I have a question about flow direction of the sewer vent for a house.
So you have a vent stack in the house that all the plumbing connects to. This vent exits the roof.
You have an onsite septic system with a septic tank that flows via gravity to a leach field. The leach field has a vent that protrudes about 4 feet off the ground.
Does the air flow into the leach field vent and exit the roof stack, or the other way around?
Simpler question (same principle):
You have a vent at elevation 10 and another vent at elevation 20. I say the airflow will exit the taller vent. Correct?
Sewer gases collect in lower areas and will displace air. A pipe would clear out from the roof vent down to the level of the vent opening in the field vent if it was open the whole way . I wouldn’t expect any kind of breeze to form that clears the pipe of the gases over it’s whole length. The pipes are there so the trapped gases will not break the water seal in the drains and burp sewer gases into your residence.
Air does not normally flow through the vent pipes at all. The vent is provided so that water draining from a fixture does not create a siphon and remove all of the water out of the fixture’s trap. As Harmonious Discord mentioned, the trap is to prevent sewer gas from entering the building trhough the drain pipe.
I’m not an expert, but I believe both are exhausts for their respective sections. Air does not flow between the two, for the same reason that you never see bubbles in a swimming pool sink to the bottom.
In currently mandated septic tanks, there is no continuity of airflow between septic leach field and building (fixture vents). Effluent enters a baffled chamber aerobically but flows under the baffle to be digested anaerobically. Venting exists to maintain atmospheric pressure and prevent aforementioned vacuum/syphonage.
Septic systems aside, as regards the stack effect (chimney effect):
"You have a vent at elevation 10 and another vent at elevation 20. I say the airflow will exit the taller vent. Correct? "
>So if you have 2 vents, one short and the other high, would air flow into the high and exhaust out the low?
If it does, the guys who spent their careers working on sewage plumbing when they could have cashed in on perpetual motion machines and free energy are gonna be mighty ticked.
I think there are a few reasons that there could be a little flow in one direction or the other in this system. There’s the chimney effect - if the entire system has air in it and is vented below and above the part of the system that is in your heated house, the air in that part will be warmed by the house around it, and will get a little lighter and start floating up through the pipes. The opposite would happen if you run air conditioning (though probably not so vigorously because AC rarely differs from the outdoors by as much as heating does). Then there’s the water vapor, which at warm temperatures can be a percent or two of the total. Since it is roughly half as dense as dry air, it will help the air in the pipes rise. Of course, sending water through the system can entrap air or displace it. The flow of the wind over the tops of the pipes, and around their different environments, can also create small breezes inside. But, as the others say, there isn’t supposed to be much going on typically.