On just about every house I've seen here in the west (CA) the vent stacks are just a bare pipe sticking out of the roof. There is no hat on them. I understand that all the vent stack does is vent the house's sewage system so that water flows downward unimpeded by suction, and since it is directly attached to the sewage system, there is no obvious reason to place a hat on it; so what if rainwater falls into it?

This having been said, when wind blows over the top of a vent stack, good old Bernoulli creates somewhat of a vacuum, depending on the velocity of the wind. The stronger the wind, the stronger the vacuum. This vacuum can draw the water out of the bowl of a toilet, which in turn would allow sewer gasses to enter the home. On some extremely windy days, I've seen the toilet bowl almost empty of water, though it has never actually been drawn down to completely empty. I imagine it would take a pretty strong wind to completely empty the toilet bowl, but I wouldn't characterize that as being completely impossible.

I hat placed on top of a vent stack that shields the top from direct wind but leaves it otherwise open to the air would prevent the Bernoulli Effect. It would also prevent birds from setting up housekeeping on top of the vent stack. So, why aren't vent stack hats more common? (I'm assuming they are used in some places.)

  • Vent stacks are also to relieve sewer gas pressure, to keep it from comically ejecting the water out of your traps, fill your house with stinky methane, and kablooey. – Harper - Reinstate Monica Feb 2 at 23:15
  • how will a hat prevent crosswind at the top of the pipe? ...... also, don't you think that the suction from the Bernoulli Effect would pull gasses from the sewer instead of pulling water from the toilet? – jsotola Feb 3 at 3:55
  • @jsotola, I've seen water missing from my toilet on windy days. If you know of another mechanism besides Bernoulli that could account for that, I'm all ears (er...eyes?). And a hat whose brim is lower than the top of the vent stack would prevent wind from blowing across the top of it. And as to my comment concerning rainwater, I said, "...so what if..." – BillDOe Feb 3 at 21:44
  • is your house connected to a septic system? – jsotola Feb 3 at 22:20
  • @jsotola, no. It's on a municipal sewer system. – BillDOe Feb 3 at 23:38

You are right about the Bernoulli effect. Additionally, I have had customers who have complained about odors in the front yard when the wind is just right.

As far as emptying the toilet bowl, the government may need to bear this responsibility. Manufacturers, in their rush to meet the demands and constraints of an out of control national legislature, requiring ultra low flush commodes, weren't able to get all the bugs out in time. They had to meet the requirement and still keep product on the shelves. Some of the early models had this problem. I think most of the later models have this problem under control. It has to do with how they flush the bowl and how they keep the bowel filled.

As to a fix, here is an idea that may work. It is an engineered concept and works in some situations. Say your problem stack is a 3" ABS riser sticking through the roof. Take a piece 5" diameter of the same material about 8" long. Fix it to the top of this stack. This sleeve will run parallel to the stack. Put equal sized spacers between the stack and the new sleeve to drive your support screws through. The sleeve should not touch the stack. Extend the sleeve above the top of the stack about 4". The sleeve will also hang below the top of the stack 4". This sleeve will break-up the wind currents and take the pressure off the top of the stack. Good Luck.

  • You're right about the toilet; it's one of the earlier low-flow models. Your solution is quite brilliant. Thanks. – BillDOe Feb 3 at 21:48

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