Roughing in Heights..

Larger diameter vent termination means less chance of snow bridging. Makes sense for air intake needs and sewer gas release reasons. I apprenticed in Maine. They used to get good snow and vent termination enlargement was not a thing there.

Hmmm, it seems by definition, water traveling in plumbing drain piping is normally in a type of two phase flow.

If slow drains can really be caused by vent blockage, you would think the the explanation for it would be easy to describe and would be taught to apprentices everywhere. Even if it was a bit complicated. Pointing to two phase flow isn't doing it for me.

I'm trying to conceptualize it: The vent provides a place for air to move out of the way in front of the water for the short piece of pipe between the trap and the vent take off. That "air in the way" event takes place just after the drain plug is pulled and once flow passes the vent it's no longer in the way. Air in front of the water could always move out of any other vent or down the drain to the sewer. Increasing pipe diameter as the water travels makes back pressure very unlikely. Horizontal flow is mostly separated exchange of space. Water below and air above in counter flow if needed. Air from the vent of the draining fixture can only be incorporated into the water flow. Incorporating more air would displace water and cause a more turbulent two phase flow. Wouldn't this mean a working vent would be more likely to cause air to be incorporated. This doesn't support the idea that a blocked vent slows draining via more turbulent two phase flow.

Ok, I woke up with a maybe plausible. If the sewer gas flow rate leaving the pipe system through the house's venting is high enough and a vent is blocked, it causes a continuous positive pressure condition inside the pipe and up stream flow gas velocity increases in the unblocked pipes. Water entering the pipe might be slowed by the countering gas leaving a working branch. The main would have a static pressure preventing gas from being pushed back into the septic or sewer. The volume of water going down displaces the gas volume and that adds to static gas counter pressure. Maybe?

I see that lots of plumbers believe the idea that blocked vents do slow draining. I really would like to know how it works. It's written about in info ads. Many use the finger and straw analogy. Some point to internal vs external pipe pressure differences as a reasoning. I haven't found any complete explanation for the belief beyond word salad sales talk meant to satisfy the customer.

I do understand that experience has its own valve beyond a scientific explanation. "I unblocked the clogged vent and water flows better again. What more proof do you want?" We humans used to kill epileptics because we "knew" they were possessed by demons. And that made perfect sense to all involved while also being completely untrue.

I get the double trap problem. That makes good sense. Most areas don't have whole house traps. Aren't double traps illegal in all but NY and some other older places. An air Admittance (meaning only letting air into the pipe) valve above another trap with no other vent in play is an obvious problem. I still don't see how a properly plumbed system with a blocked fixture vent would slow draining. Maybe if it's on septic and all vents combine to one and it's capped. If there's nothing preventing the air in the drain from moving down the drain and out another vent I don't see how it can slow water down measurably. The finger over straw analogy doesn't apply in the double trap cases. The finger prevents air admittance preventing the straw from draining.