the new boilers.

Certainly somebody’s gonna ask me about chimney size, the boilers will be 60 K each, the chimney is about 30 feet high.